Diagnose and Repair Modern Electronics: A DIY Guide: Philosophy of Troubleshooting (part 2)

<< cont. from part 1

Physical Stress

Chips, transistors, resistors and capacitors can take the physical shock of being dropped, at least most of the time. Many other parts can't, though. Circuit boards can crack, especially near the edges and around screw holes and other support points. Larger parts, with their greater mass, can break the board areas around them. That happens often with transformers and big capacitors. On a single or dual-layer board, you may be able to bridge foil traces over the crack with small pieces of wire and a little solder if the traces are not too small. With a multilayer board, you may as well toss the machine on the parts pile, because it's toast.

LCDs, fluorescent tubes and other glass displays rarely survive a drop to a hard surface. The very thin, long fluorescent lamps inside laptop screens are particularly vulnerable to breakage. If you run across a laptop with no backlight, don't be too surprised if it got dropped and the lamps are broken inside the screen. I've seen that happen with no damage to the LCD itself being evident.

If you leave carbon-zinc or alkaline batteries installed long enough, they will leak.

Not maybe, not sometimes, they will. Devices like digital cameras take a fair amount of current and get their batteries changed often, but those with low current demand, such as digital clocks and some kids' toys, may have the same batteries left in them for years. Remote controls are prime candidates for battery leakage damage, because most people install the cheap, low-quality batteries that come with them and never change them; their very low usage ensures those junky cells will be in there until they rot.

Once the goo comes out, you're in for a lot of work cleaning up the mess. They don't call them alkaline batteries for nothing! The electrolyte is quite corrosive and will eat the unit's battery springs and contacts. If the stuff gets inside and onto the circuit board, that's where the bigger calamity goes down. Copper traces will be eaten through, solder pads corroded, and those pesky circuit board layer interconnects will stop working. No shop will try to repair such damage, but you might want to give it a go if the device is expensive or hard to replace.

People sit on their phones, PDAs and digital cameras fairly often, resulting in cracked LCDs, broken circuit boards and flattened metal cases shorting components to circuit ground. It's easy to bend a case back to an approximation of its original shape, but the mess inside may not be worth the trouble.

Liquid and electronics don't mix, yet people try to combine them all the time, spilling coffee, wine and soft drinks into their laptops, and dropping their cameras and phones into the ocean and swimming pools. Good luck trying to save such items. Now and then you can wash them out with distilled water, let them dry for a long time, and wind up with a functional product. Most of the time, and especially with salt-water intrusion, it's a total loss.

Just being near salt water will destroy electronics after awhile. Two-way radios, navigation systems, stereos and TVs kept on a boat or even in a seaside apartment get badly corroded inside, with rusted chassis; dull, damaged solder joints; and connectors that don't pass current. Very often you'll see crusty green crud all over everything.

Speaking of the ocean, the beach is a prime killing ground for cameras. Most digital cameras feature lenses that extend when the camera is powered on. Any sand in the cracks between lens sections will work its way into the extending mechanism and freeze that baby up, and it’s very hard to get all the grit out. In a typical case, the camera is dropped lens-first into the sand, and a great deal of it gets inside. I've taken a few apart and disassembled the lens assemblies, cleaned half a beach out of them, and still had little luck restoring their operation. There's always a few grains of sand somewhere deep in those nylon gears, where you can't find them, and even one grain can stop the whole works.

The Great Capacitor Scandal

Around 1990, a worker at an Asian capacitor plant stole the company's formula, fled to Taiwan, and opened his own manufacturing plant, cranking out millions of surface mount electrolytic capacitors that found their way into countless consumer products from the major makers we all know and love. A few other Taiwanese capacitor makers copied the formula too.

Alas, that formula contained an error that caused the electrolyte in those caps to break down and release hydrogen. Over a few years, the caps swelled and burst their rubber seals, releasing corrosive electrolyte onto the products' circuit boards, severely damaging them and ruining the units.

This ugly little secret didn't become well known for quite awhile, until long after the warranty periods were expired. Billions of dollars' worth of camcorders and other costly small products were lost, all at their owners' expense. Any attempt at having repairs made was met with a diagnosis of "liquid damage-unrepairable." The disaster was so pervasive, and took long enough to show up, that many companies insisted the failures were random and have never to this day admitted any liability for the lost value.

