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Do-It-Yourself SERVICING So you would like to service electronic gear for yourself and your friends. Provided you have some basic test equipment such as a multimeter and oscilloscope, there is a surprising range of electronic equipment than can be repaired, provided you have the time and persistence to do it. This article gives some useful hints and tips. B y D AV I D R E I D E LECTRONICS IS A BLACK art to the uninitiated. They marvel at all those colourful little components and wonder how on earth we “experts” can navigate that secret world armed only with a schematic diagram – which to them is somewhat akin to a magician’s grimoire written in a long-dead language. That is why, upon seeing that you own a soldering iron and multimeter, your next-door neighbour may disappear briefly, only to return with his TV, DVD player, lawnmower or quite possibly, his wife’s pacemaker. “I don’t suppose you’d mind having a look . . .” You either decline and explain that it’s not really your field and that it should be seen to by an expert or you spend a couple of hours poking around inside, after which you say “look, it’s not really my field and it should be seen to by an expert”. In reality though, many fault-finding techniques are really just variations on common sense. You might be surprised at just how many repairs you can successfully complete. Play it safe Electrolytic capacitors degrade with age and heat. Inspect them carefully for signs of leakage and bulges in the case (especially the top). 42  Silicon Chip Get a portable Residual Current Device circuit breaker and “don’t leave home without it”. Never assume that you don’t need one or assume that it will magically protect you from every eventuality. It won’t. Never dismantle any unit with the 240VAC mains still connected. It is too easy to inadvertently touch exposed circuitry with the metal lid or drop a screw inside. Similarly, never power up a dismantled unit unless you are absolutely certain of the voltage distribution inside. Some manufacturers physically isolate mains and low-voltage circuitry siliconchip.com.au but it is common to find a single PC board with everything mounted on it, including the mains transformer. Tape exposed mains connections and never make assumptions. Treat every power supply as if every component were live, as there are many supplies where this is the case. If you do not find a decent-sized mains transformer in the unit, be especially suspicious, as you will probably be dealing with a switchmode power supply. In the case of colour television sets, they frequently have a live chassis. This means that it is not safe to touch any part of the electronics, including anything that looks like an earth, while the set is operating. If you feel uncertain as to the nature of a power supply, limit yourself to passive tests and measurements with the power off. Watch very carefully for high-voltage electrolytic capacitors. These are common in switchmode power supplies and 300V of DC can easily blow the tip off your favourite screwdriver! Finally, don’t work alone but ask for children to be removed from your vicinity, as they can be a dangerous distraction. Video heads are very brittle, and can be snapped by a cotton bud. Use a clean, flat piece of card moistened with alcohol. Initial observations Many faults are intermittent or become intermittent as soon as a technician enters the room, so keep your screwdriver holstered while you determine what the symptoms are. Check every function on the unit, as many faults have multiple symptoms which collectively point directly at the faulty circuit. Consider a DVD player, for example In spite of the owner’s claim that it “isn’t working”, a thorough check may reveal that the disc tray opens and closes, that the display is illuminated and that it seems to be spinning up and playing. As it turns out, most of it works and the actual fault is that there is no audio. When you dismantle the unit, you may find that the fault is gone but because you have done some preliminary work, you will have an idea where to look and it will later come as no great surprise if you discover a muting fault or a hairline fracture of the PC board next to the line output sockets. You repair the crack, hand the unit back and all that remains is to congratulate yourself on how terribly clever you are. siliconchip.com.au Don’t be tempted into thinking that you can adjust the fault out of a unit by playing with trimpots. It’s highly unlikely and can render many appliances inoperable. Sometimes, this initial analysis can save you the grief of attempting a dead-end repair. You have a friend who wants his beloved tape deck repaired. The fault is similar: no sound. However, you noticed that the meters are showing a signal on playback, so you very cleverly grab some headphones. Yes, there is audio. However, the speed is all over the place, one of the meter lamps is gone and the heads have a groove in which you could park a bicycle. Can you get the parts required? How badly does he want it fixed? Is he willing to pay what it’s likely to cost? If you are serious about saving time, see if you can get the brand and model number of the unit and hit the web. More and more schematics are finding their way online and there are some very specific fault FAQs on various products. Even if you can’t find something on the exact model, February 2006  43 