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SERVICEMAN'S LOG No magic hammers with smart TVs You don’t always win in the servicing game. I recently took on a large-screen (140cm) LCD TV set but despite my best efforts, I ended up coming off second best. I’m not usually into repairing TV sets, preferring instead to let professional TV servicemen handle those specialised jobs. However, when a client recently asked me if I “did TVs”, I answered “yes, I do”, surprising myself even as the words came out. Not wanting to obviously back-pedal, I then asked him what was wrong with it. My thinking was that he might describe a symptom that would give me the chance to bail out gracefully by claiming that, unfortunately, I couldn’t repair “that” type of problem. His response was that it worked fine at switch-on but that the screen then went dark a minute or so later. To me, that sounded like it should be quite straightforward to repair, my thoughts being that it was probably just a dud connection or something along those lines. He then told me that the TV was only about a week old, so why hadn’t he simply taken it back for a warranty claim? It turned out that he’d lost the receipt and apparently the retailer was being extremely pedantic about not taking the set back without it. While I can understand this stance on one hand, surely they could have dug around and found the relevant sales details, especially as the set had only been purchased from them the week before. However, they wouldn’t budge and so the customer was at a bit of a loss as to what to do. Dave Thompson* Items Covered This Month • • • • My first smart TV repair CPR on an Engel fridge Sobering up a groggy pH meter Penny’s air-conditioner Feeling his pain, I said I’d look at it for him but that I couldn’t promise any­thing. It was a neat little exit strategy to fall back on if necessary. If the job proved too difficult, I could simply point out that I’m really a computer technician rather than a bona-fide TV repair guy. With the customer’s expectations dealt with, it was just a matter of him hauling the TV in from his vehicle. For some reason, I was expecting a relatively small set but he turned the corner cradling a monster 55-inch (140cm) flat-screen TV. It took just one look to convince me that the tiny cabin I currently use as a temporary workshop was not going to be the best place to troubleshoot this device! Despite its size, it was as light as a feather and in fact a child could’ve lifted this TV! Nevertheless, I took one end and helped him carry it down the driveway to my garage workshop, which is a lot larger and better set up for electronic repairs than the cabin. Signs of life When we got there, I plugged the set in, switched it on and instantly saw a bright blue splash screen. After a few seconds, the TV circuitry switched in but the screen was all hash due to the lack of an antenna. However, rather than connect an antenna, I plugged in a flash drive I had hanging about the workshop and used the remote to select the USB input, whereupon the media menu came into view. I then chose a random AVI file from the flash drive and away it went. To be completely honest, it wasn’t the best picture I’d seen on a TV but 42  Silicon Chip siliconchip.com.au My first smart TV This was the first big smart TV I’d disassembled and to be honest, I expected a little more inside than I discovered. Pretty much 80% was fresh air, there being just two moderately-sized circuit boards and one smaller board screwed to pillar mounts on the rear of the screen panel. The power supply was up near the middle left side of the panel, while down at the centre bottom sat the smaller T-Con (timing and control) board. On the mid-right side sat the main audio/video/tuner board. Of these, the biggest was the power supply board and it measured just 120 x 180mm. As a result, all the boards looked rather lost in the vast, empty expanse behind the screen area. Being a “more common” type of serviceman, I reached for my hammer to begin troubleshooting the TV! Yes, my hammer and while you may well picture a large engineers’ ball-pein hammer wreaking its havoc among the chips and transistors, you’d be wrong. This particular hammer is a small, hard plastic type and is an ideal tool for gently tapping on boards and even individual components to induce any loose connections or bad solder joints to reveal themselves. I plugged the set in and leant it against my workbench, portrait style, with the screen facing a set of plastic drawers sitting under the bench. The faces of these drawers acted as a mirror, one of the vital aids real TV service people have in their workshops but which I don’t happen to have. Admittedly, it wasn’t exactly a perfect TV repair set-up but it would suffice; I could easily make out the reflected screen images on the drawers. As soon as it was plugged in, the TV again displayed the blue splash screen, then the file menu for the USB input. As before, I chose a random movie to play and sure enough, after a minute or so, the screen went black. I then switched it off using the remote (ie, to standby rather than “‘right off”), then immediately