Audio and Video Equipment [Troubleshooting and Repairing Commercial Electrical Equipment]

Stereo sound equipment enhances the listening experience by providing a realistic approximation of a live performance. The whole idea behind good sound reproduction is fidelity, which means accurate rendition of the original. This is most relevant to music. Enthusiasts go to great lengths to create as near perfect sound reproduction as possible, and while the technician may not share fully in this passion, it must be respected when working on this equipment. For a start, a quality setup is essential to get the most out of the equipment.

When finished soldering, re-tin the soldering iron tip so that it is ready for next time.

Tinning the tip prevents corrosion. If the tip is corroded, the oxide coating acts as insulation and prevents heat transfer so that it is impossible to solder. Disconnect the soldering iron and apply solder as it cools. To clean a corroded tip, heat it up and wipe it on a damp sponge. Apply flux and solder as it cools down.

The fundamental idea behind stereo is that at the time of the original recording, two (or more) microphones are placed, let us say, at opposite sides of a stage or, for an orchestral performance, at different locations among the musical instruments. Recording engineers make all of this into a high art form, with many tweaks and adjustments that make up the final product, but the basic mechanism is simply that two separate signals are recorded and reproduced so that they can be played back as two channels. For years, this was done predominantly on vinyl LPs, then cassette tapes, and currently on the wonderfully convenient and durable CDs. (A recent development is MP3, an encoding format for digital audio that permits compression by reducing the audio resolution that is beyond what most people are able to hear.) The two sound tracks, at the listener's location, are separately amplified and the two signals are sent to two different speakers, making for a superb listening experience where the spatial characteristics of the original concert hall or recording studio are recreated.

Optimum Sound

Setting up sound reproduction equipment, at the most advanced level, is a job for professionals with specialized knowledge and expertise in the area. There are, however, a few basic principles that should be observed to get good sound.

First, the speakers must be in phase. The two speakers should simultaneously make compression waves in the air that reach the listener at the same time, as opposed to one speaker pulling away as the other pushes in. Phase may be changed by reversing the wires of just one (not both) of the speakers. It is usually done by trial and error, to see which sounds better. Some advanced stereo systems have incorporated a phase switch. You can make one of these using a four-way switch in line with one of the speakers. When the stereo outputs are out of phase, the sound will appear muddy and the stereo effect is indistinct, so the difference is immediately evident. One warning: Do not change phase, either by exchanging speaker wires or by using a phase switch, with the amplifier powered up. In fact, an amplifier should never be run without speakers or a dummy load, as this will cause the output voltage, being open circuited, to rise to a dangerous level and destroy the output transistors.

In a rectangular room, speakers should be placed equidistant from the corners along the long dimension. They should toe in a slight amount but not excessively, and should be out from the wall a little. They should also be elevated above the floor. The room should not be heavily upholstered and composed entirely of acoustically absorbent material, but neither should it be all hard surfaces-rather, somewhere in between. Either of the extremes will make for an artificial or overly stylized sound. The couch or seating for listeners should not be backed against the wall, but should set out slightly to minimize the effect of inappropriate reflections. These design considerations for a stereo installation will affect the quality of the listening experience. A few minor adjustments along with correct phasing will be very noticeable to the aficionado.

As for stereo troubleshooting and repair, the equipment is easy to diagnose. This is because with two channels, if one is outputting a bad signal, it is possible to take electrical measurements at identical points within the circuitry of each channel and compare the readings. This technique applies to resistance readings, voltage readings, voltage drops across resistors, capacitor tests, etc.

A stereo amplifier is easy to fix. If the unit is dead, check for a cord, switch, or power supply problem. If one channel is out, but not the other, take readings at various points and see if there is the same reading in the other channel. Suspect a bad speaker, which can be checked by applying voltage from a dry cell. As polarity is reversed, the cone should either pull in or push out. This technique can be used to determine correct phasing in a stereo system.

If the whole unit is dead, verify that there is branch circuit power, and then check the cord. Like all appliances, a frequent fault is in the power cord where it enters the cabinet.

Next, check transformer input and output ac voltages. Shorted windings can make for bad output voltages. If they become too high due to shorted primary windings, which would change the turns ratio, downstream components can be damaged. More likely, however, a winding is simply open or grounded out. Resistance measurements are useful for checking the transformer windings, but just make sure the unit is unplugged and power capacitors are discharged through a load. A bad power supply transformer may have visual signs of overheating. Power supply diodes and capacitors can be checked as well as any in-line fuses, which may or may not appear visually blown.

