Troubleshooting Techniques [Troubleshooting and Repairing Commercial Electrical Equipment]

To a certain extent it is possible to repair a piece of electrical equipment without a clear understanding of its inner workings. You may find a failed component by visual inspection and, by replacing it, restore the machine to operation. Alternately, it is sometimes possible to perform one or more therapeutic operations such as pulling apart and reattaching every ribbon connector in order to polish the contacts or resoldering all terminations in a printed circuit board in hope of eliminating an offending cold joint. In the absence of a true understanding of the equipment, these approaches may be valid, but the success rate is low. It is better to strive for knowledge of the structure and operating modes of the machine.

Years ago, I was called upon to repair a two-way radio base station. At the time, I had only an impressionistic knowledge of such circuitry. The unit would power up, but it would not transmit or receive, and there was no background noise emanating from the speaker with the volume turned all the way up.

I removed an access panel and it was plain to see that a circuit board had become detached from its mount so some electrical terminations could ground out against the inside of the enclosure. I remounted the circuit board, plugged in the base station, and found that it was operational.

Later, I learned in a roundabout way that the piece had been dropped and that is when the trouble started. That would have provided an important clue. Many times such information is not forthcoming; however, any background history provided by the operator is of great value.

The visual inspection method worked in that case, but it cannot be depended upon.

Often components go bad without any noticeable change in appearance, and more advanced sleuthing techniques become necessary. Moreover, circuit defects such as degraded terminations or shorted circuit board traces (caused by an errant metal filing or conductive dirt accumulation) may not show up visually.

The visual inspection method, while appropriate as a preliminary measure, is accompanied by some pitfalls.

• First, especially in modern equipment, it may not be immediately evident how to open it. The quality of plastic material has improved greatly in recent years, and is now used for equipment enclosures and mechanical parts even for very advanced and high-quality products such as digital cameras and submersible pumps. Plastic enclosures typically have some odd ways of snapping together, and may be reluctant to come apart. Obviously, it is never good to use excessive force.

Specialized tools are available. In addition, expedients such as a plastic guitar pick or an old credit card to slide into narrow gaps are effective. A service manual may provide the key, and YouTube videos pertaining to all sorts of equipment are abundant.

• A second problem in relying on visual inspection is that an obviously burnt component may not be the original culprit, but instead a consequence of some other failure mode that caused excessive current to be delivered to the site of destruction. Furthermore, in opening up it may have functioned as a fuse, preventing additional damage to the circuit. Replacing such a failed component may jeopardize other elements, making for a more difficult and expensive repair.

Therefore, we see that operating from a more enlightened perspective is usually the way to go.

It is possible to damage the very equipment you are trying to fix. Some solid-state components, especially complementary metal-oxide semiconductor (CMOS) transistors (we will discuss them later), operate at exceptionally low supply voltages and input signal levels, and they are quite sensitive to static charges that may be impressed upon them in handling. A copper bracelet equipped with a connecting wire to a known good ground is available for electronics technicians and it serves to drain static buildup from the human body. At the very least, circuit boards should be handled at the edges only. Soldering irons and other conductive tools must also be protected against static charge so that they do not transfer it to the electronic components.

If a chunk of concrete or similar material has become lodged in an empty conduit, chuck up an appropriate length of fish tape in a drill and use it to break through the offending item. Then use your shop vac to remove the debris. As a corollary, save short lengths of broken fish tape as they are handy in many situations.

Electronic equipment must be protected from damage by trauma while it is in your care.

The work surface should be nonconductive material, neither grounded nor ungrounded. If a computer monitor or TV is to be laid screen down, it should be placed on a soft folded blanket that will protect sensitive surfaces from scratches and help keep them clean. If bench grinders and drill presses are to be used, they should be located in a separate room to ensure that conductive filings do not accumulate.

A strictly visual approach may be helpful, but it cannot be relied upon. A general knowledge of electronic theory plus troubleshooting techniques and practical repair methods, as well as knowledge specific to the equipment under consideration is necessary.

It is hoped that this guide will open some doors. The mistake many aspiring technicians make is thinking they are going to fix an ailing electronic device in 15 minutes. If the machine is of even moderate size and complexity, it will take longer. You need to approach the problem in a calm and orderly fashion, and not expect to find the defect immediately.

The right diagnostic and repair tools are essential and the foremost tool is knowledge.

Fortunately, this commodity is readily accessible. A vast amount of equipment documentation is free on the Internet and, more intensively, in print. A textbook may seem expensive, but one good idea gained from it will easily pay for the volume.

A further issue has come up in recent years. Increasing miniaturization and use of proprietary microprocessors has meant that a lot of equipment that is around these days is said to be unserviceable. To a limited but not absolute extent, this is true, but that is far from the whole story. With the right equipment, as we shall see later, very small microprocessors with many pins may be de-soldered and replacements installed. It is feasible to repair printed circuit boards and, once a few simple principles are understood, the soldering techniques are not too difficult. Consumer appliances may be difficult to disassemble and diagnose, but the necessary information is out there and consistent success in this area is achievable.

Entering the Field

Before proceeding, we should make some other observations. This guide has for a premise the notion that the fields of electricians and electronics technicians, while remaining separate and distinct groups of workers, are beginning to merge. This is because of several recent trends. Economic dislocations, not to say turmoil on a worldwide level, have meant at times a marked reduction in commerce and decreased demand for services. Knowledge based professions including electricians have been partially protected from the worst effects of this slowdown. While there has been diminished demand for new construction, maintenance and repair have remained strong.

Many electricians have been able to redefine their way into low-voltage work, by which is meant telecom, alarm, and data. (Low voltage is something of a misnomer because at times the voltage and power levels are sufficiently high to be hazardous.) Regardless, such a shift provides access to much more work, making the profession relatively recession-proof. For example, new commercial buildings in most jurisdictions are required to have centralized fire alarm systems. This goes far beyond simple smoke detectors even if wired in concert to go off simultaneously. Such fire alarm systems are costly and complex but very reliable in reporting fires, even if at times prone to false alarms.

Many commercial venues such as large restaurants and hotels have robust fire alarm systems, which are great life and property savers, but nuisance alarming can be disruptive to put it mildly. Such establishments rarely have an on-site fire alarm technician, so it falls upon electricians to perform routine maintenance and, in case of malfunction, to decide when to call in a fire alarm specialist.

Some states, municipalities, and other jurisdictions require licensing for fire alarm designers and installers. However, the many ways these systems interact with sprinklers, elevators, air-conditioning and ventilation, combustible gas flow, telephone, Ethernet, and other systems, to say nothing of the electrical supply, make precise boundaries difficult to delineate. Usually the electrician is called upon to make quick, critical decisions regarding rapidly unfolding events.

Here again, knowledge is the key to successful resolution in these matters. In section 12, we will take a closer look at fire alarm systems. The point for now is that they provide an opportunity for electricians looking to expand into related areas. When the fire alarm system requires attention, we enter a troubleshooting mode. The techniques are similar for all types of systems and equipment.

In troubleshooting, the first step, as mentioned previously, is to interview the operator and gather relevant information. It is especially important to determine if the problem developed gradually or appeared suddenly. Indeed, has the machine ever functioned properly? If not, there may be a manufacturing or programming defect as opposed to failure of a discrete component. The operator should also be asked if failure was accompanied by any unusual sounds, sparks, flashes of light emanating from within, dimming of nearby lighting that could indicate an excessive current drain, outside evidence of a voltage surge, burning odor or sensation of overheating, etc. Ask the operator for any thoughts on the cause of the malfunction. It is amazing how often an operator, having come to know the machine over a period of time, can have a sense of the nature of the problem. The initial interview may prove helpful in retrospect, even if definitive information does not appear forthcoming at first.

Thus far, we have been speaking of the defective unit in terms of a small machine, something that could be easily moved to the repair shop if extensive work is needed.

However, the same comments apply to any size equipment from the largest machine ever built, the North American electrical power grid, to microscopic motors that are now being constructed at the very frontier of nanotechnology.

Complex machinery, such as the huge particle accelerators now in operation and straddling international borders, are equipped with a lot of instrumentation and in many ways have self-diagnostic capabilities. Commonly encountered electrical equipment may have a power light and a temperature gauge, or much more extensive instrumentation. An elevator or fire alarm system will have a control panel with alphanumeric display intended as a human interface. Much smaller equipment, such as a household microwave oven, will have a readout that displays time and operational details, and may provide a seemingly cryptic error code in the event of failure. The error code (something like E8) may be referenced in the operator's manual. If not, you can type the error code with make and model into a search engine and instantly come up with a diagnosis and suggested repair procedure.

Let us say, for now, that a machine has been examined visually and no obvious defect has been found. What next? Based on information provided by the operator, you can attempt to power up the machine. If it runs on other than 120 volts and you do not have the proper volt and ampere branch circuit and receptacle in the repair shop, you will have to run a line from the nearest panel. It is essential to verify that you have the correct size wire and overcurrent protection. Consult the National Electrical Code (NEC) for sizing and branch-circuit construction details. If the machine is relocated to the repair shop and it contains a motor or other device where the order of the phases is critical, that becomes a consideration and must be dealt with.

When wiring a three-phase motor, it is common practice to find correct phase rotation by trial and error, observing phase rotation and switching wires at any convenient location anywhere upstream provided other motors are not affected, as in a group installation. (To reverse rotation in a three-phase motor, any two of the three wires are reversed.) However, it must be noted that some equipment, notably some types of motor-driven pumps, will be destroyed instantly if run in the wrong direction, at least to the extent that the seals will be damaged and need to be replaced. Therefore, the trial and error method is not always a good choice.

A better approach is to use a three-phase motor rotation tester, which costs about $100 and comes with an instruction manual. Alternately, you can use a small, fractional-horsepower three-phase motor to check hookups at various locations by observing rotation. (You will find a more complete discussion of motor troubleshooting and installation in the next section).

After doing the initial hookup or beginning any repair procedure, I always check the enclosure to make sure it has not become energized with respect to ground. Check again after the repair has been completed. A ground fault inside, perhaps caused by a chafing wire in conjunction with a missing equipment ground, can make the chassis and enclosure hot, resulting in shock hazard. A good way to check this is by using an inexpensive neon test light, available from your electrical distributor or any good hardware store. This is a great instrument for checking the presence or absence of voltage and approximate voltage level (120 or 240 and up to 600) and it can be used to distinguish hot from neutral. It has very high impedance because the neon bulb draws little current and there is also a resistor in series with the bulb. After preliminary measurements, you will want to set aside the neon test light and use your digital multimeter for internal work.

As for troubleshooting technique, the following comments are applicable to all sorts of equipment, but for now, we will turn to a typical premises electrical problem. This is a very simple, almost trivial, example, but it will show how the entire electrical troubleshooting process can be rationalized to ensure success without wasted effort. By this we mean analyzing the situation based on available information and proceeding in a way that makes sense, rather than drifting from test to test in a random manner in hopes of hitting on something.

Let us suppose that we are called into a commercial area where a single ceiling-mounted two-bulb fluorescent fixture is out-no flicker or dim light output. We know by experience that most likely replacing both bulbs will solve the problem. However, suppose there is a high ceiling and there is no ladder on site that will access the fixture. Before investing time in going after a ladder, it makes sense to look elsewhere first-switch, circuit breaker, etc. even though the defect is more likely a bulb or, next in line, the ballast.

When temporarily taping materials, leave an inch of tape. Make a tail by spinning it between your fingers, so that later you will not have to dig to find the end.

From this trivial example, it is possible to work out a general principle: prioritize all troubleshooting operations with regard both to probability of success and difficulty or expense in performing the diagnostic. Now back to the defective machine.

Getting Started

We have applied power by plugging it into a receptacle or, if hardwired, energized the line.

Often there is a power light. If it does not light up or the machine is completely unresponsive even though there is voltage at the input terminals, the machine is said to be dead. These problems are actually among the easiest to diagnose. Conductors and components are large and wiring is easy to trace, even without specialized knowledge of the piece of equipment and in the absence of a schematic diagram. Look for the moving parts because these are more likely to fail. Many types of laundry equipment and portable food processors found in a commercial kitchen have door interlocks. These are safety switches that prevent the machine from running if the door is not firmly closed. The switch may be a simple single-pole device in series with the incoming ungrounded conductor. Either this interlock switch is defective (open) or there is a mechanical problem in the linkage to the door or the door is not latching completely. Pressing firmly on the door may cause the machine to come alive. The problem is easily repaired by replacing the switch or repairing the linkage, door latch, or hinges. Avoid the temptation to override the switch by shunting around it to get the machine working because the switch plays an important safety role. There are many instances of this type whereby incoming power is interrupted. They are easy to diagnose and repair. Frequently a power wire is burnt at the termination. Another repeat offender is the main power switch. In addition, there could be a timer or fuse that malfunctions. Any of these series-connected items may be checked with an ohmmeter (with power disconnected). These passive devices may usually be checked in circuit because they are dead-ended when not powered up. Specifically, there should be no load in parallel that would fool the ohmmeter. When in doubt, disconnect one lead.

Another method for checking the functionality of switches, fuses, and similar devices is to take a voltage measurement across them while powered up. A switch should read absence of voltage when in the ON position and presence of voltage when OFF. This method is in some ways the more professional approach and is useful in checking fuses in an old entrance panel or controller where visual inspection is not always definitive.