More recently, similar electrolyte problems have continued to plague computer motherboards and the power supplies of various products, affecting even their full-sized capacitors with leads. Caps are dying after just a year or two of use. The higher heat of lead-free soldering also may be contributing to early capacitor failure.

Lawsuits have been filed, and remedial action has been taken by some manufacturers to purge their product lines of the offending parts. Still, it’s highly likely you will run into bulging capacitors in your repair work, perhaps more than any other single cause of failure. Even when they're not bulging, the caps may lose their ability to store energy, showing almost no capacitance on a capacitance meter.

History Lessons

A good doctor understands the value of taking the patient's history before performing an examination. Knowing the factors leading up to the complaint can be very valuable in assessing the cause. How old are you? Do you smoke? Drink? Have a family history of this illness? What were you doing when symptoms appeared? If you have access to a machine's history, it can provide the same kinds of helpful hints, often leading you to a preliminary diagnosis before you even try to turn it on.

Here are some factors worth considering before the initial evaluation:

Who made it? As discussed earlier, products from specific companies can have frequently occurring problems due to design and manufacturing philosophy.

Becoming familiar with those differences may help guide you to likely issues, especially if you've seen the problem before in another unit, even of a different model, from the same maker.

It pays to check the Internet for reports of similar troubles with the same model product. You may save many hours of wheel reinvention by discovering that others are complaining about the same failure. You might find the cure, too.

How old is it? If made before the 1990s, it shouldn't have the leaking capacitor problem. It could have a lot of wear, though, with breakdowns related to plenty of hours of use. If it was made in the '90s or more recently, those caps are a prime suspect.

Has it been abused? Dropped? Dunked? Spilled into? Sat on? Left on the dashboard of a car in the summer? Used at the beach? Had batteries in it for months or years? Had a disc or tape stuck in it, and somebody tried to tear it out? Been in a thunderstorm? Through the washing machine? Kept on a boat? Played with by kids? Cranked up at maximum volume in a club for long periods?

Each of these conditions can lead you down the diagnosis path. A stereo amplifier used gently at home by 70-year-olds is likely to have a very different failure than one cranked up to high volume levels in a club, or one run 40 hours a week in a restaurant for 10 years.

What was it doing when it failed? While gadgets sometimes quit while in operation, many stop working when sitting idle, and the problem isn't discovered until the next time someone tries to use the product. This is particularly true of AC powered machines that, like most things today, have remote controls. To sense and interpret the turn-on signal from the remote, at least some of the circuitry has to be kept active at all times. VCRs, DVRs, DVD players and TVs are never truly turned off; some power always flows. A power surge, a quick spike, or perhaps just age or-as always-bad capacitors can kill the standby supply, resulting in complete loss of operation.

If the device did crash while being used, it's very helpful to know precisely what operation was being carried out when it quit. If a laptop's backlight went dead while the screen was being tilted, For example, that's a good indication of a broken internal cable, rather than a blown transistor in the backlight inverter.

Did it do something weird shortly before quitting? Many failing circuits exhibit odd operation for anywhere from minutes to seconds before they shut down altogether. That peculiar behavior can contain clues to the cause of their demise.

In fact, it usually does, and it may hold the only hints you have in cases of total loss of function.

Was it sudden or gradual? Some causes of failure, such as drifting alignment, dirty or worn mechanisms, and leaking or drying electrolytic capacitors, may manifest gradually over time. Bad caps on computer motherboards are a great example of this, as they cause the machine to get less and less stable, with more and more frequent crashes, until bootup is no longer possible.

Parts don't blow gradually, though. While it's possible in rare cases for components, and especially transistors, to exhibit intermittent bad behavior, a truly blown (open circuited) component goes suddenly and permanently, frequently shorting first and then opening a moment later from the heat of all the current passing through its short. So, if the symptoms appeared gradually, it's a safe bet that the problem is not blown parts.

Preliminary Evaluation

Before you take a unit apart, examine it externally and try to form a hypothesis describing its failure. The most potent paintbrush in the diagnostic art is simple logic.