By its nature, the power supply is intimately connected at various points with every stage in the amplifier, so if there is an excessive power supply output voltage, there can be open or shorted components downstream. Visual inspection may reveal this damage. If the power supply is good but neither channel is operating, look for a bad common ground return.

If one channel is working, but not the other, compare electrical values at identical points on both sides. It is good to start at the speakers because the biggest power flow is here and therefore there is a great chance for mischief. There are several ways to check speakers- exchange them between channels, take out of circuit ohmmeter readings at the terminals, or connect a dry cell to the speaker inputs and observe whether the cone moves. (Similar tests can be used to check the speaker wires.) Very often there is an open coil or, especially for large woofers, a broken lead that is too short to solder. Many speakers have impedance matching transformers mounted on them. They go bad when called upon to handle too much power, particularly when the volume is turned up high. Replacement speakers often come without transformers, so when installing a new speaker it is necessary to take the transformer off the old speaker and mount it on the new one. This involves drilling holes in the new speaker's metal frame. Use a magnet to catch the steel particles. While we are on the subject, it is worth mentioning that a large discarded speaker is an excellent source for a very powerful permanent magnet, which is useful around the shop or on the job for such diverse tasks as retrieving dropped tools and hardware that have become inaccessible, cleaning up spilled nails, holding steel objects in place on the bench to aid in difficult soldering jobs, and setting off fire alarm heads in order to test them for scheduled maintenance.

High-end stereo equipment is very expensive, so great care must be taken when working on it. If an internal fuse has blown, it may be because the fuse element has become fatigued and its time has come. It is also possible that a line surge has occurred due to nearby lightning or inductive kick from a load being interrupted quickly. In these situations, a fuse can be replaced and the equipment fired up. However, there are instances where a blown fuse indicates a problem within the equipment such as a shorted lead, wiring fault, or component failure that caused the circuit to draw abnormally heavy current. If so, a replacement fuse should cut out before further damage occurs. However, this is not always the case. Therefore, when there is a blown fuse, especially in expensive equipment, the sensible course of action is to trace the circuit downstream from the fuse. Start with a careful visual inspection. If it is a printed circuit, see if there is a partial solder bridge between any two adjacent traces. In addition, some sort of conductive debris or metal filings could be the problem. A minute fracture in the circuit board can fill with dirt that may suddenly become conductive if there is the slightest amount of moisture. These problems become more acute at higher voltages. Ohm measurements are helpful.

Alternatively, capacitors and resistors often appear discolored or distorted when they go bad. Test every downstream component before replacing the fuse. Above all, avoid the impulse to go to a higher ampere value for the fuse. This can endanger a single component or the piece of equipment, or even cause an electrical fire within the building.

The Code permits ungrounded systems in certain applications. Of course, even an ungrounded system has to have a green or bare equipment-grounding conductor connected to a ground electrode system complying with the full set of Code rules.

In troubleshooting stereo equipment beyond the power supply, a useful technique is to insert a signal into the front end. A CD output will do, but the ideal is a uniform audio tone, the same level being fed into both channels. A signal generator is excellent for this purpose, and some technicians are fortunate enough to have one of these in addition to an oscilloscope. For many, this is more an aspiration than a reality, and it is necessary to work with what is available or what can be fabricated. There are audio oscillators available for download from the Internet, and it is possible to put together a computer-based oscilloscope, especially if you have a laptop you are willing to dedicate for the purpose. However, this is not equivalent to having top of the line equipment, albeit much less expensive.

You can make dummy loads out of 4- and 8-ohm power resistors, so that an amplifier being worked on will not overload speakers that might be more costly than the amplifier. A bench power supply is great to have, but alternately you can put together something from the power supply of a discarded amplifier. The plan is to power up the amplifier gradually, so that if there is a short it will not come on full force.

Audio Fundamentals

If an amplifier had been working properly but abruptly developed a fault, it may suffer from a component that has experienced spontaneous premature failure, solely due to a manufacturing flaw. Semiconductors are prone to this syndrome, but it happens to capacitors also. Another cause of equipment failure is a poor solder joint. Insufficient heat or a bit of oxide during manufacture can result in a solder joint that works for a while because of pressure contact, but eventually fails because alloying of the metals did not take place. A bad solder joint can be difficult to detect, either visually or by means of an ohmmeter. If all else fails, remelt every joint on the board with a pencil-tip soldering iron, using heat sinks where needed to avoid damage to sensitive components, especially semiconductors. In the same spirit, pull apart and reconnect all ribbon connectors and slide on terminations as this can repolish corroded metal surfaces.