A cautionary note: Certain components such as large electrolytic capacitors that are part of a power supply may be capable of storing a significant electrical charge that can be hazardous. It is common practice, after powering down the equipment, to short them out to remove the charge before servicing, but the high flow of current may damage the component. A better method is to shunt across the terminals using a low-ohm power resistor or other appropriate load.

Many machines have an in-line fuse. If it is blown or if there is a tripped circuit breaker, it is best to look into the matter further before powering up the equipment with new overcurrent protection. A shorted component may have activated the overcurrent protection, but next time damage may be more extensive. A chafed wire or transformer with shorted primary or secondary winding, or shorted parallel-connected component such as diode or capacitor could be the problem. Ohmmeter readings will suffice to check these devices. On the other hand, fuses may open due to age or in the course of performing their function of protecting against transient overvoltage events such as line surges.

"Half splitting" is another very powerful troubleshooting technique. Most electrical equipment and systems are serial in nature, which means the successive stages are connected so that the output of one feeds the input of the next. This is not to say that there are no other parallel elements. They may be identified by examining the wiring or looking at a schematic or a block diagram. The half-splitting technique involves going to the midpoint of the circuit and performing tests. By interpreting the results, it is possible to eliminate half of the equipment or system in one operation. Then, go to the midpoint of the "bad half" and perform another test. Proceed in this fashion to quickly isolate the defective component. In choosing the location for your measurement, select a point where access is easy. An example of this process would be in a premises wiring system where power is not present in a string of outlets. Rather than checking each outlet starting at either end, go directly to the midpoint. The same basic technique is useful in audio or video equipment where the output of one stage becomes the input of the next.

Experienced technicians may use intuition even where they are unfamiliar with the type of equipment. Others will want to consult a service manual and schematic before proceeding. By way of background information, we will now embark upon a discussion of various electronic components and their schematic symbols.

In recent years, there has been considerable standardization and simplification of schematic symbols and conventions, the result being that the diagrams are easier than ever to comprehend and use. In fact, they are extraordinarily intuitive. An example is the symbol for a capacitor, which graphically depicts two electrodes in the form of parallel plates with conductors attached and separated by a nonconducting dielectric material.

Schematic diagrams display the logical arrangement of components with interconnections shown, without regard to the parts arrangement on printed circuit board or chassis. The actual layout is shown in a pictorial diagram, which may be a photo. Here the parts locations are governed by a number of considerations including economy of wiring, protection from radiofrequency (RF) interference and thermal effects from other circuits, and maintaining characteristic impedance matching to prevent harmful data reflections at high frequencies. (More about this when we discuss impedance.) A fish tape is spring steel, halfway between mild steel and hardened steel. If you try to bend it too sharply, it will usually break. If the end breaks off, you can make a new hooked end by annealing it with a propane torch. Heat it red hot and allow it to cool slowly. This process softens the metal so that it can be formed with needle-nose pliers.

Electronic Components

Without further ado, we shall begin a discussion of individual components and their schematic representations. Figure 1-1 is the symbol for wires crossing and connected; Fig. 1-2 is the symbol for wires crossing, not connected; and Fig. 1-3 is another symbol for wires crossing, not connected, however this symbol is no longer used, but is still seen in old diagrams.

Wires must be sized for the maximum current they will carry, taking into consideration ambient temperature, duty cycle, type of insulation, heat produced by connected load, and other factors. When replacing a wire, it is important not to go smaller than the original.

Going to a larger size can create problems as well, altering the characteristic impedance in high-frequency applications thereby causing a mismatch with harmful reflections. Insulation type should not be changed, particularly in hot locations such as the leads for heating elements. Conductors in underground raceway must always be suitable for wet areas.

Figure 1-4 is a wonderfully intuitive symbol representing a fixed resistor. Resistors may be in series or in parallel with the power source or with the load. Resistance is measured in ohms and is not frequency dependent, except for unintended effects caused by incidental inductance and capacitance at high frequencies. The basic formula is:

E = I × R

Figure 1-1 Connected.

Figure 1-3 Not connected, old style.

Figure 1-2 Not connected.

Figure 1-4 Resistor.

where E = electromotive force (in volts)

I = current (in amperes)

R = resistance (in ohms)

For a specific current, the formula transforms to:

I = E/R

If you want to choose a resistor for a specific application, use this transformation:

R = E/I

These relations plus power formulas are presented in the Ohm's law wheel (see Fig. 1-5).

One of the power formulas is:

P = I^2 × R

where P = power in watts.

This formula is of great importance to electricians and electronics technicians. It is saying that the power in watts, for example heat dissipated by a resistor, is proportional to the square of current flowing through it in amperes times resistance in ohms.

From E = I × R, we know that with any given voltage, I and R are inversely proportional. As one of these variables goes up, the other goes down. Since

P = I^2

R, the value of I is far more important than R. As I increases, P (the amount of heat dissipated intentionally or as the result of an unintentional fault) soars.

What's the Significance?

This relationship has enormous consequences in electrical design, maintenance, and repair work. Electrical energy that disappears from a circuit reappears as heat outside the circuit, and it must be safely dissipated in order to prevent uncontrolled temperature rise accompanied by damage to equipment and possible fire hazard.

Electrical engineers speak of I^2

R heat loss, and it must be considered when sizing out all sorts of electrical equipment including resistors.

Accordingly, these passive devices are rated not only in ohms for resistance, but also in watts for power handling ability. In replacing a resistor, it is important to use the same size or larger. If the original burnt out, a larger replacement may be indicated. Never go smaller.

In electrical equipment, resistors play numerous roles:

• In series with a load to limit current through the load

• In parallel with a load to limit voltage on the load

• In conjunction with a power supply to create a voltage divider

Figure 1-5 Ohm's law wheel.

• Biasing of active components

• Bleeding of capacitors in a power supply

• Damping of LC tuned circuits

• Setting time constants in RC circuits

• Dummy loads

A defective resistor will prevent the circuit with which it is associated from working. It may take out the circuit, stage, and entire piece of equipment. Defective resistors are usually open and may or may not visually appear burnt.

There is a universal resistor color code. (See "Resistor Color Code.") Resistor Color Code

The value in ohms of a resistor is denoted by the colored bands that surround it.

There are usually three bands, then a gap, then a fourth band. (By the position of the gap, you can tell which end is the beginning.) The first three bands give the resistance.

The first band is the first significant figure.

The second band is the second significant figure.

The third band is the multiplier (number of zeros following the second significant figure.) After the gap, the fourth band is the tolerance.

The colors, with numbers, are:

• Black = 0

• Brown = 1

• Red = 2

• Orange = 3

• Yellow = 4

• Green = 5

• Blue = 6

• Violet = 7

• Gray = 8

• White = 9

The tolerances are:

• Brown = 1 percent

• Red = 2 percent

• Gold = 5 percent

• Silver = 10 percent

If the fourth band is missing, the tolerance is 20 percent.

It is not necessary to memorize the color code. Just hang it above your bench.

The schematic symbol for a capacitor makes sense both functionally and structurally (Fig. 1-6). It may be thought of as two large-area metal plates, closely spaced, separated by air or other insulating material. In the real world, you would more likely see two long strips of foil with a strip of insulating material, rolled tightly into a cylinder, with leads attached. A ceramic capacitor, for high-frequency RF applications, has the two electrodes embedded near enough to one another to make a low-capacitance device. In the electrolytic capacitor, shown in Fig. 1-7, the dielectric layer is formed chemically when electrical energy is applied. This layer is very thin, making for high capacitance.

Capacitors take many forms, the shape and material depending upon electronic and economic considerations. There are always two electrodes or sets of electrodes separated by an insulating layer that maintains the spacing between the plates and prevents them from touching and shorting out. Actually, this dielectric layer plays a far more complex and important role in every capacitor. A capacitor is similar to a resistor in the sense that it opposes the flow of current through it. (In fact, current does not actually flow "through" a capacitor. Energy is conveyed by means of an electrostatic charge on the dielectric material.

However, we can talk as though there is a current flowing through the device.)

Figure 1-6 Capacitor.

Figure 1-7 Electrolytic capacitor.

(Courtesy of Judith Howcroft.)

To understand how a capacitor works in an electronic circuit, it is necessary to know a few definitions.

Impedance is the total opposition to the flow of current. It is made up of resistance, capacitive reactance, and inductive reactance. It is measured in ohms. In a circuit, it works essentially the same as resistance and it conforms to Ohm's Law in the same way as a resistance.

Capacitive reactance is opposition to the flow of current through a capacitor or through any part of a circuit that has capacitance, either intended or unintended (called parasitic capacitance). Unlike resistance, capacitive reactance is frequency dependent. It is given by this formula:

XC

= 1/2p f C

where Xc

= capacitive reactance

C = capacitance (in farads)

f = frequency (in hertz)

As you can see, a capacitor offers more opposition to the flow of current at lower frequencies and less opposition at high frequencies. To very high frequencies, a capacitor is invisible. To dc, a capacitor offers maximum opposition and constitutes an open circuit, except that there is a current spike at the instant that the dc is switched on or switched off, at which point it has the property of a high-frequency pulse due to high rise and fall times.

Inductive reactance is a parallel concept, with the circuit properties being a mirror image of the above. We will discuss inductive reactance in the section on coils.

It is necessary to distinguish between capacitance and capacitive reactance. Capacitance is a fixed property of the capacitor or circuit. It is given by involved formulas including area of the plates (greater area means more capacitance) and distance between plates (closer proximity equates to more capacitance). Another variable is the dielectric constant of the material between the plates.

It is not necessary for the electronics technician to know or use the formulas for capacitance. The manufacturer will provide a capacitor having the desired capacitance, which will be printed on the device or indicated by color code.

When tapping out small threaded holes like in a wall box, place your tap in a cordless drill and feed it in slowly. This procedure gives better control and is much quicker than using a wrench or socket to hold the tap. Do not forget to oil the tap, preferably with thread cutting lubricant.

The other important parameter of a capacitor is working voltage (WV). If a capacitor is deployed in a circuit where it is subject to more than its WV, the dielectric layer may be punctured by arcing electrical energy and the capacitor permanently ruined.

Capacitance is measured in farads, but capacitors we encounter in the course of most work are rated at a small fraction of a farad. Microfarad and picofarad are the usual metrics.

Frequency Is All-Important

In contrast to capacitance, capacitive reactance, as we have seen, varies with frequency and must be calculated based on the capacitance of the device and the frequency of the voltage (hence current) involved. Therefore, the technician must know and be able to use the formula for capacitive reactance so that it can be figured on a case-by-case basis.

Because of its property of exhibiting different impedances at different frequencies, capacitors have numerous real-life applications. An example is their ability to separate different frequencies including dc.

A common usage involves network-powered communications systems. The power may be supplied at close to 60 volts. These two vastly different voltage and frequency levels can coexist within the same cable with no problem, and they can be separated by means of a capacitor network or a resonant circuit which, as we shall see later, involves use of a capacitor and an inductor in series or in parallel.

An interesting device is the electrolytic capacitor, which is very often seen in power supplies to smooth out and remove ripples from dc after it has been produced by rectifying ac. Another application is in motors where it may be switched online while the motor is starting.

Many electrolytic capacitors are easily recognized by their comparatively large, tank like appearance. They often have lugs for push-on spade connectors and are marked with capacitance and working voltage. The electrolytic capacitor is used where comparatively high levels of capacitance are required.

Not all electrolytic capacitors have that appearance. Tantalum capacitors are a subspecies. They are very reliable, but more expensive, and because of their small size, are well suited for cell phones and similar small portable equipment.

Electrolytic capacitors are manufactured in an unconventional way. One of the plates is made of an ionic conducting liquid. When voltage is first applied, this plate manufactures a dielectric layer that is very thin. Since the plates are therefore much closer than in other types of capacitors, a much higher capacitance is possible in a relatively compact package.

Electrolytic capacitors, because of high capacitance and high-power applications, are prone to failure. They can become shorted or open, or fail to meet capacitance specifications.

They may become defective if stored on the shelf for a long period of time, especially under hot or humid conditions. Sometimes these devices can be restored by applying a steady dc voltage. The good news is that they are easy to test and replace.

Electrolytic capacitors can be tested with an ohmmeter, although in an unconventional way. Connect the ohmmeter set on a medium to high range to the two terminals. Depending upon whether the capacitor is charged and which way the ohmmeter probes are connected, the digital readout or analog needle will either climb or descend in a uniform and stately fashion or it will remain unchanged. Electronics technicians call this meter response "counting." If it remains unchanged, reverse the leads and the meter will commence counting, indicating that the electrolytic capacitor is good. This odd behavior is because the ohmmeter has a battery that puts voltage on the capacitor leads in an attempt to take resistance readings. In the case of an electrolytic capacitor, however, the meter will either charge or discharge the capacitor.

This response indicates that the capacitor is good-not shorted or open-and has capacitance. However, the test is not definitive. Under dynamic circuit conditions, the device may not perform adequately.

A defective electrolytic capacitor may appear burnt, swollen, or distorted, or it may be leaking electrolyte. Such a unit must be replaced, but watch out for other bad components that may be putting too much voltage on it.