restarted it and found that the screen stayed black. I turned it off again and decided to wait for a few minutes before switching it on again, amusing myself in the meantime by taping a row of microswitch controls safely out of the way of the power supply board. These usually mounted into the back panel but once that had been resiliconchip.com.au LG24713 this set was a “cheapie” and given its relatively low sale price (about half the cost of a similar “big-name” branded set), it was perfectly acceptable. We watched the video for about two minutes and then the screen suddenly flickered and quickly faded to black, although the sound was unaffected and continued along happily in the background. “That’s what it does to me too!”, the customer exclaimed. I promised again that I would do my best and the customer left to allow me to carry on with more pressing work. I subsequently returned to the TV a few days later and this time, before plugging it in, I whipped the back off. This initially involved removing the relatively heavy base and stand assembly (the two weighing as much as the rest of the set put together) before removing half a dozen large panel screws situated around the perimeter of the case. It was then just a matter of cracking the clips holding it together in between the screw points, after which the back panel lifted away easily to reveal, well . . . not very much at all. PHS U1 COMMUNICATOR 16 CH EAR MUFF* TWO-WAY RADIO HEADSET PERFECT FOR FORESTRY AND OTHER APPLICATIONS. Comes with 10 ch for licence free UHF communications but can be programmed for UHF frequencies you may already have. Great for gangs, haulers, skidders or any short range comms. Rechargeable internal battery, clear line of sight range over 1km. Ideal for training or usual forestry work. + GST NZ$368 EA CH These transceivers are type approved to AS/NZ 4295. *Not compliant for hearing protection so you must use appropriate grade of earplugs where required. PHS LTD, 1172 ARAWA ST, ROTORUA 07 348 8850 021 985 958 mapinfold<at>yahoo.com www.bike2bike.co.nz moved, had been dangling just above the power supply board via a flexible connecting strap. I also checked every flying lead connection (there weren’t actually that many) between the boards and various satellite components like speakers, external sockets and controls. All seemed to be well-connected, using the same industry-standard connecting plugs and sockets that I’d seen in plenty of other devices. I paid particular attention to all the power supply connections, including a line of 2-pin connectors that fed the LED backlights. Everything seemed in good order; I could see nothing that would account for the screen “dying”. A red herring When I started the TV again after the pause, the blue splash screen came up again and then the menu as before. This time though, I didn’t select an input and instead began tapping away on the power supply board, hoping to induce the fade-out sooner so that I could isolate the cause of the problem. This approach seemed to pay off, because when I tapped on one of the small transformers near the centre of the board, the screen went dark. So was I onto something or was this merely a coincidence? I waited the prerequisite few minutes before repeating the exercise and this time, no amount of tapping would induce the problem. I tried it again with a movie playing, tapping in all directions and then once again after switching July 2016  43 Serr v ice Se ceman’s man’s Log – continued off and waiting. I even tried physically manipulating the transformer casing (it felt very solid to me) but I couldn’t replicate the fault, which meant that it was probably a coincidence. No magic hammer When the screen did eventually fade out, I powered it up again after a suitable interval and this time hit the other boards with my plastic hammer. However, nothing stood out and the screen again faded away in its own good time. As I proceeded through the testing process, I took note of how long it took the screen to fade out, in case there was a repeatable time-span. There wasn’t; it was seemingly random. Sometimes it went for just 20 seconds and sometimes for as long as three minutes. Once, after tapping the main board, it ran for almost five minutes and here was naive me thinking my magic hammer had done the business. In the end, the bottom line was that I couldn’t find any mechanical fault that caused the screen to die. This meant that magic hammers are for the movies and I’d have to look further afield and utilise my troubleshooting skills to discover the cause of the problem. Before proceeding further though, I stripped all the connectors from the power supply board and removed it from its mounts on the back of the panel. I then took it to my other workshop and proceeded to examine it under my microscope, looking for fractured or incompletely soldered joints, etc. However, it looked to be very well made and a quick Google search for the part numbers revealed that this same power supply graces many smart TVs, including models made by Blaupunkt and Seiki. A replacement power supply on eBay was only about $US30 but I wasn’t convinced that