Switchmode power supplies are potentially lethal. Some of the parts operate at 240V AC mains potential and that can include heatsinks. Never work on a switchmode supply with power applied unless you are an expert and know exactly what you are doing. something similar will often yield useful clues. Look, listen & sniff Many faults can be found by simple observation, so have a careful look around before you start exploring with your meter. You might find a plug that isn’t seated properly in its socket or a solder splash bridging two tracks on a PC board. Either way, the fault could rectify itself while you are attempting to fix it. In many cases, components actually look faulty; transistors that have cracked open, swollen or ruptured electrolytic capacitors or obviously overheated resistors. In the case of the latter, do not touch the resistor until you have had a chance to measure it. They have a nasty tendency to disintegrate at the slightest touch, leaving you with no idea of the original value. And you you’d be surprised how many technicians will take the back off a TV, take one sniff and mutter “caps”. Poor connections Poor connections are responsible for a huge proportion of faults. These come in a multitude of forms: dry solder joints, plugs and sockets (dirty or badly crimped, including edge connectors and IDC plugs), switches, 44  Silicon Chip board fractures and dirty pots. This brings us to the question of contact cleaning. There are dozens of contact sprays around and they all excel in their specific applications. However, they can cause trouble when used inappropriately. The only type of spray that can be used with confidence in most circuits is one designed for cleaning television tuner contacts or a plain isopropanol spray. Don’t fill your VCR with something designed for auto electrical systems. There are two cases where contact cleaners of any type should be used with caution: slide potentiometers and conductive rubber switches. In the case of the pots, contact cleaners can wash the lubricants out along with the dirt, causing them to stick and bind. Try to drop a tiny amount directly onto the carbon tracks. This often cures them. The rubber switches can be hard to recognise, but they are so common that it is better not to clean any switch unless you are reasonably sure that it has metallic contacts. I have had three memorable experiences involving sprays. Once, I was using a common furniture polish to restore an electronic organ. As I wiped the rag across the faces of the tab switches, the red lettering, which was embossed into the faces of the switches, disappeared! On another oc- casion, also with an electronic organ, a customer decided to try spraying the key contacts himself. However, he also liberally sprayed the plastic keys, chilling them to the point where they became brittle and snapped under tension! Finally, a totally predictable near disaster was caused by yours truly. My wife had been complaining of some intermittent fault in the washing machine. I wasn’t in the mood for any serious fault-finding, so I removed the top cover and soaked the beast with my favourite contact cleaner. I leaned up close to observe the results of my handiwork and hit the ‘go’ button. The ‘explosion’ that ensued was worthy of Francis Ford Coppola. I was unharmed and considered myself fortunate to have learned my lesson in an outdoor laundry, rather than in a customer’s living room! The ubiquitous dry joint Ubiquitous: u-bi’kwi-tus. (adj). Existing or being everywhere; omnipresent. Although common, dry joints (DJs) can be hard to locate as many perfectly sound joints look dry and often a crook joint looks fine. Fortunately, some components form DJs more frequently than others so you can usually save time by looking at the most likely culprits first: ie, any components that are physically large, conduct high currents or get hot. These three factors usually go hand in hand. The expansion and contraction associated with heating and cooling can fatigue joints, as can the vibrations from improperly secured large components. Consequently, the areas that are worth inspecting are power supplies, amplifier output transistors and drivers, servo motor drive transistors, TV line output transistors and so forth. Technically speaking, the best way to rework a DJ is to completely remove the old solder, clean the joint and then resolder it. In the real world, most of the time you will get away with reheating the old joint and adding a little fresh solder. If the joint doesn’t look right when you are through, you can always go back and do it the hard way. While we’re on the subject of soldering, one of the greatest causes of damaged PC boards is the act of removing an IC from a double-sided board. Unless you have a good vacuum siliconchip.com.au Sometimes it’s easier to remove a part by cutting its pins rather than desoldering it and it’s usually easier on the PC board tracks as well. One the part is out, you can desolder and remove the individual pins. desoldering station, the safest way is to carefully cut the component’s pins off one by one and then desolder them individually. Surface-mounted components are a different matter entirely. I’ve seen boards destroyed by salesmen who were demonstrating hot-air SMD rework stations. Many components are glued to the board, so it can be a challenge to remove even a 3-legged device. If you are desperate to have a go anyway, find a wrecked