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SERVICEMAN'S LOG The Serviceman's Curse I reckon servicemen are cursed. I don’t mean that people swear at us a lot (though they might!), I mean that we bear the Curse of the Serviceman. This means that when anything breaks down, we always consider the repair option first. It doesn’t really matter what has broken or whether we usually repair or service these things in our day jobs, it’s just that we simply can’t help ourselves from wanting to fix something that’s broken. This is mostly fine if we just have ourselves and our own household to think about, but for some, it also means that when any of our friends, acquaintances or colleagues break something, we are often expected to fix their stuff as well, just because we are servicemen! As if being our own go-to repair siliconchip.com.au guy isn’t enough. This is what I mean by a Serviceman’s Curse. While the more prosperous servicemen among us may have learned to suppress the curse and are able to chuck away the broken device and go out and buy a replacement instead, for me and many others, that is a difficult Celebrating 30 Years Dave Thompson* Items Covered This Month • • • • Car battery charger CIG Transmig 200 welder Compaq CQ61 laptop R&R MR16 LED downlight repair *Dave Thompson runs PC Anytime in Christchurch, NZ. Website: www.pcanytime.co.nz Email: dave<at>pcanytime.co.nz decision to make and we would have to force ourselves to even consider it. When something in my household breaks, my first instinct is to weigh up all possible options to repair it myself, with the very last option being to buy another one. Perhaps if I won big on the lottery, or inherited a few gazillion May 2018  63 bucks, this attitude might change – I’d sure like to test that theory! But in the meantime, I always consider repair before replacement. I know plenty of people who think the other way. They replace anything that breaks with the latest and greatest new version, regardless of whether it was repairable or not, but not many of these people are servicemen. I can’t really blame them; after all, they don’t bear the curse! Of course, there are exceptions; if a repair isn’t feasible or economically sensible, such as a dropped dinner plate or wine glass then the curse doesn’t really apply. Having said that, I have been known to glue people’s favourite plates or porcelain figurines back together. But if the broken article is even remotely within my skillsphere, then the curse awakens. My neighbour invokes the curse A recent example involves a neighbour who tried to start his car the other day while he still had a battery charger connected. He subsequently discovered that the charger no longer worked. I’ve done this myself in the past and perhaps due to dumb luck, I’ve had no problems, though it stands to reason that one probably shouldn’t leave anything connected when cranking the engine unless it’s designed to handle it. This is especially true if the car battery is dead flat to begin with and we are essentially relying on the output of the charger alone to supply enough grunt to fire up the motor. In such situations, the current draw through the leads and internal components of the charger can be considerable, and when the car starts there is even more current introduced into the circuit by the alternator’s output. Many car battery chargers are simply not designed to withstand this kind of punishment. Ordinarily, a guy would just think the charger was dead, chuck it in the bin and go out and buy another one – especially given the current (hah!) prices of chargers these days. In this case, the sticky wicket was that my neighbour had borrowed the charger from a friend, and while it was by no means new, it looked to him to be a reasonably flash model as far as car battery chargers go. He didn’t relish the thought of having to cough up to replace it. He brought it over to my workshop in a bit of a panic and asked if I could have a look at it, at least to see whether it was repairable. If not, he’d be chowing down on a large crow sandwich and splashing out for a new charger. I promised to see what I could do, mindful of the fact that this would probably end up being one of those “pro bono” jobs all servicemen get saddled with and would more than likely take up time I could ill afford to spare. That said, I couldn’t refuse a neighbour in need, especially as it was highly likely that I could fix the charger. The Serviceman’s Curse strikes again! The charger was about as simple as any electronic device can get. A mains cable enters the plastic case through a cheap-but-effective clamping arrangement and connects via a fuse to the primary of a reasonably heavy-duty transformer. The secondary is wired to a small PCB with a glass thermal cut-off switch, a couple of carelesslyplaced diodes and three LEDs. Attached to that board are a couple of cables which then exit the case through a rubberised grommet with comically-large alligator clips on each end; one red and one black. He might have thought it flash but I disagreed; it was a bog-standard battery charger. The battery connector cables looked to me to be a little on the light side, considering the size of the transformer and the cables that make up your average set of jumper leads. But I suppose these modern-style, piddly-thin wires would have the advantage of being self-limiting and besides, they’d ordinarily only have to cope with a few amps at the most for relatively short periods anyway. Servicing Stories Wanted Do you have any good servicing stories that you would like to share in The Serviceman column? If so, why not send those stories in to us? We pay for all contributions published but please note that your material must be original. Send your contribution by email to: editor<at>siliconchip.com.au Please be sure to include your full name and address details. 