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SERVICEMAN’S LOG You win some, you lose some Dave Thompson There’s often a perception about servicing stories that our repairs are always successful, no matter how bleak things might look. Like the hero in an action movie, even though everything is stacked against us, we always emerge victorious. Still, even James Bond doesn’t win them all. Let’s face it; the reality is that most jobs we do are mundane and barely worth even mentioning. It is mostly bread and butter work that we all get in on a daily basis and are usually tasks we can do in our sleep (and sometimes almost do!). However, these stories typically never get told, because who but the keenest of knowledge-hounds wants to hear or read about that sort of thing? The stories that do get relayed are those that have had particular challenges to overcome or some clever bit of diagnosis required to make it work. It is these tales that we brag about (and often rightly so). That’s essentially what this column is all about; finding solutions to curly problems (we’ve all had them) that might be a little out of the box. We relate these situations in riveting stories, possibly adding to the great knowledge pool that has been organically growing for many years. The idea of apprenticeships and other such roles is similar; to pass knowledge from one generation to another, making it easier for the new guys coming through by tapping into an already-proven knowledge and skill base. This training may or may not cover the latest technology; those teaching might not be up to speed with the most modern of devices and tech in the workplace. That was sometimes the case at the airline I worked at; historical theory and practice, often going back to the postwar decades, was no problem. Still, keeping up with the latest trends was not necessarily a priority, especially for older engineers. In all fairness, they’d likely never need any of that in the roles they were in anyway. It happens; as many of us get older, we settle into our comfort zones and routines and simply cruise through with what we know. Constant up-skilling is often too time-­ consuming to be practical, especially if we are also expected to work away at our regular jobs at the same time. Items Covered This Month • • • • Learning when to pull the plug Detecting micro-bats Fixing an anti-barking dog collar Repairing a series of solar party lights Dave Thompson runs PC Anytime in Christchurch, NZ. Website: www.pcanytime.co.nz Email: dave<at>pcanytime.co.nz Cartoonist – Louis Decrevel Website: loueee.com 90 Silicon Chip All that aside, even if we know and are comfortable with what we are doing, sometimes things don’t work out. Knowing when to pull the plug on a job is as important as knowing when to keep plodding forwards. I hate being beaten by anything, but it happens, and we all have to deal with it in our own way. An essential part of a serviceman’s skill set is being able to ‘triage’ or assess any potential job and know whether to take it on or not. Often, we hedge our bets and go for it, only to have it come around and bite us. Such is life, and over the years, I’ve had some jobs I spent way too much time on for absolutely no reward. Monetary compensation is one thing, but the satisfaction of a job well done often outstrips that for me. Perhaps that’s why I keep doing it! Built like a brick outhouse One job that came in recently was a Yamaha home theatre amplifier. The owner is someone I’ve dealt with and done computer repairs for over many years. When he saw an amplifier on my bench last time he was in, he asked whether I could have a look at his. Apparently, it would not switch on any longer. He suspected a fuse or similar, and I agreed that I’d assess it and see what, if anything, I could do. I was expecting a normal home-theatre-type amp, maybe 50-100W per channel, the sort of thing most people have in their lounge rooms. When my client backed up the drive and opened his SUV’s back door, and I went to lift it out, he warned that it was “quite heavy”. When I saw it, I understood why; it was an absolute monster! The first thing I noticed was the vast array of RCA sockets, speaker posts and other connectors covering almost the entire back panel. The second was that I almost put my back out when I tried to lift it clear! I’ve worked with some big amps in the past, primarily sound-reinforcement PA amps with huge transformers and heatsinks, but this was the biggest, baddest domestic amp I’d ever seen. The specs on the back panel claim 500W per channel into 8W with all the Dolby and DSP that anyone could ever want. The front panel boasts the requisite huge volume knob, digital display and soft-touch buttons everywhere. The guy saw me eyeballing the amp and admitted he’d probably ‘overpurchased’ a little, but my thinking is that he likes it, and that’s all that matters. I lugged it to my workbench and plugged it into the power. Sure enough, the symptoms were what he’d reported – it Australia's electronics magazine siliconchip.com.au just wouldn’t turn on. There were also no lights anywhere and the display was dark. The mains socket on the back panel, likely used to power up a turntable or other connected device, also had nothing coming out of it. Having completed the preliminary tests, I advised him to leave it with me. I told him I’d crack the case and check any onboard fuses that might be present. If we were lucky, it could be as simple as replacing one that had blown (although it rarely is that simple). I mentioned it could also be a dud power on/off microswitch, which would prevent everything else from working, but it felt like it was toggling OK to me. I also warned him that unless I could find circuits for it, I’d just be running around in the dark looking for anything untoward. He was fine with that, as the only repair agent in the country was ‘up north’, and shipping this thing all the way up there would be a major headache. He wanted to have at least a rough idea of what could be wrong with it, as that information could help him decide how to proceed. Apparently, the agents had (very helpfully!) told him that this particular model was no longer sold, the parts were not readily available, and he’d likely end up having to buy a new one. At around $4000 for the replacement model, that’s a whole lot to lose. So, no pressure then. far, it would never work. In this case, I measured 235V AC at the socket and into the transformer’s primary. A good start! The problem was, I couldn’t pick up anything at the secondaries. Power going in, nothing coming out can only point to one thing, which was bad news. Especially because this transformer had four separate secondaries that I could see, and none were live. Of course, I couldn’t find any numbers on the transformer, and I couldn’t find any circuits on the web for this unit. Likely it was a proprietary transformer, and while they might have used the same components in other large amps (it turns out there was quite a range of them), I couldn’t find anything on the usual auction and sales sites that was going to replace this one. There were no inline fuses or anything of that nature that I could see – it was all very much a classic meat and three-veg setup. So, sadly, that is where my involvement ended. I put the covers back on and told my client that if he wanted it repaired, he’d have to send it to the repair agents and take his chances that they could be bothered to locate a transformer for it. I’m sure the parts are out there; like any closed shop system, the repair agents will likely have access to all of them, and the circuits. If they’ve been doing this for a while, they might even have a suitable transformer under the bench in a dead unit. Because I don’t have access to any of those parts or information, that was pretty much it, as far as I was concerned. Sourcing parts is getting harder This happens all the time for us servicemen, even with computer repairs, especially with the likes of Dell or older HP machines that used proprietary hardware. Getting any components for them was always an uphill battle. I did manage to source a lot of stuff from websites like AliExpress and eBay, where people buy up old machines, strip them down, vet the parts and then on-sell them, but Dave and ‘Goliath’ I started the usual way, by removing the covers. It was beautifully made; on many similar devices, the covers are often like guillotines, ready to slice me open if I mishandle them slightly. Not so this one – all the metal edges were rolled and smooth. The interior was the same, with the cabling all beautifully routed via clips and channels, and the circuit boards packed in tightly. I can certainly see why it cost so much. The layout is pretty much the same as any big amplifier – power supply and transformer off to the right, preamp boards as close to the input sockets at the rear as possible and the huge heatsink and power amp board spanning the whole case near the front. The first step was to track power through from the mains input socket to the transformer. If it wasn’t getting that siliconchip.com.au Australia's electronics magazine May 2023  91 it’s not an ideal solution. I’ve bought many a rare motherboard from vendors like that, only to have it arrive and discover it is faulty or doesn’t work at all. These days, I assess and don’t even bother offering that option, mainly because of the blowback that invariably comes from going down that repair road. The last part I ordered from China took over 15 weeks to arrive, with the client calling every other day to see if their machine was finished. For most of us, this hassle just isn’t worth the grief, so we pull the plug before it gets to that point. Recently, I’ve had several e-scooters, dashcams, musical keyboards and dead laptops that I’ve passed on repairing for this very reason. Beastly laptop update One laptop I did take on warrants a mention just because of the fiasco it turned into. If this story sounds