Electrolytic capacitors are everywhere-in all size motors, computers, power supplies, TVs, electrical distribution equipment, and so on. Replacement capacitors must have the same or greater working voltage. In some applications, the exact capacitance is not critical.

It is usually acceptable to go 10 percent higher.

To summarize, capacitors, especially electrolytic, are frequent culprits in many equipment failure events. For most applications, unless it is a power factor device on an industrial scale, the replacement cost is modest.

The coil schematic illustrates the structure of this ubiquitous device (Fig. 1-8). Wire, usually copper, is wound in helical fashion, often around an iron core. Leads at either end provide circuit connections. The device is simple, but the underlying concept, inductance, is a large topic, involving some fundamental and mysterious properties of the universe.

Actually, any conductor has inductance, whether it is an ionized channel blasted across the sky by a lightning bolt or a nerve cell axon within your brain. However, usually we are talking about a segment of copper wire. Winding it to form a coil multiplies the inductance, and if these windings encircle a core made of a magnetically permeable material such as soft iron, the effect is increased dramatically. All materials are to some extent permeable to magnetic flux, and a vacuum is as well. (Permeability in a magnetic circuit is like conductivity, the reciprocal of resistance, in an electrical circuit. Magnetic flux is like electrical current.)

Any coil or conductor has inductance, and this is a fixed value, whether the coil is on the warehouse shelf or connected to a live circuit.

There are some complex formulas used for calculating this value. It is not necessary to perform these calculations because we are not in the business of designing and building inductors, or designing and building long electrical or telecom distribution lines. The inductance will be marked on the device by the manufacturer, and we may take this as a given when ordering replacement parts. Remember that inductance is a property of the device and remains unchanged under normal conditions, that is, regardless of frequency, voltage, current, or any other circuit parameters.

In contrast, inductive reactance is a property of the device that is dependent upon both the inherent inductance of the coil or conductor, and how it is deployed in the circuit- specifically the frequency of the current passing through the conductor. Inductive reactance, like the capacitive reactance that we discussed earlier, is a measure of opposition to the flow of current. It is likewise measured in ohms, and conforms to Ohm's law. The way inductive reactance differs from resistance is that it is frequency dependent.

Inductance may be said to be a mirror image of capacitance, and like any reflected image, certain elements of the reflection are reversed. Specifically, in a capacitor or capacitive load, the capacitive reactance is greater at lower frequencies (maximum at dc, 0 Hz) and less at higher frequencies, whereas in a coil or inductive load, the inductive reactance is just the opposite. It is greatest at high frequencies and less at low frequencies.

At dc, or 0 Hz, there is no inductive reactance except at the instant when power is applied and when it is disconnected, at which times the current behaves like a high frequency, with fast rise time and fall time of the waveform. In this connection, it is interesting to note that

Figure 1-8 Coil.

when power is abruptly removed from a circuit that contains a high-inductance coil in series with the load, there can be a quite significant voltage spike. The magnitude of this pulse is dependent upon the preexisting voltage level, the amount of inductance in the circuit, any series or parallel impedance, and especially the speed of the switching action. The phenomenon is known as "voltage kick" and it can be unintended and harmful, or it can be part of the equipment design. An example is the old mechanical ignition for a gasoline engine, with rapid switching provided by the points in conjunction with a spark coil.

Regularly inspect the handles of insulated tools for deterioration, which can be hazardous. An almost invisible crack can hold moisture and conducting grease. Heat shrink tubing installed on the handles of pliers will make them like new but insulated handles should never be trusted for high voltage or when you are on a wet surface or otherwise grounded.

Inductance is directly associated with the interaction between electricity and magnetism. When current flows through a conductor, a magnetic field is established inside the conductor and in the space surrounding the conductor. The strength of the magnetic field, of course, diminishes with distance from the conductor.

When voltage is applied across the conductor, accompanied by current flow through it, the magnetic field is established, and the energy has to come from somewhere. Accordingly, there is an inevitable reduction in current flowing through the conductor. We may consider that the magnetic circuit is borrowing energy from the electrical circuit. When the voltage is removed or even reduced, it is payback time! As the magnetic field collapses, energy is fed back into the electrical circuit.

There are two kinds of inductance: self-inductance and mutual inductance. It is the same phenomenon, just a different arrangement of the energy flow. Self-inductance always occurs when there is flow of electrical current. Mutual inductance takes place when a second coil, often wound around the same core, is brought in close proximity to the first coil. Mutual inductance is also relevant when two conductors are run parallel to one another for an appreciable distance. In that instance, mutual inductance is often unintended and harmful. Unwanted mutual inductance may also take place between adjacent traces on a circuit board or nearby wiring inside an equipment enclosure, this effect becoming much more pronounced at higher frequencies. This is one of the reasons that the wiring layout in a pictorial diagram differs radically from the schematic. In doing a repair on equipment that operates at a high frequency, it is important not to alter the routing of the wires. If it seems that they are not laid out in the most efficient way, this may be intentional to avoid parasitic inductive or capacitive effects.

In equipment that operates at high frequencies, it is important to avoid shortening or lengthening a wire, which could cause a characteristic impedance mismatch, resulting in harmful electronic reflections and data loss. (More about characteristic impedance in section 10.) Self-inductance, as the name implies, involves a single conductor with current passing through it. A magnetic field is established, and if the current is ac, pulsating dc, or some disorganized electrical activity that we may characterize as noise, there will always be a magnetic field surrounding the conductor. This field induces another current flow in the conductor, called "back emf." It is of opposite polarity at any given instant, and tends to oppose the original current. The bottom line is that this situation means that the conductor will exhibit impedance, specifically inductive reactance.

Inductive reactance (like capacitive reactance) together with plain old resistance, make up impedance. All of these are measured in ohms and all comply with Ohm's law. The two forms of reactance are frequency dependent, whereas resistance is not. Capacitive reactance decreases as the frequency becomes higher. Inductive reactance increases as the frequency becomes higher, and the formulas that are involved are similar but opposite in effect.

Multiple Circuit Elements

Resistance, inductance, and capacitance may coexist in a circuit, for example when one or more resistors, capacitors, or coils are placed in series or parallel, or any combination of the two. It is also possible, in fact inevitable, for any two or all three of these to be present in the same device. A resistor, for example, has a lead at either end in addition to any center taps, and these leads may function as the plates of a capacitor, the resistive material being the dielectric medium. Similarly, since the resistor is also a conductor, when there is current flowing through it there is actually a magnetic field around it, so that it is an inductance.

These properties are very slight, but at high frequencies, the effects become significant to such an extent that the performance of the device, the circuit, and the entire piece of equipment may be compromised if not rendered completely nonfunctional if the design does not consider this effect.

The equation for inductive reactance is:

XL

= 2pf L

where XL

= inductive reactance

f = frequency (in hertz)

L = inductance

Notice the similarity to the equation for capacitive reactance, and the difference between them.

It seems to me that the universe may be characterized in a fundamental way as an intricate dance of particles, and their inscrutable interactions with elemental forces.

Nowhere is this phenomenon more wondrous, in my view, than in the formulas that describe (define?) capacitive and inductive reactance.

The formulas are well established to the point of certainty, and as the decades roll on, we learn more about the fundamental processes, yet never get close to knowing why the particles and associated forces exist, as opposed to not existing.

Mutual inductance is similar, except that there is a second conductor within the area where the magnetic field is relatively strong.

If the magnetic field is fluctuating, current is induced to flow in the otherwise nonenergized second conductor. In this situation, the circuitry comprises a transformer.

The schematic in Fig. 1-9 perfectly portrays the electrical and magnetic properties of this ubiquitous device. There are no mechanical moving parts except for associated items such as ...

Figure 1-9 Transformer.

.... cooling fans or pumps, or a circuit breaker or switch that could be built inside the same enclosure. What is moving is the magnetic field that surrounds either conductors or coils, and this is what makes the transformer work.

The two conductors are generally formed as coils wound around the same core. If this core is made of a material that is highly permeable, the effect of the mutual inductance becomes more pronounced.

An important and very common transformer application is for stepping up or stepping down voltage. The two windings in a transformer are the primary and secondary windings.

The primary is the winding to which ac from an external source is applied, the input, and the secondary is where the induced voltage is available as an output. In theory, you could reverse input and output connections to convert a step-down to a step-up transformer, but such misuse would void the UL listing and could cause an unforeseen fire hazard.

Transformers are widely used in all sorts of electrical equipment. Isolation transformers make use of the fact that grounding does not pass through a transformer. If one conductor of the primary is grounded, none of the secondary conductors become grounded unless the windings are electrically connected. An isolation transformer is used in the power supply for a hospital operating room (with a line isolation monitor) to protect a patient who is undergoing invasive surgery from small ground currents that could cause injury. The primary and secondary windings generally have the same number of turns so that the voltage is neither stepped up nor stepped down.

The ratio of primary to secondary voltage is the same as the ratio of the number of turns in these windings. If the secondary has twice as many turns as the primary, the output voltage will be double the input voltage. This does not violate the law of conservation of energy. As voltage increases, the current decreases. Since volts times amperes equals watts (or volt-amps, for a reactive load) the power remains constant except for a small amount of energy lost in the form of heat, which is measured by a factor known as efficiency, expressed as a percentage.

The high-voltage winding may be identified by the fact that the wire, if accessible for inspection, is smaller. The larger wire is needed for ampacity to carry the greater amount of current. (The primary and secondary terminations should be clearly marked or self-evident.) To understand the operation of a transformer, it must be realized that when a larger load is connected to the secondary, the primary will draw a correspondingly greater amount of current.

In large equipment, these quantities may be conveniently measured with a clamp-on ammeter.

Transformers are generally reliable, but overheating or a manufacturing defect may set the stage for failure. Part of the troubleshooting process will entail looking at the transformer, especially when a power supply malfunction is indicated. There are other types of transformers in electrical equipment. These may be for coupling stages or impedance matching, and are often mounted directly on a high-quality loudspeaker. They are easy to test and, where defective, to replace.

High ohm readings indicate an open winding. In addition, when taken out of circuit, the windings should be found to be isolated from the metal core and enclosure, and from each other, unless it is an autotransformer (see following text). Beyond that, dc ohm readings are not definitive. It is possible that some turns within a winding could be shorted, changing the output voltage or overloading the primary. Either of these conditions could damage other components. The way to find out what is going on is to take in-circuit voltage readings with power to the primary. When performing this operation, it is essential to avoid danger of shock. This involves isolating yourself from dangerous voltage levels, especially when a step-up transformer is involved or when the equipment operates at 240 volts or higher.

One method is to clip your meter leads to the transformer terminals while the equipment is not energized, then power it up and observe the display from a safe distance. Remember to discharge all capacitors before hooking up.

Looking at Transformers

If the transformer is found to be defective, you have to ask why and what are the implications.

One possibility could be an external power surge. This could burn out the primary or overpower the secondary, perhaps taking out downstream components in the process.

Another scenario involves a shorted component in the output circuit, which could place an excessive load on the secondary and, by induction, on the primary.

Electrical and electronic equipment frequently have power supplies, or low-voltage motor control or sensing circuits, and these generally require transformers, which should be checked out if diagnostic procedures point in that direction. A replacement transformer must be an exact match, in terms of input and output voltages and frequency, as well as power rating. Identical physical dimensions and construction details also have to coincide so that heat is not introduced in the wrong place to damage nearby components and so that there is not an impedance mismatch.

As for large power transformers, proper preventive maintenance will prevent outages and save future material and labor costs, as well as reduce fire hazard. Routine temperature readings, entered into a log so that damaging trends can be spotted early, should be part of the preventive maintenance program. Dirt or debris should not be allowed to accumulate to impede air circulation and cooling. Individual large transformer enclosures should be vacuumed out periodically, with power disconnected and locked out, and with measurements taken to ensure there is no backfeed. No one should undertake this sort of work without thorough training and certification in the safety aspects involved.

What Is an Autotransformer? An autotransformer, as depicted in Fig. 1-10, has a single winding that serves as both primary and secondary. The autotransformer is smaller and less expensive than the dual winding version, but electrical isolation is not provided.

The most common application of an autotransformer is providing 120 volts from a 240-volt supply. When troubleshooting equipment where an autotransformer is part of the power supply, ohm readings will reflect the fact that primary and secondary windings are not electrically isolated. Grounding of one side of the primary will be conveyed to the secondary circuit, which is not the case in a dual-winding transformer.

There are safety implications in all of this.

For one thing, insulation failure can cause the output circuit of a step-down autotransformer to exhibit full primary input voltage. Moreover, if the common part of the winding ...

Figure 1-10 Autotransformer.

... becomes open at any point (due to an unintended break in the wire), full input voltage will show up at the secondary.

Autotransformers are used in high-voltage power distribution systems to permit operation of machinery when the required voltage is not available. Using an autotransformer, you could run a 480-volt motor off an existing 600-volt source, and this would be a less expensive solution than employing a dual-winding transformer.

Another common application is in audio systems where it is desired to power speakers from a constant-voltage source, and to match impedances for a low-impedance microphone connected to a high-impedance amplifier.

The diode is an incredibly useful, widespread, yet simple device that is found throughout the world of electrical equipment.

Visually, it takes many forms, but may be identified by the fact that there are two leads, one often marked with a "+" sign, but otherwise having the appearance of a small resistor.