64 Silicon Chip Celebrating 30 Years The LED indicators were mounted inside the case but shone to the outside world by way of clear plastic light tubes. I’ve seen this method used before, especially in devices like amplifiers, radios and laptops and it tends to work fine for them. But the inside of this charger gets hot then cools down, over and over, and this, when combined with the natural ageing process, causes the plastic to go opaque. Because of this, the amount of light reaching the user would likely be pretty low. I’d wager these indicators would be barely visible during the day, especially in bright sunlight. I’d confirm that theory once I’d fixed the thing… Testing the charger I plugged the charger into my lightbulb based load box and flicked the box’s socket switch to on. I’ve gotten into the habit of using this load box, largely because it is set up on my workbench permanently and this makes it the handiest power socket for any mains-powered devices that cross my desk. It can’t hurt, regardless of whether I really need it or not. The neighbour told me he’d plugged the charger in and got no indication of power, and with no specific mention of fuses blowing or circuit-breakers popping, it was unlikely that a shortcircuit was present. However, the test rig socket is right there, so I used it. The load box consists of two 250W incandescent bulbs (remember them?) mounted in a couple of lamp sockets screwed to the top of a suitably-sized, plastic hobby box. The lamps are wired in series with the Active wire and all three mains wires are then terminated into a standard dual, switched mains socket on the front of the box. The box is powered from a wall socket. I generally power any mains-powered equipment under test into the protected socket on the load box. If there is an internal short or other, similarlynasty electrical fault present, the lights glow to let me know while they limit the voltage applied to the load, giving me time to switch everything off safely without the drama of blowing fuses or the guts of the device under test flying across the workbench. I also have an adjustable auto-transformer (generally known as a Variac, but mine isn’t Variac-branded) and I siliconchip.com.au use that for similar jobs, especially those where it is more prudent to slowly bring up the voltage than apply it all at once. I mostly use the lightbulb load box for mains-powered stuff though, especially valve amps and similar devices. Both methods save replacing fuses and reduce the risk of damaging other equipment on the same circuit, and both are invaluable to the serviceman who needs to work with mains-powered devices and they are well worth the effort of building or purchasing. As expected, the battery charger did nothing on switch-on. No bright lights on the load box and nothing on the front panel of the charger. For all intents and purposes, it was as if I hadn’t plugged it in. Delving into its innards My first thought was a blown fuse. There must be one, and while some chargers have a fuse socket accessible from the outside, this model didn’t, so it had to be inside. Getting in was easy enough, with only four long, PKstyle screws holding the case together at each corner. It is refreshing to be able to open up something without having to resort to cruder methods of removing those ridiculous and unnecessary “security” fasteners that seem to be all the rage these days. It’s also getting increasingly rare to find the screws out in the open and easily accessible to normal tools, instead of being buried inside deep channels or concealed under rubber feet, plastic bungs or warranty seals. Once the screws were out, the case split apart easily and the various components were laid bare. There indeed was a fuse, in line with the Active lead and right next to the cable clamp. The fuse was obviously intact as I could see it clearly, but I popped it from its holder and checked it with my meter anyway. If it was dead, it wouldn’t be the first time a fuse appeared undamaged but was, in fact, open-circuit; a classic beginner’s trap. In this case, though, the fuse rang out OK. On to the next component in the troubleshooting queue: the transformer. This also looked OK but then again, something would have had to fail catastrophically for it to appear otherwise. I measured the secondary windings and got around 0.5W on my analog siliconchip.com.au