familiar, it’s because I initially described it in the July 2021 issue, but more has happened since then, so here’s the whole sorry saga. This machine was a gaming laptop purchased in the USA. It was a massive thing made by Dell’s skunkworks department, with a 19-inch screen, and to this day still the biggest laptop I’ve ever seen. It was not the sort of machine you’d want to carry around because it weighed a ton, and the size and weight meant that the carry bag and power supply to support it were equally massive. It was designed more as a gaming desktop replacement machine and, as such, boasted a fast Intel mobile i7 CPU, 32 gigs of RAM, two hard drives in RAID0 configuration and dual removable graphics cards – an unusual feature in a laptop. This machine had given a good few years of service after the guy brought it home, but now it had given up. It wouldn’t boot, no lights. The power supply checked out – one of the traps to look for when people bring a machine in that won’t boot is that the power supply has failed, the battery has gone flat and, of course, it won’t start up. Plenty of repair people tell a customer the machine is dead when all it has is a flat battery. Sometimes I can simply replace the power supply and off it goes. Sadly, in this case, the PSU was OK. I also had a similar (but smaller) Dell supply that would do the job, though due to how those machines worked, even if it powered it up, the laptop would likely report I had the wrong supply connected every time anyone tried to start it. That’s a real annoyance for people who need to replace their dead original supply with a third-party one! 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? It 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 cars and similar. 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. 92 Silicon Chip Yet, it still could be a battery fault causing the problem, so the next step was to crack the case, extract the battery and measure that. It seemed to be charged; this wasn’t looking good. After removing the hard drives and RAM and reseating the CPU, it still wouldn’t power up, so I could only advise the client that the motherboard was the likely suspect. Most people would have called it there and then, but as this guy had so much invested, he wanted to see if we could replace the board. And we could; while Dell was of no use at all parts-wise, I could get a refurbished board from China. It was expensive, but delivery was just a few weeks back then, so I ordered it and fitted it when it arrived. The machine booted and ran quite happily – for about four months, when it came in again, this time with a new problem: no video output. It seemed the video cards, which had been swapped into the new motherboard from the ‘old’ setup, had failed. I tried swapping them from one side to the other to no avail. The onboard video worked, which was weird, but it wasn’t accelerated and was no good for gaming. So, I ordered two refurbished graphics cards from China at a considerable cost ($400 each!). When I installed them and powered up the machine for the first time, actual smoke came out of one of the graphics cards, and there was still no video output. Great. I removed the one that let out the smoke, and the machine started. Both the client and I were getting jaded by now. Fortunately, the games he played worked well with just one card, and that’s the way it stayed for another six months. Then it was back in the workshop – not booting. Once again, it looked like a motherboard issue. By now, both my patience and the client’s resolve were wearing thin. Aside from the warranty side of things, which, to be fair, he was very philosophical about, I was done with this machine. It had cost me a lot of time and money; I hadn’t charged him what I really should have to make it worth my while, so it was time to pull the plug. I saved all his game data, broke the machine down into components, and he sold off what he could while sourcing a new replacement. Sometimes things don’t work out. Success stories are great, but servicemen and women must accept that sometimes, there’s nothing more you can realistically do. Knowing when to pull the pin is a very valuable skill to have. “Micro-bat” detector repair A. E., of Newcomb, Vic went a bit batty trying to determine whether his ultrasonic detector repair was successful. He used one key technique to determine that it was... It’s hard to repair an electronic device of an unusual type, perhaps almost unique, when it fails and there’s little or no documentation. This was my predicament when a biologist showed me a small battery-operated device about the size of a pocket radio. It was a detector that had been acquired to find very tiny bats in Tasmanian forests. Zoologists call them “micro-bats”. Though smaller than a thumb, they will hunt moths larger than themselves, clinging on to them as they try to fly away! You can find more information on them at siliconchip.au/link/abjt From the biologist’s account, the device had been made an ocean away (in North America) some time ago and was unencumbered by technical specs or a circuit diagram. Australia's electronics magazine siliconchip.com.au A selection of our best selling soldering irons and accessories at great Jaycar value! 