Your first brushstroke should be to reduce variables and eliminate as many areas of the circuitry from consideration as you can. Instead of chasing what might be wrong, first focus on what the problem can't be. By doing so, you'll sidestep hours of signal tracing and frustration. Before you open the unit, give some thought to these issues.

It's dead, Jim! "Dead" is a word many people use when something doesn't work, but often it's incorrectly applied. If anything at all happens when you apply power, the thing isn't dead! A lit LED, a display with something-even something scrambled and meaningless-on it, a hum, a hiss, some warmth, or any activity whatsoever, indicates that the circuitry is getting some power from the power supply, at least. "Dead" means dead. Zip, nada, nothing, stone cold. If you do see signs of life, some power supply voltage could still be missing or far from its correct value, but the supply is less likely to be the problem. In a product with a switching supply, you can assume that the chopper transistor is good, as are the fuse and the bridge rectifier. You can't be sure the supply has no other problem like bad capacitors or poor voltage regulation.

If the device is totally dead, check the fuse. All AC-powered products have fuses, and so do most battery-operated gadgets, though their fuses may be tiny and soldered to the board. A blown fuse pretty much always means a short somewhere inside, so don't expect much merely by changing the fuse. Most likely, it'll blow again immediately. Still, give it a try, just in case. Be sure to use the same amperage rating for your new fuse; using a bigger one is asking for trouble in the form of excessive current draw and more cooked parts, and a smaller one may blow even if the circuit is working fine. And no matter how tempting it might be, don’t bypass the fuse, or you will almost certainly do much more damage to the circuitry than already exists. Those fuses are there for a reason, and that reason is protection.

Though nontechnical types tend to think that truly dead machines are the most badly damaged and least worth fixing, the opposite is usually true. Total loss of activity typically indicates a power supply failure or a shorted part that has blown the fuse. In other words, easy pickings. The really tricky cases are the ones where the thing almost works right, but not quite, or it works fine sometimes and malfunctions only if you turn it facing south during a full moon on a Tuesday. Those are the unruly beasts that may cause you to emit words you don't want your kids to hear.

If the product has a display, is there anything on it? Although a dead display can be caused by many things, the condition usually indicates that the microprocessor at the heart of the digital control system isn't running. Micros rarely fail, except in cases of electrical abuse like lightning strikes or severe static electricity. The most frequent reasons for a stopped microprocessor are lack of proper power supply or a clock crystal that isn't oscillating.

If the display is there but isn't normal, that's a sign that some other issue in the digital system is scrambling the data going to it. If it's a simple system in which the microprocessor directly drives the display, the micro still might be stopped or damaged. If there's a display driver chip between the micro and the LCD, it may be bad. When the unit responds to commands but has a scrambled display, the micro is probably okay. If everything is locked up, suspect the micro or its support circuitry.

Does it work when cold and then quit after it warms up? Thermal behavior can be caused by bad solder joints, flaky semiconductors and bad capacitors. It usually manifests as failure after warm-up, but now and then it's the other way around, with proper operation commencing only after the unit has been on for awhile. Again, the problem is not a blown part.

Does tapping on it affect its operation? If so, there's a poor connection somewhere.

Typically it's a cold solder joint or an oxidized connector. Cracked circuit board traces used to be fairly common, but they're quite unusual now, except in cases of physical abuse. Faulty conductive-glue layer interconnects can make boards tap-sensitive. On very rare occasions, the bad connection may be inside a transistor, and I once found one inside an intermediate frequency (IF) transformer in a radio receiver.

Eliminating variables If the device runs off an AC adapter, try substituting your bench power supply, being careful of polarity, as discussed in Section 3. If the unit can operate from batteries, put some in and see what happens. The remote control won't turn it on? Try using the front panel buttons. Even if these attempts don't restore operation, at least you'll know what isn't causing the trouble.

Speaking of remotes, they can go wild and emit continuous commands, driving the micro in the product out of its little silicon mind and locking out all other attempts at operation. The situation usually occurs when liquid has been spilled on the remote, causing one or more of the keys to short out. The remote thinks a key is being pressed and sends data ad infinitum. To be sure that isn't the problem, remove the batteries from the remote and see if the symptoms disappear.

Use Your Brain

Once you've tried these preliminary experiments, think logically about their results and you will probably have a pretty good sense of where to poke your scope probe first. Let's look at some real-life cases from beginning to end, and how this approach helped get me started in the right direction.