multimeter; probably not that accurate a reading but at least it had continuity. When I measured the primary though, it appeared to be open circuit. A current surge might have burned out the windings but I could see no darkening of the yellow tape they typically use to bind transformer coils with and there wasn’t even the faintest whiff of that distinctive burnt-enamel smell that almost always goes handin-hand with high-current component failures. I’d have to dig further. After making a note of what went where, I desoldered the transformer’s wires and removed the four screws going down through the laminations and holding it to the bottom of the case. I do like these simple-to-disassemble devices. With the tranny out and sitting on the bench, I re-measured the windings with another meter just to be thorough but got the same result; the primary side was definitely open-circuit. Ordinarily, this is where most servicemen/repairmen would start looking for a replacement transformer but as I’ve already made clear, I hate throwing things away (that curse again). Anyway, without having any part numbers or any other information written on the component, I’d have to either take an educated guess as to its specifications or assess other, similar chargers to determine or approximate the voltages and ratings of their transformers. Either method would likely lead me to a replacement component that would be close enough for rock and roll – it’s a car battery charger after all – but that’s a bit too wishy-washy for my liking. Besides, I wasn’t finished with this dead transformer yet. Fixing it the hard way I started by removing as much of the yellow tape that bound the windings as possible, a task made difficult by the way the E-I core kept getting in the way. I got out one of my craft knives and slid the edge of the blade between two of the hundreds of tightlylaminated shaped metal shims that make up the core. It went in relatively easily, meaning the laminations weren’t potted or bonded together with varnish, as some are. I removed the transformer’s folded heavy-metal outer cover by bending four metal locking tabs on the base out of the way and lifting it clear. Celebrating 30 Years With the core now totally exposed, I used a sharpened flat screwdriver to carefully pry free a couple of the E and I-shaped laminations from one side. Now loosened, I could ease out the rest of the laminations one by one until there was a large pile of them lying on the bench. What remained was a hard, white plastic bobbin that held the primary and secondary windings, and it was a simple matter to strip the remaining yellow tape away from the primary side. Once gone, I could see nestled at the very top of the now-exposed windings a black, rectangular, twolegged component that I assumed to be a thermal fuse, wired in series with the primary winding. I already knew it would be open circuit, as the clean state of the windings showed nothing had burned out but I measured it anyway. It was dead as John Cleese’s parrot. I unsoldered it and measured the windings beyond it. My meter made it around 50W and whether that was about right or not I’ll leave to the mathematicians; all I needed to know was that it seemed about right to me. I located another thermal fuse in my parts store, which according to the data sheets I downloaded from the web was a suitable substitute for the original. Once soldered in, I re-bound the primary windings with similar tape and set about re-assembling the core, a grubby job as each one is coated in an anti-corrosion substance that if not May 2018  65 actually oil, has very much the consistency and feel of it. However, after I’d stuffed as many of the laminations back into the bobbin as I could, I still had about a dozen “E”s and “I”s left over. No biggie, or so I thought. After a quick megger check to make sure my insulation was good, I temporarily rigged up a mains cable to the primary and used my auto-transformer to power it up slowly. My multimeter showed the secondary voltage rising as expected as I wound up the power, but then it happened; the transformer started to buzz. Above about 150VAC, the transformer was buzzing very loudly; I guess I was going to need those extra laminations after all! By using some wood-workers’ clamps and a lot of very colourful language, I managed to shoe-horn all the remaining laminations back into the core. It was then thankfully buzz-free. That just left the simple matter of reassembling everything, giving it another insulation test and trying it out. It worked as well as it did before and while those LEDs were barely visible, my neighbour was hugely relieved and grateful. He offered to pay, but I declined; it was the neighbourly thing to do. Darn this Serviceman’s Curse! CIG Transmig 200 welder repair Sometimes it doesn't take a large fault to stop equipment worth thousands of dollars from working. G. S., of Castle Hill, NSW, recently saved a heavy-duty welder from the boneyard and here is how... My ex-neighbour Paul is into old Ford Falcons from the 1960s. He restored a '64 ute some three years ago and has two 1964 2-door coupes awaiting restoration. He