that was where the problem lay. In fact, more Googling revealed information posted on TV repair forums that indicated that the most likely cause of this issue was the T-Con board. This is the smallest board in the set and it connected directly to the screen via two large, flexible cables. I removed this board and gave it the microscope treatment but once again, nothing obvious was apparent. A web search turned up several used-but-guaranteed replacements for Servicing Stories Wanted Do you have any good servicing stories that you would like to share in The Serviceman column in SILICON CHIP? If so, why not send those stories in to us? In doesn’t matter what the story is about as long as it’s in some way related to the electronics or electrical industries, to computers or even to car electronics. 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. 44  Silicon Chip only $40, so I hit the buy button and got one on the way. This component is also used in Blaupunkt TVs and as that brand has a good reputation for quality, I wouldn’t consider it secondtier hardware. The new board arrived in just five days (not bad from the USA) and I installed it in minutes. This time, when I fired up the TV, it ran for 20 minutes but that was hardly cause for celebration. At the end of that time, the screen flickered to black. It would then recover seemingly at random but would only run without glitching for a couple of minutes at the very most. A faulty board? That was a real blow because it meant that I was dealing with something else, unless I’d had the misfortune of getting a faulty secondhand T-Con board. However, at this stage it wasn’t worth sending it all the way back to try another one, even though the vendor kindly offered to swap it for me. He was surprised at the outcome and had been reasonably confident that it would have fixed the issue. I went back to the power supply board and started taking voltage measurements at various points when the screen went dark, comparing these with the voltages obtained when the screen was working. I found that the backlight voltage remained relatively stable, although it did drop slightly when the backlighting went dark. By now, any talent I have for repairing TVs had long since been exhausted and so, clutching at straws, I ordered another power supply. During this process, I’d kept the customer up-todate with what I was doing and he’d agreed to cover the costs of any parts I needed to get it going. Because the TV was a cheap model, he figured that he had a little leeway as far as spending money on it went and thus he could afford to take a gamble and buy these less expensive parts for it. Once again, the power supply arrived quickly and I soon had it fitted into the set. And once again it worked fine at first but the screen soon went black and that was that! Love lost I was seriously starting to fall out of love with this thing by now. Not only was it taking up room in my workshop but it was also soaking up a lot of my time and it wasn’t the simple fix siliconchip.com.au I thought it would be. The input board was the only part left that I could easily replace and although I tracked down a replacement, it was new and a lot more expensive than the other parts. What’s more, from what I’d read in the online forums, it was unlikely that this board was the problem and this was borne out by even more research I did on the issue. Apparently, this board couldn’t “do” anything that would cause the backlights to go dark. It could fill the screen with artefacts and other related faults but not make it go black, according to the “experts” on the forums and the various parts guys I talked to when buying the other replacement boards. I also had a gut instinct that I’d be wasting my time and my customer’s money by swapping that board out with a new one. And then I discovered some interesting information on one of the forums. I stumbled on this while searching for scraps of information about the various boards used in this TV and learned that while many 55-inch sets utilised the exact same power supply, some of the cheaper sets shipped with bad LED arrays. When they went dark, repair technicians assumed that the power supply was the problem but it was actually a screen issue; or, more accurately, a problem with the LED backlights. This made sense; if the various circuit boards were first rate, one place left for the manufacturers of these second-tier products to cut costs would be the screen. From my research, I knew that the boards were good enough for brand-name sets, which made it even more likely that the problem lay in the one part I couldn’t easily replace – the screen! I was now going to have to break this news to the owner who was not going to be a happy camper. The fact is, we’d have to buy a whole new screen assembly to get this TV going and while there may be one available somewhere out there, I haven’t looked for one as my time and the customer’s expenses so far have edged this repair into the “not economically feasible” bracket. The best I could do was recommend that he place a wanted ad on some of the local sales sites on the web, in the hope that someone