Hi-Fi Receiver

The unit was a fairly high-end stereo receiver with a dead left channel that nobody in the shop could bring back to life. Eventually they'd given up, and the set had languished on the shelf for two years by the time it and I met. The shop's owner handed it to me as an employment test. If I could fix that one, I was in. The smug look on his face told me I was in for a challenge.

I saw no evidence of obvious damage or abuse, so I hooked up a pair of speakers, connected a CD player for a signal source, and fired it up. My initial evaluation was that the power supply had to be okay because the right channel worked fine. The front panel lit up and the unit seemed to operate normally, other than its having a stubborn case of mono. I hooked a clip lead to the antenna terminal and tried FM reception, thinking that the trouble might be in the input switching circuitry feeding audio from the input jacks to the amplifier stages. Nope, FM sounded great, but still from only one channel.

There was no hum in either channel's output, so the power supply wasn't being bogged down by a short someplace. (A loudly humming channel with no audio is classically indicative of a shorted output transistor.) I plugged in headphones, because sometimes amplifiers with bad output stages can drive a little bit of distorted signal into headphones, though they can't power a speaker. I kept the cans off my ears, as always, just in case the thing blasted me with punishing volume. There was no difference this time; I couldn't hear a hint of audio from the bad channel, even with the balance control turned all the way to that side. It was as quiet as a mouse. A dead mouse.

I'd eliminated as many variables as I could, so it was time to open 'er up. Several techs had taken their best shots at the poor thing, and evidence of their endeavors was all over the inside. The output transistors had been changed and large components in the power supply resoldered. Other solder work indicated that resistors in and near the bad channel's output stage had been pulled and tested. The focus clearly had been toward the output stage, which very often dies in audio amps and is where most techs look first. It made sense, but it hadn't done any good this time.

Thanks to the working channel, I didn't head straight to the power supply. Since the other guys had replaced the output transistors, I didn't bother to check those either. Instead, I stuck my scope probe on the signal line feeding the output stage, and there was no audio signal. Thus, the trouble was farther back in the chain toward the input stages someplace, and everybody had been hunting in the wrong place! I looked closely at a few small-signal transistors and traced their connections between stages. Some amplifiers are capacitively coupled (there's a capacitor between each stage), while others are directly or resistively coupled. The direct and resistive styles are also called DC coupling, because the voltages on one stage get passed to the next. It's a tougher type of circuit to design, but it results in superior sound. So most good audio gear works that way, and I expected to see that kind of circuitry here.

I wasn't disappointed; this baby had resistors between stages, but no capacitors.

Thus, the DC voltage levels on one stage could affect those on the succeeding stages.

A little light was beginning to glow in the back of my mind, but I needed to take a few measurements before coming to any conclusions.

I went all the way back to the first stage, finding it by tracing the line from the input jack, through the selector switches and to the amplifier board. I had a known good channel to use as a reference, so I fed the same audio signal to both sides, using a "Y" adapter cable. Setting the scope for dual trace display and the same voltage range on both input channels, I compared the outputs from the receiver's first left and right channel stages. They looked identical. Same signal levels, same DC voltage.

I went to the next stage. The good channel showed 1-volt DC at that stage's output, while the bad side only had about 0.5 volts, with the same audio signal riding on both.

Hmmm…could such a small difference matter? Half a crummy volt? In a DC-coupled amplifier, you bet it could! Transistors need a "bias," which is a little bit of DC to keep them turned on, at their bases (input terminals), and not having a high enough bias will make them cut off, unable to pass any signal. I checked the next stage in the bad channel, and its output was dead, just a sad, flat line on my scope. Without proper bias, the stage was completely cut off. There was the trouble! But why? I went back to the stage with lower DC output and checked the voltages and signals on the transistor's other terminals. They matched those of the good channel.

Only the output was different. So, most likely, the transistor was just dropping too much voltage. In other words, a bad transistor. A whole 25 cents' worth of mysterious mischief that had stymied an entire shop, simply because it wasn't the usual problem.