also recently acquired a '63 Falcon station-wagon that he's working on at the moment. Sitting in his garage among that lot is a real gem – a 1967 Ford Mustang coupe that's currently a body shell and a pile of parts. So why is he mucking around with the Falcon station-wagon and not putting all his effort into the Mustang restoration? I dunno but he'll come up with all sorts of excuses if pressed on the matter! Vehicle restoration projects invariably require rust repairs and so, about eight years ago, Paul acquired a second-hand CIG Transmig 200 welder. This is a large 3-phase machine and 66 Silicon Chip The faulty contactor shown inside the welder. is mounted on a sturdy metal trolley that can be trundled around in his garage. It probably cost around $3000 new but had been acquired for a pittance by one of Paul's mates when a business shut up shop. Having no real use for it, the mate eventually passed it on to Paul for an even lesser pittance and he subsequently used it while restoring the ute. It then sat unloved at the back of Paul's garage for three years until he got his station wagon restoration underway. It wasn't long before the welder was needed for this project and so the machine was duly trundled out and hooked up to the garage's 3-phase power outlet. Paul then pressed the trigger on the wire-feed nozzle to check its operation but no wire fed through. Instead, the 3-phase circuit breaker in the household fuse-box tripped out. Puzzled by this, Paul reset the breaker and pressed the wire-feed trigger a second time. The circuit breaker immediately tripped out again and it did so a third time after he had reset it. At that stage, Paul decided to ask my brother, who is a licensed electrician, to take a look at the machine for him. I went along for the ride and when we got there, we found that Paul had already removed the side panels from the machine. It took my brother just a few minutes to diagnose a faulty contactor. This normally pulls in and powers up a large transformer and various other parts in the machine when the wireCelebrating 30 Years feed trigger is pressed. Our snap diagnosis was that the contactor was probably full of gunk and this conclusion was reinforced when my brother demonstrated that the machine could be powered up by manually assisting the contactor to “pull in” by pressing on it with an insulated probe. Even then, the wire-feed mechanism still wasn't working, so it looked like this welder had two separate faults: (1) a faulty contactor and (2) a fault in the wire feed mechanism or in the circuitry that controls the wire feed motor (or perhaps even a faulty motor). There was no point trying to diagnose the wire-feed problem until the contactor problem had been resolved, so it was up to Paul, an electrical fitter by trade, to take things from there. He's not a man to let the grass grow under his feet and so, the very next day, he carefully labelled all the relevant connections, then pulled the contactor out and cleaned it to within a millimetre of its life. It did indeed prove to have a lot of gunk inside and when it was refitted, he was gratified to find that it now pulled in when the wire-feed trigger was pressed without tripping the circuit breaker. So that solved problem number one. Now for problem number two. With no access to a manual, circuit diagrams or spare parts, Paul figured that his next best step was to seek professional help to get the wire feed mechanism working. siliconchip.com.au The faulty tantalum capacitor on the control board was replaced with a similar electrolytic capacitor, with the adjacent 7812 regulator circled in red. As a result, he loaded the machine onto a trailer and carted it off to a specialist welder repair shop. They duly called back a week later to say that both the wire-feed mechanism and motor were OK and that they had diagnosed a faulty control board. Unfortunately, given the age of the machine (it's probably late 1980s or early 1990s vintage), replacement control boards were no longer available. And as far as they were concerned, without a replacement control board, the machine wasn't repairable. Never one to give up, Paul now figured that he would try to get the control board fixed. And that's when he flicked the problem my way. I suggested that the best approach would be to remove the board from the machine and drop it around to me. That way, I could inspect the board for dodgy solder joints and test the various semiconductors, electrolytic capacitors and other parts at my leisure. Having made the suggestion, I thought he might drop the board off in a week or two but as I said, he's not a man to let the grass grow under his feet. He turned up at my house within the hour, clutching his faulty control board with the external wiring leads all carefully labelled. I took a look at it the next day. Despite its age, it was still in good condition with no signs of corrosion. Its main parts included a couple of 555 timer ICs, an LM3900 quad amplifier IC, several transistors, a relay, two stud-mount SCRs and a stud-mount siliconchip.com.au diode. The external leads ran off to a couple of pots and