has a dead version of the same model TV but which has a good screen. Provided the dead set was cheap enough, we could swap the good bits out of his set into that one. In addition, he could then advertise any leftover good boards for sale while he was at it. You win some and you lose some; that’s the servicing business! CPR on an Engel fridge Compact fridges capable of running off mains or battery power can really come in handy. B. C. of Dungog, NSW recently resurrected one such unit that had been rescued from the recyclers . . . Deciding that it might make a good project, a friend recently bought an Engel 240VAC/12V DC (Model MT45FG4) refrigerator at the recyclers. The leaves and mud were washed away and we carefully dismantled the unit. Eventually, an enclosed metal box was removed and after the four connecting leads were unplugged, it was eased out from under the external condenser. This box, which contained all the electronics, had been beside the compressor (which is basically a solenoid pump). With a grin, he handed it to me and said “what’s inside is your department”. At the bottom end of the box was an input connector for the 240VAC, along with a 12V DC input which is protected by a blade type 10A DC fuse. Removing a number of screws and pop rivets subsequently revealed two rectangular PCBs and some flood residue inside. Everything was then thoroughly washed in warm water, after which the two PCBs and the input connector block were immersed in methylated spirits for a few hours. All these were then allowed to dry in the sun for a number of days. Switchmode supply Closer examination of the two PCBs then revealed that one was a 240VAC SMPS (switchmode power supply), while the other was the compressor driver unit. My main concern at this stage was to try to ensure that the switchmode transformer (T1) was completely free of moisture. I also removed the silicone insulator (which encased Q101) to ensure it was dry inside. The compressor driver PCB had all opentype toroidal coils fitted and it had fully dried out. At this stage, we decided to test-run the refrigeration unit from 12V DC, New WiFi-enabled Development Boards Cytron ESPresso • Value ESP8266 solution • Arduino-compatible WiFi • Onboard power regulator $12.95 inc GST RedBear DUO • WiFi and Bluetooth LE • Arduino, JavaScript and Python support • Includes Particle.io cloud services $45 inc GST Local stock! • $5 delivery • Visit tronixlabs.com.au support<at>tronixlabs.com • PO Box 313 Mooroolbark 3138 • Latest updates on twitter - follow <at>tronixlabs siliconchip.com.au July 2016  45 Serr v ice Se ceman’s man’s Log – continued Sobering up a groggy pH meter A little wine is supposedly good for the heart but not when it gets into the heart of a pH meter. P. E. of Heathcote, Victoria was able to cure the groggy instrument of its hangover . . . I was recently given a “simple” job to repair a Hanna Instruments HI 8314 pH meter, which was used to measure the pH of wine. I was told that it originally cost several hundred dollars and that it had stopped working. Well, that narrows it down! The instrument looks like a fancy multimeter and is used in a similar way. My first thoughts were that copper and wine don’t mix and while it may have been an expensive instrument, it wasn’t very well sealed from liquids. The buttons on the front were similar to a modern dishwasher and while they were well sealed, the calibration adjustment screws at the bottom would allow liquids (including wine) to enter inside the case. It was a simple matter to open it with power coming from a DC output socket in the back of his Landrover. The correct protected 12V DC Engel power lead had been borrowed from another member of the family but before plugging it in and running the test, a timber cover was placed on top of the fridge where the lid normally sits. The various leads from the compressor, thermostat, temperature sensor, 24V DC cooling fan and 12V DC input connector were then all plugged into the correct compressor driver PCB terminals. A digital multimeter set on a low AC range was then connected to the compressor terminals, after which the Engel power lead was plugged into the Landrover’s DC output socket. The multimeter immediately displayed close to 12VAC and a steady hum came from the compressor. After it had been running for a few minutes, I could feel the temperature getting colder on the inside condenser. And then, with the thermostat knob on position 2, the unit cut out after about 10 minutes. It subsequently cut back in again about 10 minutes later, so all was good so far. Now for the 240VAC SMPS. Back 46  Silicon Chip up; just undo four screws and pry the two halves of the case apart. It wasn’t clipped together like a typical TV remote though. Instead, I had to pry it because it was stuck with dry, gooey wine. Sure enough, the main PCB had been awash with wine and this wine was now dry and had done its deed on the copper tracks and on the two 100kΩ multi-turn trimpots used to calibrate the instrument. Two copper tracks had gone completely but I was able to see where they’d been due to the green solder mask. Removing the trimpots was like