board and get some practice. You need an ultrafine tip, fine gauge solder and a very narrow probe that will allow you to lift individual legs clear of the board. Use a magnifier to make certain that all the legs are clear and then attempt to rotate the component on the board, rather than pulling it off. The glue is very strong but usually quite brittle. Pulling on the part might lift tracks but twisting it will often make it come away with very little force. All of the above presupposes that you have found a dry joint to resolder. If you cannot locate it visually, a little freeze spray often helps. Gentle tapping of components or the PC board with a (non-conducting) screwdriver often helps to localise the fault to a particular area. If all else fails, you can always try tracing out the fault. solder mask from the tracks and polish the copper at the same time. Tin the tracks, using an absolute minimum of solder and tack down the finest tinned copper wire you can get your hands on, such as 3A fuse wire. spot. If gently flexing the PC board seems to have an effect on the fault, then you may well be looking for a fracture. In this case, try removing the board completely and looking at it with a bright light behind it. Many boards are translucent to some degree and frequently, when backlit, the extent of the fracture will show up clearly. Again, large components are more likely to have fractures beneath them. Look closely around any mounting holes and any components which protrude into the “outside world”, such as volume controls, input and output sockets, etc. The repair of fractures is an art in itself and a shoddy job can introduce more problems than it cures. Modern equipment often has very fine tracks, narrowly spaced. The trick is to get the tracks really clean on both sides of the fracture. An ink eraser will remove the Component failure Even without a schematic, it is possible to pin-point many component failures, once you know where to look. Those same components which I mentioned as being prone to DJs are also the most likely to fail. The most reliable electronic appliances are those with little mechanical complexity and low power consumption. Hifi tuners and equalisers are generally more reliable than amplifiers, VCRs and DVD players. To begin with, don’t get too theoretical with the problem. Check the components which seem to have some bearing on the problem, starting with Fractured PCBs are remarkably common. Backlighting helps determine the extent of the crack. Board fractures Board fractures can be difficult to siliconchip.com.au February 2006  45 Warning labels are there for a purpose. Reading them is good for the soul, among other things. easily-tested components like output transistors and rectifier diodes. Desolder at least two legs of any transistor before measuring it. Any low value resistor is worth measuring. Often these are used as fuses in low voltage rails. Electrolytic and tantalum capacitors are also worth a look. These are often used to provide localised filtering of power supplies and can present real problems when they go short circuit. A short circuit on a 5V rail can be a nightmare in a digital circuit, but always look at the bypass caps before you start removing LSI chips. Most capacitors have 38 fewer legs to desolder! You can often determine what voltages should be present in a circuit without needing a schematic if Use a quality photographic blower-brush to clean the lens of a CD or DVD player. you have a degree of familiarity with the components involved. 3-terminal regulators are wonderful devices - they have their voltages printed on them. Integrated circuit families usually work off predictable rails, so a quick peek at pin 14 of most TTL ICs will often save time. Look for other old standards, such as LM741s, 555 timers, etc. Even if you have no idea what an IC does, it should usually have some sort of rail on one of the pins! Because much consumer equipment tends to be under-engineered, you often find that filter electros in power supplies are working very close to their rated voltages, so that 4700mF 63V cap in the power supply is not likely to be filtering the 5V rail. Transistors usually fail either open or short-circuit, so don’t worry about beta, hfe and all that other stuff unless you want to make a career of one repair. Diodes frequently go leaky and this is easily checked with a multimeter but make sure you are measuring leakage and not the resistance between your fingers. Resistors tend to go open-circuit and mostly these will be either very low or fairly high in value; less than 1kW or more than 100kW. This may sound like a very simple approach and may lead you to think that there is not much science involved in electronic repair. However, the really hard repairs can present such a challenge that a busy technician won’t have the time to worry about style on the easy ones. Dead shorts PC boards with surface-mount components are easily damaged during rework due to the small pad sizes. Note that most components are physically glued to the board as well as being soldered. 