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Before you start repairing electronics, get clear on one important fact: as soon as you crack open a product's case, you have left the government-regulated, "I'll sue you if this thing hurts me," coddled, protected world of consumer electronics behind.
Once the cover comes off, you are on your own, and you can get hurt or killed if you're not careful! You've probably heard many times how dangerous CRT TV sets are to service, but don't fool yourself into thinking that today's gear is all that much safer.
Even some battery-operated devices step up the voltage enough to zap the living crud out of you.
That said, you can learn to navigate all kinds of repairs safely. Let's look at a few ways you can get injured and how to avoid it, followed by the inverse: how you can damage the product you're trying to service.
This is the most obvious hazard and the easiest to let happen. It might seem simple to avoid touching live connection points, but such contact happens all the time, because the insides of products are not designed for safety. Remember, you're not supposed to be in there! You may find completely bare, unprotected spots harboring dangerous voltage, and a slip of the tool can be serious.
Remove your wristwatch and jewelry before slipping your hand into a live electronic product. Yes, even a battery-operated one. Take off the wedding ring, too.
They don't call metal contact points terminals for nothing! In most devices, the electrical reference point called circuit ground is its metal chassis and/or metal shields. This is where old electrons go to die after having done their work, wending their way through the various components to get there. The trick is not to let them take you along for the ride! If you are in contact with the circuit ground point and also a point at a voltage higher than about 40 or 50 volts, you will get shocked. If your hands are moist, even lower voltages can zap you. The bodily harm from a shock arises from the current (number of electrons) passing through you, more than the voltage (their kick) itself. The higher the kick, though, the more electrons it forces through your body's resistance, which is why voltage matters. The path through your body is important as well, with the most dangerous being from hand to hand, because the current will flow across your chest and through your heart.
That, of course, is one electrically regulated muscle whose rhythm you don't want to interrupt. So, it's prudent to keep your hand away from the circuit ground when taking measurements, just in case your other hand touches some significant voltage.
In the old CRT TV service days, techs lived by the "one hand rule," keeping one hand behind their backs while probing for signals in a powered set. Also, don't service electronics while you're barefoot or wearing socks; you're more likely to be grounded, offering a path through your body for wayward electrons. Always wear shoes.
Switching power supplies (see Section 14) have part of their circuitry directly connected to the AC line. As I mentioned in Section 2 on the section about isolation transformers, that's a very dangerous thing, because lots of items around you in the room represent lovely ground points to which those unisolated electrons are just dying to go, and they don't mind going through you to get there. Once again, never work on circuitry while it’s directly connected to the AC line. If there's no transformer between the AC line and the part of the circuit you wish to investigate, it's directly connected. Unplug it from the line even before connecting your scope's ground clip, because where you clip it may be at 120 volts or more, which will flow through the scope's chassis on its way to the instrument's ground connection, blowing fuses and possibly wrecking your scope.
Lots of AC-operated products have exposed power supplies, with no protection at all over the fuse and other items directly connected to the AC line. Touching one of those parts is no different than sticking a screwdriver in a wall socket. It's all too easy for the back of your hand to grant you a nasty surprise while your fingers and attention are aimed elsewhere. Even if the shock isn't serious (which it could be), you'll instinctively jerk your hand away and probably get cut on the machine's chassis. When probing in a device with an exposed supply, place something nonconductive over the board when you're not working on the supply itself. I like to use a piece of soft vinyl cut from the cover of a school notebook.
Capacitors, especially large electrolytics, can store a serious amount of energy long after power has been removed. I've seen some that were still fully charged weeks later, though many circuits will bleed their energy off within seconds or minutes. The only way to be sure a cap is discharged is to discharge it yourself. Never do this by directly shorting its terminals! The current can be in the hundreds of amps, generating a huge spark and sometimes even welding your tool or wire to the terminals. Worse, that fast, furious flow can induce a gigantic current spike into the device's circuitry, silently destroying transistors and chips. Instead, connect a 10-ohm resistor rated at a watt or two to a couple of clip leads, and clip them across the terminals to discharge the cap a little more slowly. Keep them connected for 20 seconds, and then remove one and measure across the cap with your DMM set to read DC voltage. It should read zero or close to it. If not, apply the resistor again until it does.