some spade clips. So had one of the semiconductors failed? Or was it a faulty relay, a dodgy capacitor or a dry solder joint? If it wasn't the latter, the easiest approach might be to simply blanket-replace the ICs and transistors and check out the SCRs, the diode and the relay. It didn't come to that though, because I quickly spotted what was almost certainly the cause of the problem. There were five 10µF 16V tantalum capacitors on the PCB, two of them in the timing circuits of the 555 timers. Four of these tantalums were dark blue but the fifth had turned a pale blue colour with a greenish band across it. Surely it wasn't going to be this ridiculously easy? I stuck a multimeter across the capacitor and it registered a dead short! I then traced the PCB tracks from the capacitor and found that it was across the output of an adjacent LM340T-12 12V regulator. This device is mounted on a small heatsink in one corner of the board and provides a regulated 12V rail to the 555 timers and the quad amplifier (and possibly also the relay). I removed the capacitor and the short across the regulator's output disappeared. I then replaced it with a 10µF 16V electrolytic that I had on hand. Two other 10µF bypass tantalums on the PCB were also replaced with electrolytics, while those in the timing circuits of the 555 timers were left in place. In theory, the LM340T-12 should have survived since these devices are short-circuit proof. However, I replaced it with an equivalent 7812 regulator as a precaution, along with an adjacent electrolytic capacitor for good measure. At that stage, it was tempting to apply power to the circuit and check the operation of the 555 timers. However, not having a circuit diagram (no luck with Google), I was afraid that this might risk damaging something, especially if two of those external leads shorted together. The best bet would be to test the control board in the welder itself. I called Paul and told him what I had found. He was on my doorstep some 25 minutes later, collected the part and shot back home to refit it. I was cautiously hopeful that it would now work and I didn't have Celebrating 30 Years to wait long to find out. An ecstatic Paul was back on the phone an hour later and he was floating somewhere between his Mustang chassis and seventh heaven. “You're a genius”, he exclaimed. “The welder is working perfectly!” There's nothing like a bit of flattery to massage the ego but at the end of the day, this was really a joint effort. My brother diagnosed the faulty contactor, Paul fixed the contactor, the service centre correctly diagnosed a faulty control board and yours truly fixed the control board. So a part costing less than a dollar was all that prevented this valuable welder from working. It's since been used to weld some fresh sheet metal into that old Falcon station wagon but it's the Mustang restoration that I reckon he should be getting stuck into. Editor’s note: older Tantalum capacitors seem to go short circuit more often than you might expect so it’s well worth taking a good look at any such capacitors first when repairing something made before the year 2000 or so. Compaq CQ61 laptop repair and refurbishment B. P., of Dundathu, Qld, spent quite some time refurbishing and repairing an old laptop for his wife to use. In the process, he discovered and fixed some classic laptop hardware problems and also ran into some time-consuming software pitfalls... Our daughter has just started university. After five years of intensive use, her old laptop was well and truly the worse for wear, so she bought herself a new laptop and she gave us her old one when she came home during her mid-year break. When I assessed the old laptop, it was in a rather dilapidated condition. The keyboard had one key cap missing on the numeric keypad and most of the top row of keys (numbers and symbols) no longer worked. The top silver layer of the touchpad was also worn off in places, revealing the black base colour, the keyboard surround had a lot of the black paint worn off it and the lid hinges were loose. Although the laptop still worked, it was certainly beat up. I started the repair by removing the old keyboard, by undoing the securing screws on the bottom of the laptop. I ordered a new keyboard and while waiting for it to arrive, decided to conMay 2018  67 tinue working on it by plugging in a USB keyboard. This allowed me to do a factory reset so that we could start with a fresh copy of Windows 7. After that was done, I uninstalled all the trial software and I installed some additional programs that were needed. Then I partitioned the 500GB hard drive into two separate partitions so that data could be stored on the D:\ drive, separate from the operating system on C:\. I noticed some issues with the Compaq factory version of Windows 7 that I wasn't happy with, so I decided to do a fresh install of Windows 7 from an OEM disc (ie, one purchased from Microsoft) instead. This all went well, but then I noticed that the webcam driver was not installed in Device Manager; it was showing up as an unknown device. I tried to locate the correct driver for the webcam on