pulling teeth as the PCB is doublesided with plated-through holes and the pots were also stuck fast with wine goo. I ended up destroying another track on the board getting them out but it was easy to replace this and the two missing tracks with fine insulated copper wire. I then soaked the PCB in methylon the workbench, I replaced the 33µF/35V electrolytic capacitor near the M51995 IC (IC101), as its measured ESR was borderline. I then removed the T3.15AMP ceramic fuse (F101) and soldered a test lamp (240VAC/60W) in its place. That done, a dummy load (2 x 470Ω 5W wirewound resistors in parallel) was connected to the DC output terminals. The original 240VAC input connector and lead were then plugged into the other end of the PCB and an IEC power lead plugged into the outlet of a 240VAC-to-240VAC isolation transformer. Switching it on The time had now come to switch it on! When I did so, the test lamp lit briefly on the input surge and the multimeter displayed +39V DC at the output. I tried adjusting the 1kΩ trimpot at the output end of the supply but this only allowed the output voltage to be varied by about ±1V. At this stage, I decided to turn the power off and draw a mud-map of the output circuit. There were two PC123 optocouplers, one set up as an over- ated spirits for about 10 minutes and then very gently scrubbed it with a small paintbrush. Eventually, the goo all came free and, fortunately, there didn’t seem to be any more damage to the copper tracks. I also gave the second, smaller PCB that held the lead sockets a quick clean in metho (although it already looked clean) and tested the connections from the leads to this board. One of these connections was open circuit but was easy to fix by carefully bending the contacts using a small screwdriver and spraying on some WD40 for good measure. Fitting new 100kΩ trimpots was child’s play compared to removing the old ones. The battery lead was also replaced, after which the plastic case was given a good clean with soapy water and everything allowed to air-dry in the sun. After reassembling it, I was greeted with an LCD screen with numbers on it so it was all looking good. I had no idea how to use it but the owner subsequently reported that it works. I advised him to keep wine well away from it! voltage protector via a 47V zener diode (ZD103) and the other controlled by an LM431 precision voltage reference and an 18V zener diode (ZD102). There was also a voltage divider set up on the input pin of the LM431. I “googled” for an LM431 data sheet and browsed through the pages to find this important formula: Voltage Out = 2.5(1 + R1/R2). Using a calculator, the resistor values were plugged into this formula and I found that these gave an output voltage of close to +38V. So I had to assume that the previously measured +39V DC was close to the correct output voltage! That established, fuse F101 was then refitted and the 240VAC SMPS with its 240V/240V mains isolation transformer connected to the Engel fridge. This meant that the +39V DC was now going to go through the compressor driver PCB! The 240VAC power was turned on and the multimeter, which was still connected across the compressor terminals, displayed about 13VAC. Somehow on the way through the compressor driver board circuitry, this had all occurred correctly without any of that magic blue smoke escaping! siliconchip.com.au The fridge was then allowed to run for about an hour and it performed perfectly. After that, the two PCBs and the input connector block were shoehorned back into their box with all the associated covers, screws and pop rivets. It was then just a matter of refitting the box and soak-testing the unit for a number of hours. My friend tells me that the resuscitated Engel fridge will be used as a drink fridge in his shed and for shopping trips. As a postscript, there appears to be some conjecture about what is really inside the $8 Engel 3AG glass fuse, located inside the cigarette plug end of the 12V DC Engel power lead. I took the opportunity to find out and found a 168°C 10A thermal fuse in series with a 10A Pico fuse. These Engel fridges can also run from +24V DC but it is not recommended to run these units on 240VAC from the cheaper square-wave type inverters or from the unregulated 12V DC outlets on generators. Finally, always check for the correct polarity if you are making up DC extension leads or doing other lead modifications. There are many tales of woe on this subject on the grey nomad online forums! Penny’s air-conditioner A split-system air-conditioner that’s only seven years old shouldn’t have to be replaced if it breaks down. D. P. of Faulconbridge, NSW recently did a friend a favour and got one such unit going again . . . Recently, my wife and I were discussing the evils of the throwaway society with a friend. In particular, we were lamenting the fact that consumers are often forced to replace faulty appliances because of the difficulty and high cost of even getting a fault diagnosed, let alone repaired. Who knows what otherwise potential gems with minor faults are now languishing in the nation’s landfills, or worse still, have been shipped off to third-world countries where they are dismantled for scrap in dangerous sweatshops? Gone are the days when your dead toaster could be taken to your local friendly electrical shop, where the element would be replaced, on the spot, for a few dollars. It was during this conversation that Penny mentioned the case of her air-conditioner. It was a split-system siliconchip.com.au and had simply stopped working one day. Their children were in the room when it failed and reported that they had heard a loud “pop” from the indoor unit. A short time later, our friends asked an electrician who happened to be at their house doing other work if he would look at the air conditioner. He took one look at it and his reply was short and to the point: “Nope . . . too old!”