Before discharging a cap, look at its voltage rating, because the voltage on it will always be less than the rating. If a cap is rated at 16 volts, it isn't going to be dangerous. If it's rated at 150 volts, watch out. Even with the low-voltage part, you may want to discharge it before soldering or desoldering other components, to avoid causing momentary shorts that permit the cap's stored energy to flow into places it doesn't belong. Most of the time, low-voltage caps are in parts of circuits that cause the capacitors to discharge pretty rapidly once power is turned off, but not always.
The capacitance value tells you how much energy the capacitor can store. A 0.1 µf cap can't store enough to cause you harm unless it's charged to a high voltage, but when you have tens, hundreds or thousands of µf, the potential for an electrifying experience is considerable at the lower voltages you're more likely to encounter.
CRTs, especially in color TV sets, act like capacitors and have low enough leakage to store the high voltage applied to their anodes (the hole in the side with the rubber cap and the thick wire coming from it) for months. There isn't much capacitance, thus not a lot of current, but the voltage is so high (anywhere from 25,000 to 50,000 volts!) that what there is will go through you fast and hard enough to cause a large, sad family gathering about a week later. CRTs are going the way of the dodo, so you probably won't work with them anyway. Just be very, very careful if you do. The terminals at the back of the tube carry some pretty high voltages too.
The backlighting circuits of LCD monitors and TVs, along with much of the circuitry of plasma TVs, operate at high enough voltages to be treated with respect.
It's unwise to try to measure the output of a running backlighting circuit at the point where it connects to the fluorescent lamp tube unless you have a high-voltage probe made for that kind of work. Without one, you may get shocked from the voltage exceeding the breakdown rating of your probe, you're likely to damage your DMM or scope, and the added load also may blow the backlighting circuit's output transistors.
Speaking of lamps, the high-pressure mercury vapor arc lamps used in video projectors are "struck," or started, by putting around a kilovolt on them until they arc over, after which the voltage is reduced to about 100 volts. Keep clear of their connections during the striking period, and don't try to measure that start-up voltage.
The outsides of products are carefully designed to be user-safe. Not so the insides! It's easy to get sliced by component leads sticking up from solder joints, by the edges of metal shields, and even by plastic parts. Move deliberately and carefully; quickly shoving a hand into nooks and crannies leads to cuts, bleeding and cursing. That said, it still happens often enough that my years of tech work led me to coin the phrase, "No job is complete without a minor injury."
CD and DVD players and recorders (especially recorders) put out enough laser energy to harm your eyes, should you look into the beam. Video projectors use lamps so bright that you will seriously damage your vision by looking directly at them. The lamps and their housings get more than hot enough to burn you, and hot projection lamps are very fragile, so don't bounce the unit or hit anything against it while it's operating. An exploding lamp goes off like a little firecracker, oh-so-expensively showering you with fine glass particles and a little mercury, just for extra effect.
Speaking of eyes, yours will often be at rather close range to the work. Much of the time, you'll be wearing magnifying lenses offering some protection from flying bits of wire or splattered molten solder. When the magnifiers aren't in use, it's a good idea to wear goggles, especially if you don't wear glasses. Excess component leads clipped with diagonal cutters have an odd, almost magnetic tendency to head straight for your corneas at high speed. Solder smoke also likes to visit the area, and it can be pretty irritating.
You can hurt your ears, too, particularly when working on audio amplifiers with speakers connected. Touching the wrong spot may produce a burst of hum or a squeal loud enough to do damage when your ears are close to the speakers. That sort of thing happens mostly with musical instrument amplifiers, because their speakers are right in your face when you work on them, and those amps pack quite a wallop. Even a 15 watt guitar amp can get painfully loud up close. Don't think turning the volume knob down will protect you; there are plenty of places you can touch that will produce full power output regardless of the volume control's position.
Other opportunities for hearing damage involve using headphones to test malfunctioning audio gear. Even a little MP3 player with just a few milliwatts (thousandths of a watt) of output power can pump punishingly loud noises into your ears, particularly when ear buds that fit into the ear canals are used. If you must wear headphones to test a device, always use the over-the-ear type, and pull them back so they rest on the backs of your earlobes. That way, you can hear what's going on, but unexpected loud noises won't blast your eardrums.