I popped in a new transistor, and voilà! The entire channel came to life and worked perfectly. Just to be sure all was well, I checked the previously dead stage's output levels, and both the signal and DC level matched those of the good channel. Case closed. I got a few open-mouthed stares from the other techs over that repair, along with an offer of full-time employment at the shop. I decided not to work there, but the episode left me feeling like Sherlock Holmes solving a perplexing crime. All I needed was a pipe and an English accent. "Elementary, my dear Watt-son!" Silent Shortwave

A friend brought me this set after buying it for very little, knowing it didn't work but badly wanting it to, as he'd always longed for one of these models and they were hard to find. One of the better digitally tuned shortwave receivers, this portable radio had no reception at all. It wasn't dead, though; the display came up normally, and a little hiss came from the speaker. Logic brain, spring into action! Where should I start? The first thing I did was try the various bands. AM, nada. Shortwave, same. FM… hey, the FM worked! Sounded great. The FM band is at a much higher frequency and uses a different kind of signal than AM and shortwave (which is also AM), so all multiband radios have separate stages dedicated to FM reception, and clearly they were fine. The audio and some other stages are shared, though, so the working FM also confirmed that the power regulation, digital control and audio stages were all functioning. Thus, the trouble had to be in the RF (radio frequency) or IF (intermediate frequency) stages of the shortwave section, which also handled AM, or in the digital frequency synthesizer that controlled the tuning.

I discounted the frequency synthesizer because the FM worked. There still could have been trouble there, but it wasn't suspect number one. Let's see, the synthesizer generates the oscillator signal that mixes with the incoming radio signal from the antenna, resulting in the IF signal, which is then amplified by the IF stages. Then, in good radios like this set, that signal is mixed with yet another oscillator, this one of fixed frequency, to create a lower-frequency IF signal that passes through yet more amplifier stages before it’s demodulated into audio.

The trouble could have been anywhere along that chain, but experience reminded me to check that the fixed oscillator, called the second local oscillator, was actually running. Back in the '70s, when I'd worked in the service department of a large consumer electronics chain, tons of CB radios had come in with dead receivers, thanks to a bad batch of oscillator crystals. We'd change 'em and be done in a jiffy, fixing the units without even having to troubleshoot them, since they always had the same problem. I did so many of them that the issue of a dead second local became permanently embedded in my mind. I looked for this set's crystal and touched each end with my scope probe, checking for a nice sine wave of a few volts. Nothing. The oscillator was not running. Aha! Sometimes weak crystals can be jolted into operating by adding some capacitance to one end, increasing the voltage drop across the crystal because of the extra load and making it vibrate a little harder. So, I touched my finger to each lead of the crystal, one lead at a time, with another finger touching circuit ground via a metal shield, employing my hand as a capacitor. This was all very low-voltage, battery-operated stuff, and it was safe to do that. The first try, nothing. The second, wham! The radio sprang to life and the BBC boomed in loud and clear from thousands of miles away. I let go and silence filled the room again. Ah, a bad crystal, and this one would have to be ordered from Japan. I tried resoldering it, just in case it had a cold joint. No luck.

Then, glancing at my friend's glum expression of disappointment that a new crystal would have to be procured from halfway around the world-restoration of the radio would be months away-I decided to grab a magnifying glass and take a close look at the surrounding components. I spied a tiny surface-mount capacitor connected from one end of the crystal to ground, performing essentially the same function my finger had. The solder joint on that one looked awfully dull. I resoldered it and the radio starting playing its little heart out. "This is London calling. And now the news…." Cost: zero. Grin on elated friend's face: priceless.

The Broken Projector

How about a nice, high-resolution DLP video projector, with plenty of lamp life left, for $20? Sure, we'd all go for that, right? Oh, there's one small catch: it doesn't work! I snapped up this craigslist puppy because I knew from the history of its failure exactly what was wrong before I ever saw it. The owner told me that it had started turning itself off randomly and becoming difficult to turn on. Eventually it stopped responding altogether. Now what could possibly cause that? Obviously, it couldn't be a blown part. You guessed it: a classic electrolytic capacitor failure. I could picture just what it would look like with its bulging top. I figured it'd be at the output of the internal switching power supply, probably near the DC output end of the board.