. They subsequently found that it was impossible to get anyone to even come and look at the air-conditioner! In each case, the advice was either that the job was too small or that the unit was too old to even consider repairing and should be replaced (it was about seven years old – plainly a dinosaur!). So our friends were left with the prospect of replacing the whole system at considerable cost, despite the possibility that it may have had only a minor fault. As well, this was one of two identical NEC units that had been installed in the house at the same time and both had done about the same amount of service. The other unit was still functioning normally, so the catastrophic failure of a major component in the faulty unit seemed unlikely. Understandably, they hesitated to make a decision and so the dead air-conditioner languished where it was for quite some time. When I heard this, I volunteered to have a look at their air-conditioner, to see whether or not it was fixable. I didn’t promise anything but from what the children had reported, it sounded suspiciously like something electronic which meant that the repair could be straightforward. I knew that these indoor units contain an electronics module that controls the fans and the compressor and that this module usually includes a microprocessor, various sensors, some relays and other electronic-type bits and pieces. There is also typically a switchmode power supply with its highly-stressed electrolytics, as well as various other capacitors and relays carrying serious current in there. In short, there are plenty of things that might go “pop”! When I arrived at the scene, the first thing I noted was that the air-conditioner wouldn’t even try to start. In fact, there was no sign of life at all. I checked the circuit breaker on the power board and it was on, so I turned it off before The component side of the bottom PCB in the NEC air-conditioner looked pristine but the underside was a different story! The black rectangular component at top right is the compressor relay, while the grey cylindrical part next to it is the T2 fuse. starting work on the unit. Dismantling the indoor unit to get at the electronics module was easy enough. The outer cover was secured with three screws along the lower edge and after undoing these, the cover then had to be detached from snap-on mouldings along the top edge. I managed to do it without breaking anything but it would be great if more manufacturers would include markings, such as arrows, to show us where the snapon mouldings are. Having removed the cover, I then turned the circuit breaker back on so that I could check that power was getting to the air-conditioner. This proved to be the case so whatever was wrong was definitely in the unit itself. I then turned the circuit breaker back off so that I could take a closer look at what was going on. Several layers The electronics module was in a plastic box which contained several layers, the top layer carrying only mains terminals. The next layer consisted of a PCB which carried a microcontroller and numerous surface-mount components, while the bottom layer consisted of a second PCB, this time carrying through-hole components. This second PCB looked like a switchmode power supply and carried several electrolytic capacitors, several AC-rated capacitors and various relays – in short, all the usual suspects! I was expecting, or more precisely, hoping, to see something obvious like July 2016  47 Serr v ice Se ceman’s man’s Log – continued This view shows the underside of the switchmode supply after it had been cleaned up. The burnt-out circuit board track with the relay pin in the middle is circled in red. an exploded electrolytic capacitor but as I delved deeper into the innards, my hopes were rapidly fading. Everything looked pristine but when I removed the bottom PCB and turned it over, it was a completely different story. The top of the board was completely clean but about a third of the underside was covered with thick black soot. It was impossible to determine the source of the soot at this stage and all the parts on the top of the board looked perfect. Perhaps something had once been underneath the board which was there no more? Missing solder pad It all became clearer once I had cleaned up the underside of the board, however. There was a relay marked “RY – COMP” (compressor relay) on the