On the subject of audio equipment, we might as well take a look at the superheterodyne radio receiver. A receiver is an amplifier (often stereo because some FM transmitters offer two-channel stereo programming) combined with a tuner and radiofrequency (RF) amplifier.

These units are not normally repaired unless it is a superficial power supply or speaker problem, although some high-end receivers are valuable enough to justify a full-scale tear down and extensive parts replacement. Many receivers are combined with a stereo CD player, graphic equalizer, alphanumeric display, and other features. These units offer advantages in troubleshooting, if not repair. If the CD player produces beautiful sound but the receiver is silent, you can immediately eliminate large amounts of circuitry as being at fault.

Even if you do not foresee a situation where you will be servicing radios or TVs in the future, it is still instructive to know how these things work, and the knowledge will provide insight into other types of electronics.

The superheterodyne receiver was a remarkable invention with its origins in the early twentieth century, and today virtually all radios and TVs operate in this way. With separate RF, intermediate frequency (IF), and audio frequency (AF) amplification, these radios are highly amenable to the diagnostic procedures that we have been discussing.

A radio antenna produces an output in the microvolt range. This RF signal, after tuning by means of a resonant circuit and detection by diode action, is barely audible without amplification through headphones. At the transmitter, the RF carrier wave is modulated by an audio signal. The process may be amplitude modulation, frequency modulation, or phase modulation. At the receiver, this modulated signal is amplified in successive stages, the output of the first stage becoming the input of the second, and so on.

Then, through a process of detection and filtering, which resembles rectification in a power circuit, the RF component is removed, leaving an AF that is intelligible to humans.

That signal, in turn, is amplified until it is strong enough to drive a speaker. This generic radio worked after a fashion, but had some severe drawbacks. For one thing, many stages of amplification were required, which translated into the need for many tubes, at that time quite expensive. Moreover, the signal was not very stable; whistles and noise accompanied the final product.

About the time of World War I and in response to allied military needs, a new product was developed, known eventually as the superheterodyne. It used a technique of mixing frequencies to produce a more usable resultant. If two musical tones, ideally pure sine waves, are present in the same air space, they will mix, their own identities intact but with a portion of the energy going into two additional tones. These frequencies are equal to the difference and sum of the original frequencies. The sound made by the difference of two tones that are close together is a very noticeable low-frequency "beat," which you have probably heard. The same thing happens at much higher frequencies, electrically, when radio frequencies are mixed. The resulting IF, still modulated, is more amenable to amplification for several reasons, as we shall see.

The earliest arrangement was for the transmitter to broadcast both the modulated signal and an oscillator-generated RF signal. The two could be combined to produce the desired IF. Soon a superior protocol emerged whereby the required frequency was created locally, meaning within each receiver. This was accomplished easily--an oscillator is just an amplifier that is biased so that its output becomes a pure sine wave. You may have noticed that radios with knob-controlled variable capacitors have two of the components attached to a single shaft. One of them tunes the modulated RF while the other varies the locally generated oscillator output. When the two combine, a single unchanging IF is produced, and it is this signal that is amplified prior to demodulation, or detection as it is known. The advantages of a single IF as opposed to the varying RF for amplification are twofold. For one thing, the frequency is lower than the RF as broadcast by the transmitter, so there is not as great a battle against unwanted capacitance and attendant losses. This means fewer stages are needed. Second, since there is a single IF rather than the varying RF, resonant frequency filtering can be accomplished by a single set of components rather than one for each broadcast channel. Because of these inherent advantages, superheterodyne technology remains the dominant methodology at present, even in video reception.

What does this mean for troubleshooting? A signal can be injected at the input of the first IF stage and voltages or oscilloscope readings can be checked at the output of each stage. Some schematics show an oscilloscope graphic at various test points, and this facilitates locating the circuit and component that is at fault. At IF frequencies, the process is rather more manageable than in the varying high-frequency regions upstream.

When wiring a three-phase motor, it is important to balance the phases. The idea is that one leg of the supply will likely have a slightly higher voltage than the others do, and one leg of the load may draw a little more current than the others do. Hook up the motor with the desired rotation and then take clamp-on ammeter readings. Then roll the connections without reversing any two legs, which would reverse the rotation. Take readings for all three configurations and whichever set of readings is most uniform is the right hookup.

After connections are finished in the motor terminal box, the wiring has been stuffed in, and the cover is screwed on, go back to an upstream access point such as the three-pole breaker in the panel, and with power off take ohm readings. The resistance in all three phases should be substantially equal, say 3.9 ohms, and there should be no resistance to ground at a high megohm range setting. That means wiring is correct, except that rotation may still be wrong. Often it is safe to try out the motor briefly to check rotation, but beware of some pumps, which can be instantly damaged (seal ruined) when run the wrong way.