Breathing in solder smoke, contact cleaner spray and other service chemicals isn't the healthiest activity. Keep your face away when spraying. When you must get close while soldering, holding your breath before the smoke rises can help you avoid inhalation.
Sure, electronics can hurt you, but you can hurt the equipment too. Today's devices are generally more delicate than those of past decades. It was pretty hard to damage a vacuum tube circuit with anything short of a dropped wrench hitting the glass.
Today's ultra-miniaturized circuitry is an entirely different slab of silicon. Here are some ways you can make a mess of your intended repair.
Working with powered circuits is essential in many repair jobs. You can't scope signals when they're off! Poking around in devices with power applied, though, presents great opportunity to cause a short, sending voltages to the wrong places and blowing semiconductors, many of which cannot withstand out-of-range voltages or currents for more than a fraction of a second. One of the easiest ways to trash a circuit is to press a probe against a solder pad on the board, only to have it slip off the curved surface when you look up at your test instrument, and wind up touching two pads at the same time.
Any time you stick a probe on a solder pad, be very aware of this potential slip. At some point, it'll happen anyway, I promise you. Luckily, many times it causes no harm. Alas, sometimes the results are disastrous. If you experience this oops-atronic event and the circuit's behavior suddenly changes, and toggling the power doesn't restore it to its previous state, assume you did some damage.
Another common probing problem occurs when a scope probe is too large for where you're trying to poke it, and its ground ring, which is only a few millimeters from the tip, touches a pad on the board, shorting it to ground. Again, sometimes you get away with it, sometimes you don't. In small-signal circuit stages, it's more likely to be harmless. In a power supply, well, you don't want to do it, okay? It can be helpful in tight circumstances to cut a small square of electrical tape and poke the end of the probe through it, thus insulating the ground ring.
When operating a unit with your bench power supply, there are several things you can get wrong that will wreck the product. First and foremost, don't connect positive and negative backward! Nothing pops IC chips faster than reversed polarity.
Products subjected to it are often damaged beyond repair.
Some devices, especially those intended for automotive use, have reverse-polarity protection diodes across their DC power inputs. The diode, deliberately connected backward with its anode to - and its cathode to +, doesn't conduct as long as the power is correctly applied. When polarity is reversed, the diode conducts, effectively shorting out the power input and usually blowing the power supply's fuse, protecting the product's sensitive transistors and chips from backward current. If the power supply has a lot of current available, the diode may rapidly overheat and short, requiring its replacement, but the rest of the unit should remain unharmed.
Few battery-operated products have protection diodes. Reverse-polarity protection is usually accomplished mechanically in the battery compartment by a recessed terminal design that prevents the battery's flat negative terminal from touching the positive contact. The AC adapter jack probably isn't protected either, because it's assumed you will use the adapter that came with the product.
So, be very careful to connect your power supply the right way around, and never hook it up while power is turned on, lest you even momentarily touch the terminals with your clips reversed.
Be sure you've set your power supply's voltage correctly, too. Undervoltage rarely causes damage, but overvoltage is likely to do so if it's applied for more than a few seconds. We're not talking millivolts here; if you're within half a volt or so, that's usually good enough. A decade ago, most items ran on unregulated linear-type adapters and did the regulating internally, so they were fairly tolerant of having excessive voltage coming in, and you were fine if you were within 2 or 3 volts. These days, more and more products are using regulated, switching-type AC adapters with very steady voltage outputs, so the gadgets expect a pretty accurate voltage.
It's possible to cause electrical damage when taking measurements, even if you don't slip with the probe. While scopes and meters have high impedance inputs that won't present any significant load to most circuits, sometimes you may be tempted to connect resistors or capacitors across points to gauge the effects. That can be an effective diagnostic technique in some cases, but it should be used with caution because you can pull too much current through some other component and blow it.
Other times, you may want to connect a voltage to a point to see if it restores operation. That, too, can be useful, but it requires consideration of the correct voltage and polarity, the amount of current required, and exactly where that energy will go.
Get one of those things wrong, and you could let some of that magic smoke out of the unit's components, with predictable consequences.
When your scope is set to AC coupling (see Section 6), it inserts a capacitor between the probe and the rest of the scope. After you probe a point with a DC voltage on it, that voltage remains on the capacitor and will be discharged back through the probe into the next point you touch. The amount of current is very, very small, but if you touch a connection to an especially sensitive IC chip or transistor that can't handle the stored voltage, you could destroy the part in the short time it takes to discharge the scope's capacitor. When using AC coupling, touch the probe to circuit ground between measurements to discharge the cap and prevent damage to delicate components.