In larger sizes, there is a prominent heat sink or even cooling fins. When forward biased, a significant amount of heat appears, and this energy must be conveyed away from the device so that excessive temperature rise does not occur.

The first diodes, other than cat's whiskers found in crystal sets, were power-hungry vacuum tubes, but in our solid-state epoch, the device is much simpler, more efficient, trouble-free, and less expensive.

A single diode will operate in one of two modes. Usually, with exceptions noted below, if it is forward biased, it will conduct. If it is reverse biased, it will not conduct. Biasing refers to the polarity of the voltage that is applied to the diode. Looking at the schematic, we see that of the two leads, the anode is connected to an arrow pointing from the lead into the device (Fig. 1-11) The other lead, the cathode, is connected to the other side, represented by a line segment perpendicular to the flow (or nonflow) of current. These terms, anode and cathode, are taken from the old vacuum-tube diodes. The bottom line is that when the anode is connected to the positive pole of a dc power source of appropriate voltage and the cathode is connected to the negative pole, the device will conduct, rather like a switch that is in the ON position. Then, the diode is said to be forward biased. If the polarity is reversed so that the anode is connected to the negative side, the diode is reverse biased and will not conduct. Accordingly, the diode may be considered a one-way gate, resembling a check valve in a water system.

You can build a very simple circuit to demonstrate the operation of a diode, as shown in Fig. 1-12 and Fig. 1-13.

Figure 1-11 Diode.

Figure 1-12 Forward-biased diode.

Figure 1-13 Reverse-biased diode.

In place of the pilot light, an ammeter may be deployed. If an ammeter is used, since it is a very low-impedance device, it is necessary to have a resistor or other load in series lest the diode be fried. Depending upon the relative polarities of the diode and dc source, the ammeter will indicate either the presence or absence of current through the circuit.

To make the circuit work, diode, battery, and resistor ratings must be coordinated to avoid destroying the diode. Relevant information is in the manufacturer's data sheet, which can be downloaded off the Internet or obtained from the electronics parts distributor. As an exercise, you should acquire a data sheet for any active device that crosses your path, as these are easy to read and provide insight into how various devices work in circuits.

Of course, a diode is a sealed unit impossible to be repaired. However, it is instructive to understand its inner workings because such knowledge is useful in understanding and troubleshooting circuits containing one or more diodes.

Most semiconductors are made from silicon, which in its pure state is not much of a conductor.

An n-type silicon results from the application of a minute amount of phosphorous gas in a process known as doping. An atom of silicon has four electrons in its outer shell so that it bonds with four adjacent silicon atoms making a stable crystal. The outer shells of two adjacent silicon atoms in undoped state have eight electrons. This situation changes when a trace amount of phosphorous is added. Phosphorous has five electrons in its outer shell, and the fifth electron gains mobility so that it is free to move within the confines of the silicon crystal when voltage is applied. Since electrons are negative, the silicon that has been doped with phosphorous gas is known as an n-type material.

A cracked circuit board can be repaired. Straighten the board and reinforce it by gluing strips of plastic as needed. Solder light copper jumpers across any cracked traces. Be sure to use heat sinks on sensitive electronic components.

P-type silicon, on the other hand, is doped with boron, again in a gaseous state. Because boron has three electrons in its outer shell, it can bond with only three of the adjacent silicon atoms. Accordingly, one silicon atom has an empty space in its outer shell. This empty space is aptly called a hole. Unbelievably, these holes are charge carriers as well. The silicon that has been doped with boron gas is known as a p-type material because the hole is, in effect, a positively charged particle.

Inside a Diode

A diode is manufactured by fusing thin wafers of n- and p-type silicon to one another. The disc or cylinder so formed is equipped with leads at the two ends. Where the n-type and p-type material join is known as the junction. What is important in understanding any diode (or other semiconductor including the many types of transistors and integrated circuits) is the junction, and we have to consider what happens at the junction and on either side of it.

If the p-type side (the anode) is connected to the positive terminal of the dc power source, the charge carriers-holes-are repelled and pushed toward the junction. In this configuration, the negative pole of the dc power source is connected to the n-type side, the cathode, so the charge carriers are repelled and are pushed toward the junction. Then there is an abundance of charge carriers on both sides of the junction, and conditions are right for current to flow. The diode is said to be forward biased.

Conversely, if the connections are changed so that the positive pole of the dc power source is connected to the cathode (n-type material) and the negative pole of the dc power source is connected to the anode, the two types of charge carriers-holes and electrons-are attracted to the dc power source poles. Then, there is an absence of charge carriers in the region of the junction. The diode is said to be reverse biased and it will not conduct, with an exception noted below. The junction becomes what is known as a depletion region.

As mentioned, there are two exceptions to the general explanation of the diode presented above. First, the diode does not immediately start to conduct upon being forward biased. The junction must see a certain minimum voltage before entering the conduction mode. This quantity is called the forward breakover voltage. It is generally less than 1 volt.

The other exception is known as the avalanche effect. What this interesting bit of terminology means is that in reverse bias mode, if the voltage rises above a certain level, the electrons will barge, so to speak, through the depletion region, and the diode will conduct reverse bias notwithstanding. This avalanche effect may be unintended and destructive, or it may be purposely incorporated in the circuit design, as in the case of the Zener diode, which we shall consider soon. Forward breakover voltage and avalanche voltage amounts are found in manufacturers' data sheets.

You should be aware that a diode might also act as a capacitor, with the junction becoming the dielectric, particularly at high frequencies. This effect may be unintended and harmful, or it may be part of the design.

That is how a diode works. There are variations depending upon how the diode is configured, packaged, and biased. Some of the applications are as follows.

Since the diode conducts only when forward-biased, that part of an ac waveform either above or below the x-axis will be eliminated, depending upon which way the anode faces in the circuit. This operating mode is enormously useful and easy to implement. Just connect the diode to either leg of the input in series with the load and you have built a half-wave rectifier (see Figs. 1-14 and 1-15).

The output is pulsating dc, with no electrical energy 50 percent of the time. This output would make quite the ac hum in audio equipment, but it can be put to good use charging a battery. A half-wave rectifier, while cheap and easy to build, has the disadvantage that it uses only half the power that is present in the ac circuit because it conducts during half the cycle.

Figure 1-14 Half-wave rectifier.

Figure 1-15 Half-wave rectifier output.

It can be used as a light dimmer, an improvement over the old rheostat-based device because excess energy is not dissipated as heat. There is just the heat loss involved in the forward-biased diode.

Another disadvantage is that the output of a half-wave rectifier is difficult to filter effectively due to high harmonic content. Furthermore, it is demanding of the diode because all the heat is concentrated in one place. In addition, it is harder on the transformer. For these reasons, a full-wave diode rectifier is often chosen (see Figs. 1-16 and 1-17).

Two diodes, with anodes connected to the legs of a transformer, are configured in parallel and the cathodes are brought together to make a positive dc output. The negative output comes from the transformer center tap. Most equipment with a dc power supply operates at less than line voltage, so there will be a transformer. Since the full-wave rectifier requires a center-tapped secondary and two diodes, it is a little more costly to implement, although the filtering is simpler.

Another variation, a full-wave bridge circuit, is shown in Fig. 1-18.

Notice that the cathodes of two of the diodes are connected to the + output and the anodes of the other two are connected to the - output. A center-tapped transformer is not necessary. Because there are four diodes, the heat dissipation is less problematic. The bridge circuit is also an improvement over rectifiers that are more primitive because it makes more efficient use of the transformer.

When troubleshooting, if in your opinion a power supply problem is indicated, look to the diodes. Where there is large current flow, consider excess heat, temperature rise, and resulting individual component failure as a cause of the equipment malfunction.

Figure 1-16 Full-wave rectifier.

Figure 1-17 Full-wave rectifier output.

Out, In

Figure 1-18 Full-wave bridge rectifier.

Lacking the schematic, you can tell a lot about a power supply by looking at the components. The power supply will be located near where the conductors enter the enclosure. If a significant amount of power is consumed, any diodes will be heavily heat sinked. If there is a single diode, it is a half-wave rectifier. Two diodes with three secondary leads from the transformer indicate a full-wave center-tapped rectifier circuit. Three or six diodes may indicate a three-phase source. (Did you know that modern automotive alternators produce three-phase ac of varying frequency? Six diodes, one in each line inside the alternator, result in a reasonably smooth dc further refined when it is connected to the battery, which provides regulation.) Four diodes indicate a full-wave bridge rectifier.

In troubleshooting electrical equipment, power supply problems are frequently found, and diode replacement is often the answer. Besides rectification, there are other applications, but these are less frequent offenders because the power levels are less intense.

Damage is still a possibility, however, because overvoltage may be applied for a variety of reasons. Applications are inside and outside the power supply proper.

Zener diodes take advantage of the avalanche effect. The avalanche voltage lies in the normal operating range so that when reverse biased above a certain value, say 60 volts, they will conduct without damage. This property makes the Zener suitable to function as a voltage regulator (see Fig. 1-19).

Thyrectors protect valuable electronic equipment, such as desktop computers and peripherals, from sudden, brief powerline transients or spikes (see Fig. 1-20).

You will notice that the thyrector consists of two diodes in series with polarities reversed. They are constructed as a single unit and connected between ac lines and ground.

These devices are incorporated within a short plug-in strip with a resettable circuit breaker and power light, and sold as surge protectors. If the overvoltage is greater than a certain magnitude or duration, the thyrectors will be destroyed, sometimes making the electronic equipment go down as well.

Figure 1-19 Zener diode.
Figure 1-20 Thyrector.

Thyrector failure is seen frequently in areas of high lightning activity. Visual inspection and ohmmeter tests will often determine the status of the device. If there is any doubt about the condition of the device, replacement is recommended.

Some additional diode applications include:

• Electronic switching

• Envelope detection

• Step recovery

• Frequency multiplication

• Hot carrier devices

• Signal mixing

• Amplitude limiting

• Laser diodes

• Photo diodes

• Photovoltaic cells

• Optical isolators

• High-frequency oscillators (Gunn diodes and Tunnel diodes)

Sometimes a defective diode can be identified by its burnt appearance, but more often meter tests are needed.

Testing a Diode

The simplest way to proceed is to use a standard multimeter. It will be noticed that in ohms measuring mode, the meter will read either open circuit or circuit continuity (high resistance or low resistance) depending upon which way the meter probes are connected to the diode. This is because in ohmmeter mode, your meter of necessity applies a dc voltage across the component that is being tested. This dc voltage suffices to forward bias or reverse bias the diode, a rather unscientific but adequate way to proceed. The actual ohms values do not mean anything.

Most but not all meters apply positive voltage to the red probe. You can determine the polarity by connecting a known good diode to the meter. When the positive pole is connected to the diode's cathode, as indicated by marking, the ohmmeter will read low resistance. Then, if necessary, you can color code your meter probes again.

You can make a dc polarity tester by soldering a diode into a spare probe lead. This accessory makes a useful tool for checking circuitry, particularly automotive, boat, and aircraft wiring.

Of course, the ohmmeter test is not fully definitive. For one thing, it does not check for forward breakover and avalanche voltage to see if they are within specifications, as shown in the data sheet. Furthermore, real-world operating parameters at elevated voltage, current, and frequency conditions may not be acceptable.

Multimeters with advanced features sometimes include a "diode-check" function, whereby a low-level current is passed through the diode so that voltage drop can be displayed.

Fortunately, most low-power diodes are cheap enough so that you can afford to install a new one to see if that is the problem. This does not address the problem of corollary damage to some other component accompanying diode failure.

This completes our discussion of diodes for now. The next topic builds upon the foregoing subject matter. More advanced semiconductors, including transistors and integrated circuits (ICs), are semiconductors like diodes but operate in a somewhat more complex manner. Instead of one p-type and one n-type material meeting at a single junction, there are three or more layers having various characteristics and multiple junctions. Using these concepts, researchers (especially in the mid-1900s) developed a multitude of different active components, which made possible advanced types of audio and video equipment, computers, telecom networks, and much more. These active components can be diagnosed and replaced once the basic principles are understood. The place to start is with an understanding of solid-state electronics. This may be acquired through practical experience in the field, in books that are readily available, and, of course, on the Internet.

Like diodes, transistors are composed of p-type and n-type material. A bipolar transistor has three layers with two junctions. The pnp transistor is shown in Fig. 1-21 and the npn transistor is shown in Fig. 1-22.

Figure 1-21 pnp transistor.

Figure 1-22 npn transistor.

The center region for both types of bipolar transistor is called the base. The line having the arrow is always the emitter, so in a schematic diagram it is not necessary to label the terminals. If the arrow in the emitter points in toward the base, it is a pnp transistor. If the arrow points out away from the base, it is an npn transistor.

In most applications, npn and pnp transistors work the same, with one major difference-all polarities are reversed and the direction of current flows are opposite, both inside and outside the device. As long as the transistor specifications (as given in the manufacturer's data sheet) are equivalent, pnp and npn transistors are usually interchangeable provided voltage polarities are reversed.

Transistors Have Multiple Uses

Transistors have many applications. The most basic and commonplace is amplification, as in an audio amplifier. An example of amplification in everyday life is when you are driving a car and slight changes in the position of the gas pedal due to foot pressure are instantly translated into variations in engine rpm and vehicle speed. In fact, motion of the gas pedal is the result of a still slighter level of electrical activity in your brain and nervous system.