board. And where one pin of this relay had once been soldered to a PCB pad, there was now just a hole. The pad was missing! The pin was still there though, apparently undamaged, but was now surrounded by empty space. There was no sign of solder, this apparently having been completely vaporised along with several millimetres of copper surrounding the pin. This track had once connected the relay to the compressor, so it carried the full compressor load. As a result, I bridged the gap with copper wire and plenty of solder, making sure that the solder had wetted everything properly so that I had a good, low-resistance connection between the relay and the compressor. The question was, what had caused the failure of the original soldered 48  Silicon Chip joint? Was it a power surge, due possibly to a lightning strike (quite common in this area)? Or had the relay pin never been properly soldered (a “dry” joint) so that over time, a cycle of progressive heating and oxidisation had eventually produced a high resistance joint which generated enough heat to melt the solder completely? Or had the heating originated in the relay itself, due to increasing contact resistance? The other question was, whatever the process, had the relay itself survived? There was no sign of its plastic body overheating but I reasoned that its contact resistance could be high without any damage being visible on the outside. I was not able to find any data on this particular relay but looking at the specifications for similar devices, it seemed that the typical contact resistance for this type of relay was less than 100mΩ (milliohms). I didn’t have anything that could measure such a low resistance value but my DMM has a 200mV range, so I figured that if I passed some current through the contacts, I should be able to measure the voltage across them and accurately calculate the resistance. After some thought, I decided to use a 12V DC supply with a car tail-lamp in series to limit the current through the contacts and to also use this same 12V supply to activate the relay. This set-up gave a contact current of 1.85A, while the voltage drop across the contacts measured 9mV. Using Ohm’s Law, this then gave a calculated resistance reading of just 4.9mΩ, so it looked like the relay was OK. However, the open circuit to the compressor did not explain the general lack of signs of life. Had the control circuitry survived the trauma? It was not going to be possible to test the entire control circuitry with it removed from the air-conditioner but I could test the power supplies. If these had survived, then there was a reasonable chance that the logic circuits were OK. There was a switchmode supply with a +12V output, which supplied the various relays. This also fed a +5V linear regulator which supplied the logic circuits. In order to check these rails, I would need to apply 230V mains to the switchmode board itself. Fortunately, I have an isolated (float- ing) mains supply in my workshop, which dates from the “hot chassis” (AC/DC transformerless) radio days and still comes in handy from time to time. It consists of two 240-110VAC transformers connected back-to-back, with a lamp socket in series with the output. I can also take the output from the first transformer only to get 110VAC if required and various incandescent lamps can be plugged into the lamp socket to limit the current to the required level. A shorting plug can be plugged into the lamp socket if full current is required. Open circuit fuse The switchmode regulator is fed from the mains via a slow-blow 2A fuse marked “T2 250V”. This fuse is a small, grey, cylindrical, vertically mounted component and looks like an inductor at first sight. Apparently these have very specific delay characteristics and are soldered in, so presumably are not expected to blow very often. I checked the fuse and it was open circuit, so apparently it had received a fair surge and it remained to be seen whether it had effectively protected the switchmode supply. I didn’t have one of these specialised fuses on hand but I figured that by bridging it out temporarily and by using a 15W lamp to limit the current, I would be able to determine whether or not the power supply was working. I connected it all up, connected the control board to the supply and applied power. The lamp didn’t glow, no smoke came out, the switchmode regulator produced a nice clean +12V rail and the linear regulator produced a steady +5V. It was all looking good, so I decided to order a new fuse and proceed to the next step – a smoke test in the air-conditioner! Thankfully, the final smoke test went well and our friends have a cool house again. But the question remains as to what caused the original failure. I am inclined to think that it was simply due to a bad solder joint on the relay pin, because internal heating in the relay would have damaged the contacts, while a power surge would probably have tripped the circuit breaker on the power board. But what caused the fuse to blow? The likely answer is that it was due to a current spike that was produced by arcing as the soldered connection was SC burning out. siliconchip.com.au