the internet, however, this proved difficult. I did eventually find the driver and installed it but when I went to test it with Skype, Skype said that it was already in use by another program. It wasn’t, though. I thought I would try uninstalling the webcam driver in Device Manager and then scanning for new hardware. This did not work, as the webcam then showed up as an unknown device again. I decided to do another factory reset to try to get the webcam working correctly but because I'd altered the hard drive partitioning, the restore partition was no longer present. I still had the set of three restore discs that we'd made when the laptop was new, so I decided I would just use those. I set about restoring the laptop from those discs, but when I got to disc three, it had a read error, so the factory reset failed. This left me with a blank hard drive. That meant using the OEM disc to reinstall Windows 7 again. So I was back to square one, with the webcam still showing up as an unknown device. I then realised that we had a related Compaq laptop model, a CQ42, that uses the same HP-101 webcam. I checked what driver the webcam used and it was a standard Windows driver. I'd recently installed Windows 7 on the CQ42 without any issues and Windows had found and installed the webcam, so I wondered why I was having so many problems with the CQ61's webcam. 68 Silicon Chip I then tried to tell Windows to use the driver that it should be using, but this idea didn't work out and I was still in the position of not having the webcam working. At this point, I decided to give the laptop a rest and do something else. I went to my shed to look for something and when I opened a box to check inside it, I discovered another Compaq CQ61 that I'd been given some time ago that I'd forgotten about. This CQ61 was a cheaper variant with much lower specifications than the one I was working on, so I would use it for parts. I started dismantling it with the intention of using the top case half to replace the well-worn top case half on the one I was fixing. The touchpad still had its original silver colour and the keyboard surround was still in asnew condition. These new parts would make a huge difference to the original one. Unfortunately, the keyboard on this laptop was also faulty and unusable. After unscrewing a multitude of screws, I had the donor laptop fully dismantled and I retrieved the parts that I wanted to use for the refurbishment. Then I turned my attention to the laptop I was working on and I dismantled it. I was now ready to reassemble it, using the better parts from both laptops. Before proceeding further, I decided to give the interior a good clean because there was a considerable amount of dust inside it from the many years of heavy use. I used a small paintbrush with natural bristles in order not to generate static electricity while brushing the dust off the motherboard components. When I got to the heatsink, I removed the fan for cleaning and I noticed that there was a thick layer of dust on the inside of the heatsink's fins, where the fan had been blowing air through it. This dust was removed and the fan was then thoroughly cleaned, before being refitted. This is a very common problem with older laptops and often causes them to overheat and either lose performance or become unstable. It was fortunate that I had stumbled across the other laptop and as a result, decided to take apart the one I was Celebrating 30 Years working on because if I hadn’t done so and cleaned it out, chances are I would have run into some of these problems later. At this point, I decided to also transfer the lid assembly from the donor laptop (including the display and webcam) because it was in slightly better condition than the original lid. By this time, I was also suspecting that the original webcam may be faulty, so this was the perfect opportunity to test this theory. With the laptop partly assembled, I propped the lid against a box, so that I could connect it up and test the screen and the webcam. Once booted, I checked Device Manager and the webcam was now installed. I loaded Skype and tested it and it now worked, indicating that the original webcam had been faulty all along. I then noticed that there was a problem with the replacement screen, as it had several dead pixels in the lower right-hand section of the screen. This blemish was not that bad and it would not make a huge difference to the laptop, but seeing that the original screen was in better order, I would swap them over. I dismantled the lid and fitted the original screen to it and then I reassembled it and continued with reassembling the laptop. It's very important to take particular note of which screws go where when reassembling a laptop. There are around five different length screws in some laptops and each screw must be used in the correct location to prevent damage (when putting a long screw in where a short one should go). siliconchip.com.au After a bit more work, the laptop was reassembled and back in working order again, except for the keyboard. I was still waiting for the replacement. In the meantime, my wife could just