The totality is a type of multistage amplifier, where the output of the first stage is coupled to the input of the second stage. Unlike the step-up transformer that we considered earlier, where there is no increase in power ( just a rise in voltage with decrease in amperage), in amplification the power is actually increased. The power is multiplied many times from the input of the first stage to the output of the final stage. Here again, the law of conservation of energy is not contradicted. At each stage, power is fed into the system from an external power supply.

In the automotive example, it is the gasoline engine that powers the car. The motion of the gas pedal controls the output level, but the actual energy comes from elsewhere.

The three terminals of a bipolar transistor allow for a two-wire input and two-wire output, with one terminal common to both circuits.

The varying input provides changing bias on one junction, and a larger dc power source in series with the transistor output provides bias for the other junction. When both junctions are forward biased at the same time, the transistor conducts at a level greater than the input.

In common emitter configuration, the control circuit terminates at the base and emitter.

Here a small current controls the larger current that flows in the collector-emitter circuit. For both pnp and npn transistors, the flow of electrons is in the direction opposite the arrow.

The direction and placement of the arrow tell you the type of transistor and identify the terminations.

The common emitter configuration is fundamental and easy to understand. There are common collector and common base applications as well. From the schematics or visual inspection of the equipment, you can ascertain which lead is common to the input and output circuits, but this approach is limited by the fact that there is no universal standard for marking, color-coding, or positioning the three leads of a bipolar transistor. You would think that the middle lead would go to the base, but this is not always the case. Fortunately, there are tests that can be performed with an ohmmeter. There are full-featured transistor testers as well. These fall into three categories.

A quick-check in-circuit transistor tester measures the transistor's ability to amplify a signal. Without removing the transistor from the circuit, a rough measurement is made so that you can decide if replacement is warranted.

A more sophisticated instrument is the service-type transistor tester. It ascertains the forward current gain (known as beta) of the transistor. It also checks for base to collector leakage current with no current going into the emitter. Additionally, some service-type transistor testers identify the leads-collector, emitter, and base.

The high-end transistor tester is known as a laboratory-standard transistor analyzer. It will simulate a real operating environment, providing voltage, current, and signal inputs and looking at the output.

These testers come with extensive operating instructions, and you will find all of them user-friendly. Let us assume for now that all you have is a simple multimeter with ohms function. It will be possible to check a transistor, although the amount of information provided regarding its actual status under dynamic operating conditions would be less than total. However, it is a good way to proceed.

Transistors are incredibly reliable and rarely go bad. Many transistors have been in service for over 50 years, and they are still as good as the day they were soldered in place.

Nevertheless, it is possible for a transistor to fail if excessive voltage or excessive heat is applied to it. Static electricity, line surges, high ambient temperature, failure of a ventilating fan, inadequate heat sinking, or external circuit conditions may cause transistor failure.

Simple ohmmeter checks will let you know the status of the device on a go, no-go basis.

As mentioned in the discussion of diodes, a multimeter in the ohms mode is useful for testing a solid-state device because the dc power supply serves to bias the NP junction.

Naturally, the full power supply voltage is not applied. The ohms value as shown in the readout is not to be taken as a meaningful resistance measurement, but it does indicate whether the holes and electrons have migrated to the junction so that the particular junction can conduct, given the amount of bias provided by the meter.

The first task is to determine the polarity of your meter's probes when in the ohmmeter mode. Most meters are configured so that the red probe, when connected to the ohmmeter function socket, is positive, and the black probe, connected to the common socket, is negative.

However, meter manufacturers have not been consistent, so you have to test the tester. The procedure is to find a known good diode with polarity marked on it. The diode could have a diode schematic symbol printed on it, a single band on the cathode, or + and - signs. Orient the diode so that it is conducting according to your meter. The probe that is connected to the cathode will be positive because it is repelling the holes and pushing them into the region of the junction. Then, permanently identify the polarity of your meter so that you can make meaningful diode and transistor tests and distinguish between npn and pnp transistors, and identify the leads if they are not marked. We have outlined the procedure for testing a diode.

Testing a transistor is more complex. Keep in mind these points:

• Solid-state component manufacturers package, terminate, and mark their products in a variety of ways, so make no assumptions. A bipolar transistor will have three leads, but it may have no identifying color code or markings. It is even possible that the middle lead will not be connected to the base, although that would be logical. Assuming that you do not have an overall schematic of the piece of equipment or data sheet for the component, you have to start from scratch. (If you do have the schematic, you will know that the terminal with the arrow is the emitter and if the arrow points inward, then the component is a pnp device.) By the circuit wiring or printed circuit board traces, you should be able to identify connections and biasing of the other terminals.

• If the component is in circuit, adjacent components can cause misleading meter readings, so it will be necessary to temporarily cut or desolder some terminations. You can always leave one lead attached as it will not comprise a complete circuit path and your readings will not be compromised. When desoldering or resoldering these connections, remember that heat is the enemy of solid-state components. Use a minimum of heat to accomplish the operation and protect the component by using a clip-on heat sink. (More about this later in this section when we talk about repair methods and materials).

• When working with solid-state components, you must protect them from static electricity.

Your body can acquire a static charge from time to time. This can be controlled. A copper grounding bracelet connected to the system ground will take care of this charge.

Similarly, your soldering iron or any other metallic tool could introduce an unacceptable voltage. Frequently touch it to a known ground.

To test a bipolar transistor using a multimeter, now that you have ascertained the polarity of the ohmmeter probes, proceed as follows.

A transistor may be thought of as two diodes, back to back. If the transistor is npn, the two anodes are connected and the base connection is tapped from where they join. If the transistor is pnp, the cathodes are connected and the base is tapped from where they join (see Figs. 1-23 and 1-24).

Figure 1-23 Diode equivalent of npn transistor. Figure 1-24 Diode equivalent of pnp transistor.

It must be emphasized that these diode hookups will not function as working transistors. They are just models that indicate the meter connections for testing transistors.

It is possible that you do not know whether the transistor is npn or pnp, and it is also possible that you do not know the identity of the three leads if you do not have the schematic or the manufacturer's data sheet. You can nevertheless perform tests with an ohmmeter, taking advantage of its ability to simultaneously bias and measure continuity of each pair of leads.

Since there are three leads, there are three pairs that may be tested, and since each pair can be biased in either of two directions, it means that there are six possible readings that will provide the only information to be acquired using your ohmmeter.

It is possible to draw some conclusions. The collector and emitter should read open regardless of which way they are biased, because the two conceptual diodes are pointing in opposite directions. As long as the avalanche voltage is not reached, they will not conduct.

If there is low resistance on all three pairs, regardless of biasing, the transistor is shorted and defective. If there is high resistance on all three pairs, regardless of biasing, the transistor is open and defective.

Assuming that the transistor thus far tests well, you have identified the collector-emitter pair, so you know the remaining lead is connected to the base. The base will conduct to either of the other leads when forward biased, and will not conduct when reverse biased.

For these two readings, the biasing is opposite. In other words, to make an emitter-base junction conduct, the anode must be connected to the base if it is a pnp device. If you know whether it is an npn or a pnp transistor, you can determine which lead is the emitter and which one is the collector. If you know the identity of one of these leads, then you will know the identity of the other because the base has already been identified. From this, you can deduce whether the device is npn or pnp. (All of this assumes that you know the polarity of your ohmmeter.) It is not possible with these simple tests to identify the leads, aside from the base, if the transistor type is not known, nor is it possible to determine the transistor type if you do not know which lead is the collector or which is the emitter.

Tests that are more sophisticated will reveal both of these unknowns, taking advantage of the fact that the emitter material is more heavily doped and so will have a lesser voltage drop.

With the tests outlined above, however, we have obtained enough information to decide quite often whether the transistor is defective, if not all of its parameters. These tests are preliminary and are not to be construed as providing a definitive dynamic analysis.

However, transistors usually fail all at once if at all; they do not gradually weaken.

Using the Diode Check Function

Some meters incorporate a diode check function, and this displays voltage drop rather than the pseudo-resistance reading you get from an ohmmeter. Therefore, where possible, you will want to use the diode check function.

As you might expect, a transistor will amplify the input voltage only up to a certain point. After that, the output levels off and the transistor is said to have reached its saturation point. In analog applications, where it is desired to amplify a signal without distorting it, the transistor is kept below its saturation point. In contrast, digital circuits usually involve transistors that are biased beyond saturation when in the high state, and biased so as not to conduct when in the low state. These two states represent the binary numbers 1 and 0.

Two very important transistor parameters are alpha and beta. In a bipolar transistor, alpha is the collector current divided by the emitter current with base at signal ground. This quantity is the dynamic gain of the transistor. In contrast, beta is defined as collector current divided by base current when the emitter is at signal ground. Both of these parameters assume a small electrical signal applied at the input. Two formulas that illustrate the relationship between alpha and beta of a given bipolar transistor are:

Alpha = Beta/(1 + Beta) and Beta = Alpha/(1 - Alpha)

The underlying current relationship is:

I C = I E = I B

where I C = collector current

I E = emitter current

I B = base current

Bipolar transistors may be connected in any of these configurations: common emitter, common base, and common collector. Each of these can be designed to operate as an amplifier.

The common emitter configuration, when used as an amplifier, provides high gain. It must be remembered that the output is always 180 degrees out of phase with the input, but this is not a problem as long as we know what to expect. (An even number of successive stages will result in a noninverted output.) Aside from the phase change, a common-emitter amplifier provides excellent fidelity when operated below saturation.

The common-base configuration is similar, however, with base at signal ground. Output current appears at the collector so that amplification occurs. The output is in phase with the input, not 180 degrees out of phase as in the common emitter configuration.

The common-collector hookup, also called the emitter-follower circuit, involves placing the collector at signal ground. There is no amplification. The circuit is used to provide isolation between other stages, and for this reason, it is known as a buffer. Input and output signals are in phase. The only configuration where output is out of phase with respect to input is the common-emitter circuit.

Amplifiers are divided into four classes, depending upon how they are operated: Class A, Class AB, Class B, and Class C.

Class A amplifiers are biased so that they are never operated below cutoff or above saturation. Thus, collector current flows during the entire input. Class A operation is appropriate for many applications including audio and RF amplification.

Class AB operation is characterized by the fact that the forward bias voltage is less than the input signal peak voltage. The base-emitter junction is reverse biased for part of each cycle. Collector current, accordingly, flows between 180 degrees and 360 degrees of the input signal. Class AB operation is employed in push-pull amplifiers because it mitigates the harmful effect seen in Class B operation known as crossover distortion.

Class B operation is biased so that the collector current is cut off for one-half of the input cycle. Its application is for audio amplifier final stages, where the output power is very high, and for transmitter power-amplifier stages.

Class C operation occurs when collector current flows for less than one-half of the input cycle. It is employed for transmitter radio-frequency amplification.

Thus far, we have been talking as though the bipolar transistor were the only type. This is not at all the case. In fact, there is a great variety of transistors and other semiconductor devices that you will see in all types of electrical equipment. They operate on the same fundamental principles as the diode and bipolar transistor, but typically the structure is a little more complex or, in the case of ICs, vastly more so. There are often four or more terminals, making for more complex circuitry inside and outside of the device, with enhanced functionality. We shall look at a few of the varieties frequently encountered in troubleshooting and repairing electrical equipment.

Other than the bipolar device that we have been discussing, another widely used type of active device is the field-effect transistor (FET). Two types are the junction FET (JFET) and the metal-oxide-semiconductor FET (MOSFET).

The JFET, like the diode and bipolar transistor, is composed of n-type material and p-type material. The parts of the JFET have highly descriptive names-gate, channel, and drain. There are two varieties of this device-the n-channel and the p-channel.

Electrons or holes, as the case may be, move along the channel between the source and drain. This comprises a flow of current that is of the same magnitude at the source, at the drain, or at any point along the channel. A fluctuating electric field within the solid-state material causes the channel current to fluctuate. Small fluctuations in the gate voltage are what cause the field that envelopes the channel to also fluctuate. The bottom line is that there are corresponding changes in the channel current that occur at a much higher level due to external biasing. Actually, the higher the gate voltage rises, the lower the channel current falls, so it is an inverse relationship.

To summarize, the voltage at the gate produces an electric field that chokes off and limits the channel current-electrons for an n-channel JFET and holes for a p-channel JFET. Looking at Figs. 1-25 and 1-26, you can tell whether it is an n-channel device, with the gate arrow pointing inward, or a p-channel device, with the arrow pointing outward.

In some schematics, the arrow is missing, but you can discover the JFET type by looking at the power supply polarity.

A depletion region forms in the channel in response to any increase in gate voltage. For an n-channel JFET, this will be in a negative direction and for a p-channel JFET, it will be in a positive direction. Either way, increased voltage at the gate means decreased current along the channel and at the output of the device. This is because a depletion region forms in the ...

Figure 1-25 JFET, n-channel. Figure 1-26 JFET, p-channel.

... channel as a result of the more intense electric field. When the field intensity reaches a certain level, the channel current falls to zero. This event is known as pinchoff.