46  Silicon Chip A short circuit on a power supply rail can be one of the most difficult faults to find. Imagine a shorted bypass capacitor on a 5V rail on a good-sized logic board. In this case, practically every single component will be connected directly to the rail and any one of them could be the culprit. The real trick is not be overwhelmed by the magnitude of the task. First, disconnect every single item which is not soldered in - plugs and sockets, edge connectors, socketed ICs, etc. Check the resistance to ground after each removal. Next, desolder the “easy” components; large electros, regulators, transistors and so on. If you have access to a schematic, use logic (the intellectual type, not the electronic kind!). If your short circuit measures only a few ohms, then a transistor which has a 1kW emitter resistor siliconchip.com.au is highly unlikely to be the cause of the fault. However, if the same transistor had a 10mF capacitor across that 1kW resistor, then it would be conceivable for the capacitor to have developed an internal short, which in turn, could cause the transistor to go shorted. Thirty seconds taken to establish that a component could not possibly be the cause of the fault can save a lot of fruitless soldering. There is another approach which is a little unorthodox and if you decide to try it, you should be aware of the possible side effects. It involves attempting to ‘smoke it out’. By applying a power source to the rail (at no more than the rail’s rated voltage and preferably somewhat less) at a reasonably high current, it is often possible to heat the component enough to cause it to smoke or at least enough to make it hot. However, you should know that you could be forcing the board tracks to carry considerably more than their rated current. In most cases, I am reluctant to use more than 500-odd milliamps. As long as you do not exceed the rated voltage of that particular rail, the other components on that rail should be safe enough, even if the short should actually burn itself out. Of course, this assumes that you have access to a current-limited supply. If you cannot set the current separately from the voltage, give this technique a big miss. You will need to use your own judgment on this one, as it does involve potential risk to the PC board. Know your test gear I am continually surprised to see experienced technicians making fundamental mistakes in taking measurements. Because we mainly use high impedance digital meters these days, many of us tend to be blissfully ignorant of the bad old, “20kW/V” days and usually, this is a blessing. However, a 10-megohm input impedance is still a long way from being an open-circuit and loading effects can happen. Similarly, few crystal oscillators function properly, if at all, unless your oscilloscope probe is set to the X10 position and even then, they might not run. Nevertheless, I saw a very senior Technical Officer employed by a major manufacturer who was unaware of the loading effect. A typical multimeter will have poor frequency response on its AC volts siliconchip.com.au Do basic safety checks before and after every repair. Is there earth continuity? Is the appliance lead damaged? Is the polarity correct? This mains lead has obvious damage and must be replaced. ranges. However, if you have access to a meter with a good AC bandwidth, you can use it to trace an audio signal through an amplifier. To fix or not to fix It’s a sad fact that many appliances today are not economically repairable. DVD players are essentially throwaway items, VCRs aren’t far behind and even TVs are cheaper than ever before in real terms. If you get a sense of personal satisfaction from being able to get something going again or if you are fixing your own appliances, then it is probably worthwhile spending a few hours tinkering. However, be careful not to get into a vicious circle in doing repairs for friends. The scenario can go like this: you agree to look at something. A week goes by and you still haven’t got around to taking the covers off. The guilt kicks in, so you spend a while and get some idea of the fault. You order a part and a week later you fit it. And it still doesn’t work. With every day that goes by, you will feel more pressure to complete the repair, although the chances of you being able to fix it are decreasing at the same time. The trick is to be able to cut your losses and cut them early. Final checks There’s one important aspect to repairing an appliance which is easily overlooked. You must be certain that you have not overlooked a fault which might render it unsafe or worse, introduced one. For a non-professional, this is primarily a moral responsibility. Although there is an Australian Standard (AS/NZS 3760:2001) which details the tests required after repair of any appliance, it is unrealistic to expect full compliance from someone who is performing a “homer” for a mate. Nevertheless, basic tests can (and should) be performed which go some of the way towards keeping your friends and family intact: Inspect the power lead for any damage. • Check the wiring of the plugtop for correct polarity and anchoring of the cable. • Check the strain-relief where the cable enters the appliance. • Check that external fuses are of the correct rating. • Ensure that the resistance of the earth connection from the power plug to any exposed metal surface is less than 1W. • Measure the resistance from the Active pin to Earth and from the Neutral pin to Earth. Both should measure open circuit. Finally, note that these tests are not a substitute for a proper high-current earth-bond test, nor for a high-voltage leakage test, but they will give you an indication of gross conditions and a warm and fuzzy feeling when you hand the appliance back. SC February 2006  47