A static charge from rubbing your shoes across the carpet, or just the dry air of winter, can put hundreds of volts, or even a few thousand, on your fingertips and any tools you're holding. If you think you could be charged, and especially any time you handle CMOS chips, MOSFET transistors, memory cards or other sensitive semiconductors, touch a grounded object first. The metal case of your bench power supply or analog scope should do the trick, as long as it's plugged into a three-wire outlet. I don't recommend using a digital scope as a discharge point because it has plenty of sensitive chips inside, and you sure don't want to damage those!
There are lots of ways you can break things when you're inside a machine. One of the easiest is to tear a ribbon cable or snap off a critical part while disassembling the device. Some products pop apart easily but may have hidden risks. I once serviced a video projector cleverly designed to snap open without a single screw, but a tiny ribbon cable linked the top and bottom, and I was lucky that it popped out of its connector without being torn in half when I removed the top case. Had I pulled a little harder, I'd probably have done damage difficult to repair. As careful as I was trying to be, I still didn't see that darned ribbon until it was too late.
Small connectors of the sorts used on laptop motherboards and pocket camcorders can be torn from the circuit board. Today's products are soldered by machine, and the soldering to connectors isn't always the greatest, because they are a bit larger than the components, so they don't get quite as warm during soldering. A little too much pressure when you disconnect the cable, and the connector can come right off the board. Depending on the size scale of its contacts, it may be impossible to re-solder it. Most ribbon connectors have a release latch you must flip up or pull out before removing the ribbon. Always look for it before pulling on the cable. See Figures 1 and 2.
Your soldering iron, that magic instrument of thermo-healing, can also do a lot of damage, especially to plastic. The sides of the heating element can easily press against plastic cabinetry when you have to solder in tight places, melting it and ruining the unit's cosmetics.
Finally, be careful where you press your fingers. Most circuitry is fairly hardy, but some components, including video heads, meters, speaker cones, microphone diaphragms, DLP projector color wheels and CD/DVD laser optical heads, just can't stand much stress and will break if you push on them even with moderate force.
You Fixed It! Is It Safe?
After repair, it's your duty as a diligent device doctor to ensure the product is safe to use. One common error resulting in an unsafe repair job is neglecting to put everything back the way it was. If you have internal shields and covers or other items left over after you close the unit, you'll need to open it back up and put them where they belong.
Manufacturers don't waste a single penny on unnecessary parts, so you know they're important!
It's easy to touch wiring and melt insulation with the side of your soldering iron while you're concentrating on soldering components. In units with lots of wires, it can happen, despite your best intention to be careful. You might not notice doing it if you're focusing on the action at the tip, but the smoke and smell will alert you. Should you do this, fix the damage immediately; don't wait until the repair is over. For one thing, you may have created a short or a lack of insulation that could cause damage or injury when power is applied. For another, you might forget later and close the unit up in that condition.
Patching melted insulation can be as easy as re-melting it to cover the wire, in the case of low-voltage, signal-carrying wires with only small melted spots. Or it might require cutting, splicing and heat-shrink tubing if the wire handles serious voltage, or if the damage is too great. Remember, electrical tape will come off after awhile, so never depend on it for long-term safety.
If you've replaced power-handling components like output transistors or voltage regulators, be sure to test the unit for proper operation and excess heat. Older stereo amps and receivers, for instance, sometimes require bias adjustments when the output transistors are changed. If you don't set the bias correctly, the unit will work for awhile, but it may overheat badly. Let it run on the bench for a few hours at normal listening volume and see how hot it gets. Be sure what you've fixed is really working properly before you close it up.
When the product has an AC cord, take a look at it and run your hand along its entire length, checking for cuts. Naturally, you want to live, so unplug it before doing this! You'll be amazed at how many frayed, cut and pet-chewed cords are out there.
Replace the cord or repair it as seems appropriate for its condition, paying extra attention to a good, clean job with proper insulation. If the damage is only to one wire, it's easier to fix than if both sides are involved, because at least the two wires can't short to each other. With a damaged AC cord, I like to use both electrical tape and heat-shrink tubing over it.