All of this sounds like the performance of a bipolar transistor, with the difference being primarily in terminology. However, there are a few substantial differences. FETs are unipolar rather than bipolar. The channel is a homogenous substance without a junction, and the current flows from one end to the other. Looking at Figs. 1-25 and 1-26, you can see that these two terminals-drain and source-appear the same. In fact, in many applications, it is possible to connect them either way and the device will perform in the same manner.

A major difference is that whereas in a bipolar transistor the input is a fluctuating current, in the JFET the input is a fluctuating voltage. The amount of current required to establish and fluctuate the electric field in the very small channel is minute. Therefore, we may say that the input of the device is very high impedance. It loads the circuit that feeds it to a very small extent, and is practically invisible to it. That property makes it a very useful device. For example, it may be used in a voltmeter to create a high-impedance instrument, so that the circuit being tested will not be loaded and altered because of the measurement being taken.

What Is a MOSFET?

Like the JFET, the MOSFET can be used either for switching or for amplifying signals. It further resembles the JFET in that it has a gate, source, and drain. Moreover, there can be either a p-channel or an n-channel. MOSFET, in a way, is obsolete terminology because metal oxide is no longer always used. Another term, perhaps more descriptive, is insulated gate field effect transistor, but most technicians still use the original acronym.

The main difference between a MOSFET and a JFET is that in the former the gate is not directly connected to the semiconductor material. Separating them is an extremely thin silicon dioxide insulator. Therefore, the connection is actually a capacitor, passing higher frequencies with less impedance and capable of holding the very small charge involved for some time.

There are two types of MOSFETs-enhancement mode and depletion mode. The enhancement mode is far more common. The difference between the two is all in the biasing. The enhancement mode is off when the voltage between gate and source is zero, and on when the gate voltage moves in the direction of the drain voltage. A depletion-mode MOSFET, on the other hand, is on when the voltage between gate and source is zero, and will be turned off by the application of voltage.

Like the JFET, the MOSFET has very high input impedance, practically infinite. While this is useful where you do not want to load the circuit connected to the input, it also means that the device is very sensitive to static charge, which can destroy it without changing its outward appearance. MOSFETs come packaged in conductive foam in which the terminals are imbedded, effectively shorting out the internal elements so that there can be no damaging static charge buildup. It is a good practice to leave the MOSFET in the conductive packaging up until the time you are ready to install it. Moreover, do not touch the leads if your body may be carrying a static charge.

In testing a MOSFET, you have to remember that if voltage is applied to the input, the MOSFET will be turned on or off (as the case may be), but if the voltage is removed, the MOSFET will not revert to the nonenergized state due to internal capacitance. To make it revert, you have to bleed off the charge using an appropriate resistance.

Most multimeters, in the diode check mode, will apply about 3 volts, which will not harm the device and yet will be sufficient to activate it.

To begin, connect the negative probe to the source. Then connect the positive probe to the gate. The next step is to shift the positive probe to the drain. You will read low because the meter has charged the gate capacitance. Leaving probes connected as above, short out the source and gate, using your finger. The gate will become discharged with respect to the source and the meter will read high, indicating that the device is not conducting. All of this activity indicates that the device is working.

The test does not reveal all the parameters given on the data sheet, but it will let you know if the device has failed.

Often a defective MOSFET will have a burnt appearance. When these devices short out between drain and gate, relatively heavy current may be fed to other MOSFETs and drive transistors, so it is necessary to check out everything.

Diodes and transistors are rugged and well protected within sealed packaging.

However, external forces, including environmental heat and excessive current or voltage, may damage a solid-state component. It is usually cheap and easy to replace.

Many electricians and electronic technicians simply replace an entire circuit board, and this may be the best option if time is of the essence and money is no object. However, complete circuit boards are costly and there is the serious problem that if you go this route, essentially handing off the repair to another organization, you have less control. It is possible to invest in a new circuit board and replace the old one, only to find that the problem persists. For the cost of a few circuit boards, you could keep an inventory of a great many transistors, diodes, capacitors, and resistors. With the proper techniques, soldering new components in place is not difficult, with the exception of ICs. We will discuss actual repair procedures later.

Thus far, the focus has been on discrete components-devices packaged individually with leads or terminals attached to internal elements to facilitate external connections. There is another category known as the integrated circuit or microchip. It is widely used and, in fact, our highly wired world of today depends heavily upon it. ICs are everywhere, from a simple household appliance to the International Space Station.

This ubiquitous item is a very small plastic or metal solid with conductive pins attached, sometimes 14, sometimes more or less, so that it may communicate with the outside world.

ICs are made up of many very small electronic components connected together to form an electrical network. The components, transistors, resistors, capacitors, and so on are placed on a monolithic semiconductor slab that forms one pole of some of the components, while others are electrically isolated from it by means of an insulating layer.

A process known as photolithography creates the whole thing. The circuitry is enormously complex, sometimes involving over 1 billion components. The design process, of necessity, is highly automated because no one human could conceive of complexity on this scale. An IC could never be opened up and repaired, although replacement is a routine task for electronic technicians. If you can isolate the fault first to a board, then to a circuit, and finally to a defective IC, a replacement should get the equipment up and running unless one or more other components were overloaded concurrently or preceded the demise of the IC.

There is a problem replacing ICs when they are soldered onto a circuit board, as opposed to being plugged into a socket. To take out the old IC, it is necessary to melt the solder joint for every pin simultaneously, in order to pull the component. The same is true for installing the new one. So much heat can damage the semiconductor. Fortunately, there are tools and procedures to facilitate the operation, and we will discuss them later.

There are limits to how an IC can be used. For one thing, inductors cannot be built inside the chip, so any coils have to be deployed outside the package, and wired appropriately. Second, ICs can be used only in very low-power applications because any heavy current within the component would produce excessive heat in such a small volume and the temperature rise would quickly destroy the IC. Otherwise, these chips are very useful and found in all types of equipment. Heat sinking and cooling by means of a fan or natural convection help control temperature rise.

ICs may be linear, for analog circuitry such as found in a stereo amplifier, or nonlinear, for data processing. The physical principles are the same, in terms of semiconductor behavior, but circuit details and biasing are different.

Some of the applications for ICs are:

• Voltage regulator, part of a power supply that ensures the voltage level will remain constant despite loading variations, within specified limits. To identify a voltage regulator IC, look for three terminals.

• Timer, often seen in conjunction with a potentiometer, which may be adjusted to control the delay. The timer consists of an oscillator with an electrical output at a specified frequency and means for counting the pulses so that the length of the delay may be set.

• Multiplexer, combining two or more signals so that they may be distributed on fewer lines. There are numerous multiplexing schemes, each associated with a demultiplexer.

Telephone transmission upstream from the branch exchange involves this technology.

• Computers rely on random-access memory (RAM) and read-only memory (ROM). These are two distinct areas that coexist within most computers. A detailed knowledge of how they interrelate and work together is necessary in computer repair work, and we will look into some of the details in section 8. ROM exists within dedicated ICs.

• Operational amplifiers work in a rather unique way, and because of their properties, they are quite plentiful in electronic equipment (see Fig. 1-27).

A single output derives, at an amplified level, from two inputs, one inverting and the other noninverting. There are two power supply connections. One is connected to the emitters and the other to the collectors.

Outside the package, a resistance is connected between output and noninverting input, making for positive feedback and, at a high level, oscillation. Between the output and the inverting input, circuit elements are introduced with values chosen to cause amplification to vary with frequency.

This occurs because the feedback line contains sufficient capacitive reactance to vary the feedback in accordance with the signal frequency.

The inverting and noninverting inputs are what make an operational amplifier function as it does, and these properties are well suited for treble-bass adjustment.

Figure 1-27 Operational amplifier.

ICs may be tested, but the procedure is complex and requires specialized equipment. As a final stage in the manufacturing process, ICs are tested and certified. When equipment is first placed in service, it is assumed that all ICs will operate within specifications.

Subsequently, excessive voltage or heat, whatever the source, may damage a sensitive component inside the package or indeed the entire semiconductor substrate. The damage may or may not be visually apparent. In the latter case, we must find ways to verify the operation of the IC in-circuit.

The best test of an IC is functional. That means observing it in circuits that will provide inputs and measuring the output voltages or looking at the waveform.

Most ICs cannot be effectively tested with a multimeter. There are highly specialized testers that are capable of testing a number of ICs, but this equipment is very expensive and you probably will not have access to it when you need it, unless you work in a shop or laboratory that does advanced work.

However, the good news is that many IC manufacturers have data sheets that include simple test circuits that you can build.

There is a test circuit for the very important 555 timer IC (see Fig. 1-28).

Of the many ICs available, the 555 is one of the simplest, yet it is very useful. A lot of electrical equipment has blinking LEDs that indicate operating states or a failure mode, and it is likely the 555 is at work. It was designed in 1971 and at present, there are variations from different manufacturers. Typically, the silicon substrate has 25 transistors, 2 diodes, and 15 resistors. The package is an 8-pin dual-in-line metal or plastic rectangle. There is no need to comprehend the internal circuitry, but to successfully troubleshoot electrical equipment containing a 555, you should know the operating modes, pin connections, and associated external circuitry.

There are three operating modes: monostable, astable, and bistable.

In the monostable mode, a single pulse is generated. One very useful application is the bounce-free switch, where if a switch such as a keypad element should inadvertently create two or more bogus pulses, the 555 makes it right.

In the astable mode, the 555 functions as an oscillator with a very wide range of intervals between pulses, determined by external circuit components. This is useful for making an LED blink.

In the bistable mode, the 555 becomes a flip-flop, with output in the form of rectangular pulses, the frequency determined by the external components.

Figure 1-28 Test circuit for 555 timer IC.

Varieties of Test Equipment

Testing of ICs is best done on a case-by-case basis. This means that you cannot have a single test circuit or meter that will work for all ICs. Some ICs are quite expensive, and it is not feasible to substitute them on a trial-and-error basis. Remember that heat or static electricity will destroy these chips, so proceed with caution.

We have discussed some of the electronic components that you will encounter and how to recognize their failure modes.

In doing this sort of work, test equipment is essential. Without it, one would be in darkness, as far as what is happening electrically. For quick measurements of incoming power, to determine presence or absence of voltage, and to get a general idea of the level, for example, 120 or 240 volts, the neon test light is a great little tool. It is inexpensive and clips onto your shirt pocket like a ballpoint pen, so you will have it when you need it. The principle is that two electrodes inside the small glass envelope ionize the inert gas, so that when voltage is applied, a faint yet distinct glow is emitted. It is a very high-impedance device. A small internal resistor further limits the current flow. If the probes are touched to a 120-volt hot leg and ground, the bulb will glow and the light is bright enough to be seen outside in full daylight. If the probes are touched to 240-volt terminals, the neon bulb will glow twice as brightly, so you can jump around among terminals and quickly find out what is what.

A common failure mode in large commercial appliances as well as in distribution boxes, meter sockets and entrance panels is loss of one phase, which will make equipment work erratically if at all. The neon tester is excellent for finding a dead leg.

You can touch one probe to a hot terminal and the other to your finger, and the bulb will indicate presence of voltage. This is an intrinsically safe operation because the neon bulb plus resistor has such high impedance that virtually all the voltage appears across the tester and minute current flows through the body. However, the electrician must always be aware of issues that may be involved, such as standing in water or introducing current through a cut or abrasive wound on the hand. (Dry, clean skin, especially of a worker with toughened hands, is a partial insulator.) In addition, if the tester has become shorted, you will be zapped, so you should touch the free probe to a known ground rather than to your finger.

The technique just described is useful for determining which line is hot and which is neutral.

In addition, before and after completing a repair on a tool or appliance with a metal case, you can check to make sure the enclosure has not become energized due to an internal fault.

A typical neon tester is rated for up to 600 volts, but that is a little too close for comfort.

A pencil eraser will remove high resistance oxide coatings on electrical contacts in a motor controller, TV remote, or anything in between. Rub the eraser vigorously on clean paper until the shelf-life glaze is gone, and then polish the contacts. This will not remove severe pitting or burning but it works for early stages of oxidation and is great preventive maintenance.

Another convenient tool for checking for presence or absence of voltage is the humble appliance bulb in conjunction with a plug-in bulb socket. This device is good for checking receptacles to see if they are live, whereas voltmeter probes are problematic for this measurement because you never know when they are getting a good connection inside the receptacle. Plug-in bulbs can be left in one or more receptacles while you work elsewhere on connections some distance away, providing indication when the branch circuit goes live.

The next step up in order of increasing sophistication is the so-called Wiggie, also known as a solenoid voltage tester. It has several advantages, making it popular with electricians. Besides a visual voltage indicator, it makes a buzzing sound on ac and a loud click on dc, helpful when it is difficult to watch the indicator and place the probes simultaneously. Moreover, the buzz or click can be felt if you are holding the instrument-a good feature in noisy environments. The Wiggie does not require batteries for operation-a definite plus. Wiggie is a Klein Tool registered trademark, but other manufacturers such as Square D carry a similar model.

The circuit analyzer, made by several manufacturers, plugs into a receptacle to indicate presence or absence of voltage. Additionally, three LEDs indicate whether wiring is intact and connected correctly. Major wiring errors are displayed by the LEDs, with a key printed on the plastic case. These include open ground, open neutral, and reversed polarity.