25W Soldering Iron TS1465 $16.95 Build, repair or service with our Soldering Solutions. We stock a GREAT RANGE of gas and electric soldering irons, solder, service aids and workbench essentials. 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Jaycar reserves the right to change prices if and when required. Fixing an anti-barking dog collar At least the principle of its operation was known – a microphone that could register ultrasounds up to perhaps 100kHz was followed by an amplifier, then a frequency divider that would shift the ultrasounds down into noises that we could hear. The battery checked out OK, and there was even a reasonable current drain of over 10mA when the detector was turned on. But the only sound emitted was a very faint hiss. Where was I to start? Tracing the input signal showed that the small electret microphone fed the signal into a two-transistor preamplifier and then into a comparator that was followed by either a mixer or frequency divider. Checking the collector voltage of the first transistor, I found that it was less than half a volt, well and truly in saturation. I wondered whether the detector’s builder had chosen the wrong biasing resistances. Had the wrong transistor been inserted, or had some component values drifted over time? A simple change to the biasing turned out to be all that was needed. When a change to the bias of the input stage raised the collector to a few volts, the detector started to show some signs of life; there was a hissing noise from the little internal speaker and even some crackling sounds. So then the question was, would the detector actually betray the presence of 40-50kHz squeaks? I can whistle a bit, but not anywhere near that frequency. The answer wasn’t long in coming, though. A co-worker in the lab entered a long corridor nearby and pulled a ring of keys from his pocket. The bunch of keys, although more than twenty metres away, and around a corner, caused a riotous burst of sharp sounds from the detector. The keys only made a faint jangling sound, but the detector made their ultrasound components loud enough to be uncomfortable to human ears! If you ever need a broadband ultrasound source, there’s the key. The happy owner of the device subsequently took the detector out into the Tasmanian bush and reported later that he was able to find some of the elusive little bats. 94 Silicon Chip D. S., of Maryborough, Qld had a repair job that turned out to be very obvious and very easy to fix, which made the customer happy and put a smile on his bank manager’s face... Sometimes, but not often, a job comes in that turns out to be a simple fix. Also sometimes, we miss the obvious and look for a much more challenging solution. The job was an anti-barking collar for a dog. This collar detects the dog barking through a small microphone adjacent to the dog’s throat and then does a couple of things to deter or stop the barking. Firstly, the collar vibrates when it detects barking. If this vibration does not deter the dog, it vibrates again a lot stronger after a 10-second delay. Continued barking will force the third stage, which is an electric shock. The shock is delivered through two metal prongs that press against the dog’s throat. Before I am told how cruel this is and that there are many other less cruel deterrents, this collar is not mine! As the owner of two large dogs, I understand the folks that would not want to put their dogs through this, including myself, but many councils now have very strict laws regarding barking dogs, especially nuisance barking, where a dog barks for long periods. Continued nuisance barking can bring harsh penalties to owners and, in extreme cases, result in the dog being put to sleep. So, a small shock from an anti-barking collar could be preferable to the alternative. This collar is powered by a small 3.7V lithium-ion rechargeable battery inside the collar that can be recharged through a small USB-C socket on the collar’s body. The USB socket is protected by a tight-fitting silicone cover that seals dirt and moisture out when fitted. The battery lasts for several days unless the wearer is barking a lot. The collar has various options, such as vibration duration, shock strength and sensitivity, controlled via two touch-sensitive dimples on the front of the collar body, which also switch it on and off. The option and settings are shown on a tiny two-digit display. When I opened the collar, I found the battery to be fully charged but the collar was lifeless. It is made up of two PCBs. The main PCB contained all the power and charging circuitry along with the boost circuitry and vibrator motor, while the other was the controller. Shock Probe MIC Shock Probe The internals of the dog collar and where the shock probes are located. Australia's electronics magazine siliconchip.com.au ONLY 289 $ QM1493 Specialty meters combined with multimeter functions. 