Next on the agenda is the subject of video, which is several orders of magnitude more complex than simple amplifiers or radio receivers. This is especially true where color reception is concerned. While it is beyond the scope or intent of this guide to provide the reader with any significant knowledge or expertise in color TV servicing, we can describe some of the basic principles and indicate some resources that would be helpful, if that were your inclination. To become adept at this trade, a rather thorough knowledge of electronic theory is required, plus a large amount of TV-specific learning, and perhaps several years' experience, preferably working with an experienced technician who is willing to share information.

Electronics courses with a TV servicing slant vary widely in quality and cost, and the two appear to have little correlation. Of course, there is a lot of free information on the Internet. If you have a specific question regarding a project you are working on, type the make and model into a search engine and put your concern in the form of a question. Most likely, you will get a focused answer.

TV Repair Procedures

Television, as mentioned, is fairly complex. For a start, there are potentially lethal voltages that supply the picture tube, and anytime you go inside the cabinet, you have to be aware of that fact and protect yourself accordingly. It has to be emphasized that certain components and wiring remain energized at much higher levels than the 120/240 volt system that supplies the TV, even when it is turned off and unplugged. This is because capacitors, including the picture tube, which can function as a capacitor, have the ability to store a very high charge for a long time. Here are some methods of protection:

• At the bench, place a large, thick, dry rubber mat on the floor, so that when you work on equipment you are not grounded. You can still get a shock, but it will not be as severe. Your bench should be made of insulating material, that is, not metal, and there should be no grounded surfaces that can be inadvertently contacted, such as grounded metal light fixtures.

• Be aware of which parts within a TV carry high voltages. A clear indication is thick insulation and heavy rubber boots where the wires terminate. Do not touch them, even if you think they are de-energized. Use insulated tools, not your bare hands.

• Discharge all large electrolytic capacitors and high-voltage wiring by shunting with a power resistor that is known to be good. After you think the voltage is down, shunt them out with an insulated copper wire, just to be sure. Do not forget that this process has to be repeated each time the equipment is fired up.

• If it is necessary to take meter readings on a high-voltage circuit, de-energize and discharge everything, and then connect the meter using alligator clips, as opposed to touching the live terminals with hand-held probes-that is a recipe for disaster.

• If you get a significant electrical shock, seek medical attention. An individual can be shocked and feel fine afterward, only to expire several hours later due to heart damage.

These cautionary notes apply to all electrical work, but especially for television servicing because of the presence of the high voltage necessary to deflect the beam of electrons within the picture tube. Generally, the bigger the picture tube, the higher the voltage.

The bench should be spacious and uncluttered, if you are to work safely and efficiently.

When the chassis is removed from the cabinet, the power cord normally becomes disconnected. Technicians who use a cheater cord to override this safety feature should be knowledgeable about the hazards involved and capable of protecting themselves and others from contact with high voltage. Do not leave a live, undischarged chassis unattended. A bench-mounted mirror is helpful so that you can work at the back of the set while observing the screen.

Some problems are temperature dependent. If a fault consistently surfaces some time after the TV is powered up, it is possible that a component has a temperature-dependent fault, such as a minute crack that opens up. To locate the faulty component, Cold Spray or Circuit Chiller works quite well. After the fault appears, direct the spray very briefly at suspect components, one at a time, to see when the fault goes away. This is not a fix, just a diagnostic technique. Similarly, a heat gun with a small metal funnel attached to direct the airflow can bring out a quiescent defect.

On the wall above your bench, you can mount a 4×4 surface box with two receptacles wired in series, preferably on a dedicated circuit. A plug-in bulb socket with an appliance bulb can be inserted into one of these receptacles and it will serve to limit the current available at the other receptacle, protecting equipment that has a dead short. The lower the bulb wattage, the more the current is limited, that is, the greater the protection. An appliance bulb is a good choice for a start. Also in series, at a convenient location so you can hit it quickly, install a single-pole toggle switch, or a normally open push button.

An isolation transformer is valuable. Remember that grounding does not pass through a transformer, so use of this device will temporarily remove the ground connection from a TV chassis. You can fabricate an isolation transformer by connecting the secondaries of two identical power transformers from discarded TVs. Choose small TVs that have transformers with clearly marked ratings so that you do not get involved with excessively high voltages, and carefully insulate the wiring.