The Ideal SureTest Circuit Analyzer is a much more full-featured instrument that sells for over $300. It is actually a U.S. version of the European loop impedance meter. It applies a load to a circuit under test and reveals various circuit parameters. The meter will provide a wealth of information about circuit status including wiring that is concealed behind walls such as improper grounds and intermittent faults such as occur when a nail has partially penetrated a wire causing circuit failure only under heavy load. It also reads out line, ground, ground to neutral, and peak voltage, as well as frequency.

Megger makes an excellent loop impedance tester that checks the integrity of the equipment-grounding loop and generates a report that can be downloaded to your computer via a USB port and printed for purposes of documentation and certification (see Fig. 1-29).

Figure 1-29 Megger's loop impedance meter. (Courtesy of Megger.)

The common multimeter is the electrician's most-used measuring tool by far. Older models were analog, with a needle pointing to the correct value when the relevant scale had been selected. The analog meter is more difficult to read and perhaps less rugged, but it is still preferred by some electricians as it may perform better outdoors in very cold weather.

The digital version, as the name implies, has a numeric readout that responds to input from the probes.

Some models have diode check, capacitor check, and other capabilities. The most basic model reads voltage, resistance, and low-level current. FETs on the front end of the voltage function ensure that the circuit under test will not be loaded to any appreciable degree, so that if it is a high-impedance source that is being tested, the voltage will not be dropped.

An inexpensive meter will test up to 600 volts, but great care must be exercised when measuring live circuits. There are several ranges and the idea is to start on a higher range and work down if there is any doubt as to the maximum possible voltage. Some meters are auto-ranging, so you do not have to worry about that. For dc voltage measurements, polarity must be observed, although some models will sense polarity and switch poles internally if necessary. If OL appears in the readout, it stands for overload and means that you have to select a higher range. However, sometimes OL will appear when the meter is not connected to a live circuit. How is this possible? Many beginning electricians are puzzled by the phenomenon of phantom voltage. The fact is that there are static charges at times in every conductive body that is or has been in moving contact with a nonconductive body, including dry air. There is electrical potential everywhere, even in the meter case and probes, and a very high-impedance instrument such as a digital voltmeter may see a high voltage drop for no obvious reason. As soon as you connect to a live circuit, the phantom voltage reading will go away and your meter will lock on to the real quantity.

If your meter reads current, it will only be in milliamp ranges. This is because the current has to pass through the meter in order to be measured. The power that has to be dissipated is equal to I2 R, so with any large current, the meter could not handle it. For this reason, the current function is rarely used.

Do not forget that to take a current measurement you have to temporarily cut open the circuit in order to place your meter in series with the load. This is in contrast to a voltage measurement, which is taken in parallel with the load or with the source. In current mode, the meter has very low impedance and is invisible to the source and load. In voltage mode, the meter has very high impedance and is invisible to the source and load.

A very useful function, perhaps employed more frequently than any other, is the ohmmeter mode. There should be no mystery in how to use it. For ordinary resistance measurements, where the object of interest is not an active component that needs to be biased, polarity does not matter. However, be sure the test voltage injected into the component or circuit, usually about 3 volts, will not cause damage. It is good to have two meters so you can test this voltage on both meters.

Some meters have an audible beep or buzz indicating that the reading is fewer than 20 ohms. This is very convenient as a continuity tester. For 0 ohms, the readout may say OL, which in the ohms mode means Open Loop.

In the resistance mode, any voltage may damage the meter. Some instruments have an audible alarm or internal protection, sometimes in the form of a fuse that would have to be changed.

The only drawback for this instrument is that it depends upon an internal dc power source, often a 9-volt battery. With moderate daily use, the battery will last about a year.

Immediately before measuring hazardous voltages to see if equipment is indeed deenergized and safe to work on, it is necessary to check the meter by measuring a known live voltage. The battery not only supplies test voltage for the ohms function, but also biases for internal electronics including the readout.

When running multiple parallel conduits across a ceiling or wall, make a plywood template based on the originating panel knockouts to aid in maintaining uniform spacing. Secure conduits to struts without fully tightening strut clamps at first, and then check alignment at each strut and tighten. Use the same template for drilling holes through an existing wall.

To do any serious troubleshooting on a commercial level, you will definitely need a clamp-on ammeter. Older models were analog and did not need a battery. These work well, but a digital model is now available and it is very popular with electricians because it includes voltage and resistance functions, and a hold button so that the readout will clamp on the highest current since the feature was activated.

Unlike a multimeter, the clamp-on ammeter will measure very heavy currents, as seen at an electrical service.

The clamp-on meter, rather than measuring current directly, measures the magnetic field surrounding a conductor. Just open the jaws, pass the conductor into the space between them, and close the jaws. The readout is surprisingly accurate, in either the old analog type or the new digital model. It does not matter if the conductor is centered in the space between the jaws, and if you move it around, the readout remains consistent.

It is necessary to realize that you cannot take a reading off a supply cord that contains two hot legs or a hot and neutral. This is because the current flows in opposite directions, so the magnetic fluxes surrounding the conductors cancel out, giving a zero reading. In this situation, you may be able to clamp onto an individual current path inside the equipment, or back at the entrance panel. Alternatively, you can make a splitter. Make up a short extension cord with a male plug and a female connector at the two ends. Slit the outer jacket for about 8 inches near the middle and pull out the wires. Trim back the cut insulation and you have a very useful accessory to the clamp-on ammeter. You can make noninvasive current checks of tools and appliances.

For motorized equipment, the current draw should be less than the full load current on the nameplate, except while starting. We will have much more to say about motors in the next section, but for now, it will suffice to note that as a motor ages, it will draw an increasing amount of current. This is because the insulation begins to break down. As the process continues, there is more heat and the deterioration accelerates until motor failure occurs. Large motors in a commercial or industrial setting should be monitored on a regular basis. Either the temperature or current while running at full load should be measured and the results posted on a log so that a damaging trend may be spotted before there is an outage and expensive down time.

Clamp-on ammeters with the hold feature are useful in many ways. If you have three of them, you can leave them hooked up at a three-phase service to see how well the loads are balanced over a time span.

Ground electrode conductors can be checked to make sure that excessive current is not being faulted.

The instrument is valuable in troubleshooting a submersible water pump system prior to making a decision to pull the pump.

There are many other specialized meters, especially for power quality measurements and low-voltage network certification.

The oscilloscope, especially when used in conjunction with a function (signal) generator, allows the technician to see into the operation of electronic equipment in a specialized way.

An oscilloscope draws a graph of voltage against time. Voltage will be on the vertical or y-axis and time is on the horizontal or x-axis.

The voltmeter will let you know the ac or dc voltage at a terminal, with respect to ground, but the oscilloscope provides much more information. It displays a picture of the waveform for one or more cycles. If the correct waveform is present, you will know that the equipment is functioning correctly at that point and at that time. One way to do this is to have a healthy specimen of the equipment on your bench next to the one that is being repaired. You could compare signals at various points and thereby find the defective stage, connection, or component. However, it is not realistic to think that you would have a good piece on hand. Alternately, other approaches will work.

Some schematics include small graphics that depict the correct waveform at various points. Many of these are available on the Internet. A Sylvania Srt 2232x color TV service manual is readily available there and it shows 16 waveforms as they should be observed at key points.

Such information is available for all sorts of electrical equipment. If you cannot find exactly what you want, you may find the information for a similar piece of equipment.

More Advanced Testing

For a stereo receiver or amplifier, if one channel is not working, you can take readings at various points on the good channel and use that information as a guide. Moreover, if you know the purpose of a stage, such as providing video synchronization, you should be able to figure out what the waveform should look like. Every time you observe a good waveform, you learn something, and if you care to keep a record of readings on each job, you will definitely become adept.

This presupposes that you can operate the oscilloscope. The problem is that waveforms cannot be displayed as picked up by the probe. If you tried to do that, the waveforms would flash across the screen in an indistinct blur. For the waveform to display, it is necessary for the instrument to trigger properly. There are many adjustments on the front panel, and they have to be set properly to get a good display.

More PCB Repair Techniques

There are other concerns as well. For high-frequency signals, probe capacitance becomes critical, and there is the whole matter of impedance matching so that you do not have a chaotic mix of harmful reflections. There are many variations in the ways that an oscilloscope has to be set up, depending on the make and model and the intended use.

The key to working successfully with an oscilloscope is spending a lot of time with the owner's manual so you know how to key in various voltages and frequencies. Many operators' manuals are available free on the Internet, so you can look them over in advance of purchasing an instrument.

The oscilloscope should be dual trace, meaning that it is capable of displaying two inputs simultaneously so that they can be compared. The frequency response should be high enough for the use intended.

The sad reality is that a good oscilloscope is enormously expensive. The model you would want will likely cost more than $5000. There are plenty of old scopes on the used market for under $100, but they are useless. If they work at all, they will be far out of calibration. Servicing and parts are no longer available.

If you work in a facility that has a good oscilloscope and function generator, that is a stroke of good luck and, in time, you can become an expert.

Now that we have looked at some common electrical components, their failure modes, and instruments used to test them, we can examine in more detail the troubleshooting process for commercial and industrial equipment.

First, we have to get past the throwaway mentality that says it is not economically feasible to repair equipment-that once a piece of equipment has malfunctioned it is time to scrap it and buy new. This is in part an inevitable trend, and yet not at all healthy in my view. It is not good for the environment, the economy, or our technological development as a world community.

In the nineteenth century, even as the age of electricity was dawning and industrialization permeating the land, North America was still primarily agricultural.

Generations of young people grew up learning to keep steam tractors and sawmills running. This was the setting for a great age of experimentation and invention. Will we lose all of that? It is sometimes stated that nothing is worth repairing-scrap it and buy new. For small residential appliances and home electronics, that may be appropriate. However, in a commercial or industrial facility, where there is a maintenance department with skilled and knowledgeable personnel, a very different agenda makes sense. A well-managed parts inventory and abundant documentation, together with burgeoning replacement costs, mean that equipment should be kept in service much longer if not indefinitely, given adequate preventive maintenance and informed repair procedures. That approach is advocated in this guide.

Residential equipment is tightly engineered, with plastic enclosures that snap together in strange ways. When some of them are opened, parts fly in many directions, or at least expand in a disorganized way so that it is difficult to get them back in place after the repair has been made. When you succeed in snapping the enclosure back together, there is the potential for pinching a wire or worse.

Commercial equipment, while more complex and having extensive subsystems, is actually more amenable to preventive maintenance and repair. A huge washer or dryer in a hotel or commercial laundry is easier to maintain and repair than its household counterpart.

Access panels are fastened in place by a large circle of machine bolts that can be quickly removed using your cordless tool. Then you can practically walk inside and look around.

Moreover, there is usually adequate documentation so that you can examine the schematic and troubleshooting guide before diving in.

So where does this leave us? The basic troubleshooting steps are:

• Interview the operator.

• Check the incoming power supply.

• Examine the equipment visually, and check for abnormal sounds and smells.

• Based on the symptoms, isolate the defective stage and start taking measurements including testing individual components.

• Still at a loss? Many manufacturers have a toll-free tech support line, and it is likely to help you identify the faulty component. If all else fails, try resoldering key joints and sliding out and in ribbon connectors to re-polish the contacts.

This guide contains sections on specific types of equipment. This material should help in troubleshooting them and provide insight into other, sometimes unrelated, equipment. At all times, you should try to develop insight and intuition, until you get to the point where you can instantly see where defective components will most likely be found.

First, we will look at repair methods and materials.

In the event that it is necessary to access the inside of a machine, you will want to take a critical look at the enclosure.

As mentioned earlier, some equipment is notoriously difficult to disassemble. Earlier Apple laptop computers, for example, were quite easy to service. Battery and hard drive were readily accessible behind simple access covers, and they could be easily removed if changes were necessary. The latest version, in the interest of an ever-slimmer profile, has everything including the battery miniaturized and cemented in place in a most user unfriendly fashion. Is it possible to service this type of equipment? Absolutely. It is just a question of finding the right approach. A wonderful resource is YouTube.com. Type the make and model into the search bar and you will most likely get a how-to video that will demonstrate the best method for opening the reluctant model, plus troubleshooting and repair information. Another good Internet site with a lot of free information is ifixit.com.

If none of the resources we have mentioned is helpful, you must rely on experience and intuition. There will be situations where you just stare at a piece of equipment and cannot see a way in. However, there has to be an answer. Sometimes paper stickers will cover screw heads holding a chassis together. The purpose is to provide evidence of tampering in order to void the guarantee. (If a unit is still under guarantee, that is usually the way to go anyway.) Another frequent scenario is that screws are recessed above rubber feet, which have to be removed.

Using a table saw, make a V-block from a 24-in. 2 × 2 and mount it on a short-legged sawhorse. This greatly aids in cutting conduit in the field where a bench vise is not available.

I once had to delve into an older Acer laptop, and could find no obvious way to open the case. Finally, I found the exact model the subject of an electronics technicians' Internet chat room and I learned that the secret was to slide a sharp pointed tool alongside a certain numeral on the keyboard, whereupon a latch inside was released and the case split nicely.

Access to various parts on the inside was also difficult, bringing new meaning to the word "refractory."