use the laptop with the USB keyboard. Eventually, the replacement keyboard arrived and I fitted it; the laptop was then fully refurbished and it has a new lease on life. This laptop is now around seven years old and it would be classified as being quite outdated. But it's still quite suitable for light duty work, such as web browsing, emailing, letter writing and other general duties. For a bit of work and less than $20 for the replacement keyboard, it's as good as new. That’s a lot cheaper than buying a new laptop, even a basic one, and it’s still perfectly adequate for most jobs and uses less electricity than a desktop computer. Having the donor laptop on hand certainly saved me quite a bit on replacement parts and resulted in a more cosmetically appealing end result with the new top case shell to replacing the well-worn old one. This was my first major laptop repair and I was surprised that it was nowhere near a difficult as I had imagined it would be. It's simply a matter of proceeding with caution and paying close attention to details. There are also plenty of videos on YouTube which go into considerable detail about laptop repairs but I managed to do the refurbishment without referring to any. MR16 LED downlight repair D. M., of Toorak, Vic, had a 12V MR16 LED downlight fail far short of its claimed 20,000+ hour lifespan. It would have been cheaper to just buy a new one but he wanted to know what had gone wrong so set to taking it apart… I wanted to know what had failed inside the MR16 LED lamp as I find the claims for LED downlight life expectancy, typically of 20,000 to 50,000 hours, quite unlikely. At eight hours per day, that would amount to a service life of 7-17 years. Although LED downlights have not been available for 17 years, in my experience, most such lamps don’t even last seven years. My particular light was a Muller-Licht (house brand) Reflektor rated at 320 lumens and 5W. I carefully examined siliconchip.com.au the light to determine how it might be disassembled without damage. I found I could gently pry the top retaining ring off the body with a knife which also caused the release of the light diffuser, revealing the LEDs and their heatsink. Two Phillips-head screws could then be removed from the heatsink, allowing the separation of the top and base portions of the light assembly (see photos at right). LED lights typically contain a driver which delivers a constant current to the LEDs. I quickly determined that it was the driver that had failed as when power was applied directly to the LEDs, they lit up. I decided to obtain and install a suitable replacement driver. I found one online that was rated at the same current as the LEDs, 650mA and only cost about $2.50. This driver has pins attached with the correct size and spacing to plug into an MR16 socket. So these could be used to replace the existing pins on the lamp body, or alternatively, they could be desoldered from the driver if necessary for other applications. The driver utilises a PT4115 chip. The website where I bought it has details of the circuit; see siliconchip. com.au/link/aajr I removed the old driver from the LED body and then the old MR16 pins. Be careful removing the old pins as it is easy to break the plastic body. In this lamp, the pins were hollow. You may be able to cut them and then drill through to remove them (with a very small diameter drill). I tried twisting them with pliers which ended up cracking the body of the case. Alternatively, it might be easier to leave the pins in place and desolder the old driver, then solder them to the new driver board. The driver is housed in the otherwise empty “well” in the base of the lamp, just above the pins. Having soldered the new driver in place, I glued the ends to ensure the pins would not move. Make sure any conductive parts that might contact each other to cause a short circuit are appropriately insulated. The lamp worked fine after that and so I put it back into service. This particular lamp replaced an existing halogen downlight and is driven by an old-style iron core transformer Celebrating 30 Years Prying off the cover revealed the retaining ring, diffuser, LEDs and their heatsinks. After removing two screws the LED enclosure could be separated showing two leads attached to a driver PCB. The LED driver shown above is the replacement one and is mounted differently to the old driver. Left: the replacement MR16 driver. Right: the failed MR16 driver. supplying 12VAC. These transformers are not as efficient as more modern electronic ones but they do work with LED replacement lamps. Some modern electronic “transformers” designed for use with halogen lamps will not work with LED replacements as they do not draw enough current (or perhaps it’s because they’re a non-resistive load). There are many different designs of LED downlight. Some such as the one described here might be relatively easy to disassemble, others might be more difficult or impossible. Note that type of repair is not very economical, especially if you include your time in the calculation but I found the job to be both fun and educational. SC May 2018  69