Got it home, opened it up, and there was the cap, precisely as I'd pictured it, bulge and all. It was even where I'd expected it. I changed the part with an exact replacement I found on one of my scrap boards, a power supply from a computer.

Fired up the projector and she was good to go, with a sharp, bright picture.

While I gave it the bench test, I checked online and found numerous complaints of the same problem in this model, along with various lay diagnoses, including some wacky guesses and the correct answer. The design kept the power supply turned on at all times, stressing that particular cap and causing it to fail after a couple of years, regardless of how much use the projector got. I keep my unit unplugged when I'm not running it, so it should last for a long, long time.

How can you beat a $20 video projector? And that, gentle reader, is why repairing electronics is not just fun, it's incredibly economical.

Noisy DVD Player

This portable DVD player came from the carcass pile at a repair facility for which I worked part time. The machine, one of the better brands, had been a warranty claim, and nobody could fix the thing, so it had been replaced and kept for parts. With its 5-inch widescreen LCD, the player looked kinda cute, and it seemed a shame to cut it up. The shop's owner didn't care if I took it home, so I did. I had no idea what might be wrong with it, but the price was right.

It appeared intact, so I hooked up my bench supply and flipped on the juice. The screen lit up and the mechanism immediately started making a noise like a machine gun! I killed the power in a hurry, because I knew what that rapid-fire sound was.

Disc players use leaf switches to sense when the laser head has returned fully to the initial position at the inside of the disc, where it needs to go to begin the start-up sequence leading to disc playback. The "rat-a-tat" noise was a clear indication that the micro didn't know the mechanical limit had been reached. The unit was cranking its sled motor indefinitely, grinding the nylon gears against each other until they slipped, over and over. I could just imagine the toothless mess it might make of those delicate plastic parts if I let it run for very long. Yikes.

On opening the player up, I looked for the typical leaf switch assembly and couldn't find one! Did this model use optical sensing? There was no trace of that either. I gently turned the sled motor's gear and moved the head away from the starting position, but I still couldn't see a switch. Finally, I removed the entire spindle assembly, and there it was, a tiny leaf switch hidden underneath the disc motor. It looked fine, though. Why wasn't it being tripped? Or maybe it was, and its signal wasn't getting back to the micro for some reason. Or perhaps the micro was bad….

I forced myself off the trail of wild imagination and back onto a path of pursuit.

The simplest explanation was that the switch must not be getting pressed far enough to work. I disconnected one wire from the leaf switch and connected my DMM across it, watching for the resistance to change from infinite (an open circuit) to near-zero (a closed one) as I slowly turned the gear to move the head back toward the switch.

The head hit its mechanical limit and would go no farther, but the switch never closed. That was the problem, all right.

After moving the head away again, I could see why, and it was so silly that I couldn't imagine why nobody had caught it. The little metal arm on the laser head that pressed on the switch was bent-not a lot, but just enough to keep it from pushing the leaf far enough to contact its mate. I bent the arm back ever so slightly, and I had a DVD player! Almost. Alas, the disc spindle assembly's three mounting screws also worked to set the disc alignment perpendicular to the optical head as it traversed the radius of the disc, and I'd had to unscrew them to remove the spindle. Any significant tilt would cause the reflected laser beam to miss the center of the head's lens, resulting in poor tracking and skipping. And, with the alignment scrambled, it did. I found the proper test point to use for observing the head's output signal (we'll explore how to do that in Section 14), scoped it and redid the alignment, carefully adjusting those three screws until I got a good signal no matter what part of the disc I played. Making me mess up a critical alignment to reach the leaf switch-talk about poor design! I won't mention the manufacturer's name, but I'd seen flimsy metal parts in some of their other products, so it wasn't terribly surprising to find one here too. This particular player went on to develop a baffling, chronic problem with the ribbon cable going to its disc motor, causing it to fail to spin the disc fast enough, resulting in an error message and no playback. I kept cleaning the ribbon's contacts and reseating the connector at the circuit board end, and it would work for a few months before failing again. Finally, I checked the other end of the cable, which had looked okay, and that was the real trouble; I'd just been wiggling it a little while working on the wrong end, and the movement had helped its connection for a short time. I cleaned and reseated that end, and the unit works to this day. Another mystery solved, another lesson learned in never assuming anything, and another fun freebie.