Often accessing the circuitry inside is more difficult than the actual repair. Afterward, it is necessary to put the whole thing back together without scratching the surface or damaging the latching mechanism. It is also essential to keep track of all the small parts and make sure they go back together as intended. An egg carton or a plastic tray with dividers works well. Start filling sections at one end and progress to the other end. For reassembly, reverse the order.

Often outside cabinet screws have a different finish or a different type of head so that they may be easily distinguished. A major problem with screws is that some of them may be different lengths, and not without reason. If you use a longer screw in the wrong place, you can damage a component or ground out a terminal. Therefore, make notes to yourself or mark the chassis with a felt tip pen, whatever works.

With a little experience, you will find disassembly/reassembly operations much easier.

As far as taking out components and replacing them, what is needed is good soldering skills. Fortunately, soldering is easy. The right tools and materials, some basic knowledge, and a very few hours of practice are needed.

Prior to the 1960s, electrical connections for wiring in buildings were soldered.

Electricians twisted the wires together (making sure the copper surfaces were clean), and then applied flux, solder, and heat. Afterward, the joints were wrapped first in electrical tape, and then friction tape to guard against abrasion. The connections were low impedance and reliable, but the process was time-consuming and the taped joints were bulky, making for excessive box fill.

An Essential Skill

A far better way was developed. Solderless connectors ("wire nuts") make for a more efficient workflow and, if properly deployed, result in quality connections every time. An additional advantage is that for testing or alterations, the wire nut can be removed and replaced easily.

Soldering is still essential in electrical work although it is used less frequently. If you need to make a splice where space is limited, such as inside an appliance or hand tool, soldering is the way to go. Printed circuit board components are soldered in place when first manufactured and for repairs. In motor enclosures where the leads have been cut too short or where there could be excessive vibration, soldering is appropriate. Stranded wires that must go under a screw terminal may be tinned to good effect, and inside old light fixtures that are being rewired, soldering is sometimes the best answer. Crimp-on connections for high-current applications (such as three-horsepower and over submersible pumps) gain conductivity and reliability by having solder run into the crimps on either side of the slide-in connectors.

Soldering differs from welding, where the metals to be joined are heated sufficiently to melt to some depth to achieve the good penetration required for great strength. A soldered joint usually consists of two copper surfaces with solder adhering to the surfaces and absorbed below the surfaces to a very short depth. The copper is not melted. Not all metals are equally suited to this process. Copper works very well. Stainless steel cannot be soldered. As for brass, it depends upon the alloy. Some brass solders just like copper, while some is not suitable at all for soldering. Aluminum can only be soldered using specialized materials and techniques. One of the problems with aluminum is that you can get a very nice looking joint that is not good at all. Not all of this is a problem, however, because the only soldering you will ever need to do, most likely, is copper to copper. It is worth noting that the NEC prohibits solder joints in certain applications, notably ground electrode conductors, where a heavy surge could melt the connection.

When soldering wires, they must first be twisted or looped together to make a strong joint that cannot wiggle. Then the soldering process adds strength and conductivity. For a solder joint to succeed, the right materials must be chosen, along with the correct technique, appropriate to the scale of the work.

As for materials, flux must always be applied, prior to making any solder joint. When heat is applied, or after sitting unused for a period of time, any solderable metal including the soldering iron tip will acquire an oxide coating, which comes from oxygen in the air especially in the presence of moisture. This happens instantly when the temperature gets close to the melting point of solder. This oxide layer is very visible. There is a dull or dark appearance to the metal to be soldered and to the soldering iron tip. Moreover, where there is oxide, any attempt at soldering will be unsuccessful. This is because the oxide acts as a thermal barrier or insulation, preventing the heat from getting where it needs to go.

Typically, if the soldering iron tip and one or both of the metals to be soldered is oxidized, the solder that you feed in will just crumble and fall away no matter how much heat you apply, the oxide layer becoming thicker and more intrusive all the while. If there is an excessive amount of oxidation, start by cleaning the surfaces with steel wool or scraping them with a knife.

The soldering tip is prepared by tinning it. This involves bringing up the temperature and wiping it on a damp sponge while hot. Give it a couple of swipes on each side, being sure to get the edges and the end. Then quickly, before more oxide has a chance to form, apply a little flux and melt on a small amount of solder. Allow the iron to cool, adding more solder while it will still melt. You should see a nice silvery coating on the tip, indicating that it has been tinned. This is how it has to look prior to any soldering job. The tip usually has to be re-tinned from time to time, and this should be done after the final usage of the day so that the tip will not oxidize while sitting idle. Remember that oxidation takes place instantly at high temperature and more slowly at room temperature. As long as the tipped is tinned, it will not oxidize.

The correct flux must be chosen. Acid-based flux used by plumbers and for automotive radiator work is too strong, and will reoxidize (corrode) the work after the fact. Moreover, the flux must be nonconductive so that adjacent traces on a printed circuit board or closely spaced electrical terminals will not be shorted out. Furthermore, the flux must not leave a paste residue that would attract and retain dirt that might be corrosive or conductive, or act as thermal insulation. The wrong flux will appear to work at first but is likely to make problems in the future. Buy and stock a resin-based flux that is labeled for electrical work, never acid based. Some solder has a resin flux core that is dispensed onto the work as the solder is melted. This works fine, or you can put the flux on first and use solid solder, whichever you prefer.

Precleaning with steel wool or a knife is useful for getting rid of a heavy coat of oxide, paint, or any other contaminant, but in all cases the flux is essential. Without flux, you cannot solder. Avoid the use of sandpaper as this may leave grains of abrasive embedded in the metal, which would create dead spots in the solder joint, high impedance, heat, and eventual degradation of the connection.

As for solder, there are two variables-wire thickness and alloy composition. You should have several rolls of solder on hand, with different thicknesses for various sizes of work.

If the wire is too thin, it will take a long time to complete the operation and in the process, a harmful temperature may be reached. If the wire is too thick, the large amount of solder will overwhelm the joint, spilling over into unwanted areas and again requiring excessive amounts of heat. With experience, you will choose the right wire size for the task.

Solder is available as alloys of various metals and in differing percentages. They all work to join the metals, but since the melting points vary, the right choice must be made.

Alloys with higher melting points are generally more difficult to work with because more heat has to be applied, meaning that nearby sensitive components are at risk. SN60 and SN63, with 40 and 37 percent lead, respectively, are commonly used for electrical work.

Lead-free solder is used in plumbing for potable water systems, but the higher melting point makes for problems in electrical work.

In addition, the correct tip size and heat output must be chosen, appropriate to the task.

A solder gun (these come in a range of sizes) may be chosen for soldering 14 AWG and larger wires, whereas pencil-tip irons (of various sizes) are right for printed circuit boards (see Figs. 1-30 and 1-31).

If you have chosen the right iron, flux, and solder, and have figured out how to prevent oxide contamination, the soldering job will come together nicely.

Let us start by joining two wires.

After the wires are cleaned and twisted or looped together, suspend and brace them so that the joint is in free air, away from any metal that would pull out the heat and not on wood or any material that would heat up and contaminate the solder joint. Bring the clean, well-tinned iron tip up to temperature while it is in contact with the wires. If they are of unequal sizes, apply more heat to the more massive of the two, so that they reach the

Figure 1-30 Gun-type soldering iron. (Courtesy of Judith Howcroft.)

Figure 1-31 Pencil-tip soldering iron. (Courtesy of Judith Howcroft.)

melting point of solder at the same time. If possible, it is better to apply heat to the underside of the joint. Avoid breathing smoke or fumes because you never know what impurities the metals could contain.

When you begin to approach the critical temperature, touch the end of the solder wire to the joint, preferably on the side of the joint opposite from the source of heat. The solder will serve as an indicator so that you know when the soldering temperature has been attained.

At this point, the soldering iron and solder should be held at angles so that they do not impede your view of the joint. Abruptly, the solder will melt and flow wherever there is heat and flux, even uphill. A poor joint would result from insufficient heat as well as from too much heat, which would start to burn the flux and reoxidize the metal. Therefore, good judgment is necessary as far as knowing when to remove the heat. If you apply solder to the cold side of the joint (away from the heat), wait until the solder melts, and then hold the heat another two or three seconds; you should have a good joint.

After you remove the heat, be sure the joint is not moved until the solder has thoroughly solidified. If it is allowed to wiggle while in an intermediate state, the joint will have fractures. Do not do anything foolish like spraying water or blowing on the joint to hasten cooling. That would crystallize the solder, making for a weak joint.

That is just about it as far as soldering two wires or, for that matter, any two pieces of solderable metal. Practice on scraps of copper. After they are soldered, try to break them apart. Strive for nicely proportioned solder joints, not overly large but feathering out at the edges rather than terminating as big blobs. The result should not have the appearance of a cluster of wild grapes.

Printed circuit board work is based upon the same physical principles. It is a little more sensitive to error and, of course, there is the potential for ruining an expensive circuit board.

For this reason, it is a good idea to find a discard and use it for practice. Try removing components and replacing them. Take resistance readings, before and after. Do not forget to use heat sinks and limit the amount of heat used, especially around semiconductors. While active devices are out of circuit, do meter tests as described earlier.

After melting connections and pulling a device, often the hole in the printed circuit board remains plugged with solder. There are tools designed to deal with this problem.

While the solder is in a molten state, it can be removed by using a solder sucker, which has a hand-operated squeeze bulb. In addition, fine-braided, resin-impregnated copper solder wicks work well when heated in contact with the plugged hole. You can use these tools in tandem, first the solder sucker and then the braid. Finally, if the hole will not clear, the correct-sized drill bit inserted in a handheld bit holder would work, but beware of metallic grindings that could stick to the board and short out close-spaced traces. Again, excessive heat is ruinous. Besides damaging sensitive components, too much heat can delaminate the board or lift traces.

We mentioned earlier that IC removal and replacement could be problematic. This is because all pins have to be heated and melted out simultaneously so that the component can be pulled free of the board. So much heat would surely damage semiconductors or the board itself. There are ways to work around this problem. You can heat the pins one at a time, then with repeated solder suction and wicking, free up the component. There is such a thing as an IC extraction tool, which can be helpful. This would be if you wanted to save the IC, taking it out of circuit for testing. If you knew the IC was bad from circuit analysis or if it runs abnormally hot to touch, and if you want to save the board, here is an alternate method that works even with a 40-pin DIP.

Using a small, sharp pair of side-cutting diagonal pliers, cut each pin close to the IC body. Then you can melt out and pull each pin individually. A soldering iron with a magnetic tip is helpful. After the holes are cleaned out, the replacement IC or an IC socket can be installed. Alternatively, you can leave the pin stubs in place and solder the new IC directly to them, provided there would be enough vertical clearance when everything is put back together.

There are other circuit board repairs besides removing and replacing components. A frequent repair involves a cracked circuit board. The first thing to ask is why did it crack? If there was traumatic damage to the overall piece of equipment, all stages must be checked out to make sure that some latent weakness does not cause a problem in the future. The cause of a cracked board could be that it became dismounted from the chassis. If it is held in place by screws, make sure they are all in place and tight. The board may go into a slot, and an edge could be damaged. On the other hand, the crack could be the result of age and brittleness, perhaps due to repeated heat cycles that did not reach sensitive components.

After the underlying cause is identified, evaluated, and remedied, the actual repair has to be made. Superglue epoxy works well for repairing broken boards. In addition, you might try any of the Krazy Glue varieties. Plastic glues generally work as solvents, dissolving the parted surfaces so that the materials can mix, fuse, and harden. Cold-press wood glue, which works by seeping into the wood pores and then hardening while clamped, will never work on printed circuit boards, nor would a plain epoxy, which is at its best when there is a space to be filled. A construction adhesive would also not be suitable.

After each stage of the repair, thoroughly clean the board with isopropyl alcohol, finally wiping the surface with a tack-free cloth.

Another method that can be used in addition to cementing the cracked board is to drill small holes on either side of the crack and to insert staples. Flip the board and bend over the ends. Of course, these holes and staples have to be in areas where there are no conductive traces.

If the crack ends in the board without going through to the edge, it is necessary to drill a small stop hole centered on the crack and just beyond where it ends, so that the fissure will not spread in the future. These holes have to be away from traces.

After the board has been repaired structurally, it is necessary to restore any traces that have been broken, nicked, weakened, or otherwise compromised. To start, gently scrape away the insulating solder mask, exposing but not affecting the copper trace. Then, if the damage was at one point only, apply liquid resin flux and resolder with a fine tip, being careful to avoid bridging to an adjacent trace.

If the damage to the trace is for any length, cut out the damaged part and lift it away from the board, using tweezers. Replacement trace material is available from your electronics supply house. Choose a piece the correct width, cut it to length, cement it in place, and solder it in place at both ends. Finally, recoat the area with a nonconductive coating.

An alternate method is to solder a segment of point-to-point wire of appropriate ampacity to the terminals at either end of the broken trace. Whatever you do, where there are high-frequency circuits, you have to be careful not to alter the conductor size or routing as this could alter the characteristic impedance (which must match input and output impedances) thereby causing harmful reflection and data loss or unintended capacitive coupling.

It is hoped that the ideas in this introductory section will get you started, at least to the point where you can learn by doing. To diagnose and repair electrical equipment, it helps to know how it works. In the sections that follow, we will look at a variety of different types of equipment and larger systems. After gaining some experience by reading about and working with them, you should be able to extrapolate into previously unknown areas.