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Jaycar reserves the right to change prices if and when required. I began by tracing the battery voltage across the main board where, after a degree of filtering, it left the main PCB through a ribbon cable over to the control PCB, which also contained the microcontroller. However, there was no power to the micro. In fact, no power was reaching the control PCB at all. It did show on the PCB connector for the ribbon cable on the main PCB, so I checked the control end of the ribbon cable for power. I found a lot of corrosion on both the ribbon cable and the PCB connector on the control PCB. So out came the alcohol, cotton buds and a small, stiff brush. With such small components, it took me about 20 minutes to remove the corrosion from the cable and its connector. I also removed the control board from the top shell to check for any damage below the connector, and the main PCB to check for the same. The main PCB was a bit tricky to remove due to the seals around the openings where the shock prongs went through the case. In the end, everything checked out fine, and I refitted the ribbon cable and gave everything a quick squirt of electrical sealant. I did notice a failure of the case seal where the two halves of the case joined, so after cleaning it, I resealed the case with a little neutral-cure silicone sealant. Touching the power dimple brought forth a pleasing beep, and the display showed the remaining battery time. To test the unit, I barked at it (much to the surprise of my dogs), and it did indeed vibrate. A second bark gave a much stronger vibration. At this point, my dogs decided that I was barking at them, and they barked back at me. This meant the collar did as programmed to do, giving out a shock. As I was holding the collar (and not intending to bark for a third time), I received the shock! The shock was nowhere near as powerful as the one you might receive from an electric fence, but it is still a shock, and is worse when you are not expecting it, as I wasn’t. 96 Silicon Chip The resulting “yowch “ from me and the bang of the collar hitting the surface of my bench was enough for my dogs to turn tail and make for the house. I gingerly picked the collar up and turned it off. I have to admit, when I told the customer the story, we both laughed, and he was more than happy with the repair cost (which was very little, as I hadn’t done a great deal). I can still hear my wife laughing when I told her what happened, and I believe both dogs received an extra treat that night. Lights out for the solar party A. R., of Greenbank, Qld decided to take on one of those repairs that seemed like it would be something simple, but it actually turned out to be a rather confusing manufacturing fault... My son called to ask if I could fix his solar panels. I hesitated, knowing they are not easily repairable. I asked him what kind of solar panel; he explained that his solar-­ powered party lights weren’t working, so he presumed the solar panel was faulty. I offered to take a look. The solar party lights consist of a control box about 140 × 100mm with a solar panel embedded in one face. A 200mm-long cable emerges from one side with a sealed plug/socket on the end. A string of light fittings is connected to the plug. Each ‘light bulb’ consists of a socket with an Edison screw bulb, and the whole assembly is well sealed and quite well made. The ‘bulb’ is a small glass envelope with an LED bar inside. I removed a bulb and noted 24V printed on the side. That was not quite what I expected, so I carefully applied a DC voltage to the lamp, assuming positive went to the tip, and it lit up nicely with a reasonable power draw, confirming it was indeed a 24V lamp. The control unit has two sealed switches on the rear, on/off and mode, and a small dark device shaped like an LED. I unplugged the light cable assembly from the unit, removed the eight screws holding on the rear panel, and opened the case. This revealed a control board and a sizeable 3.6V lithium cell. The solar cell and battery cell connecting wires were soldered to the board. I found the battery voltage to be about 3.2V. My next step was determining if the solar panel was charging the battery. I took it out into the sun and measured the battery voltage, which was slowly increasing. So the solar panel was OK, and so was the charging circuit on the card. With no lights plugged in, I pressed the on/off switch to turn the unit on. There are two small green SMD LEDs marked W3 and W4 at the top left of the board. These started flashing alternately. Pressing the mode switch changed the flashing pattern on the LEDs, which were obviously mirroring what was supposed to be happening with the light string. I tested the continuity of the cable to the external connector. This and the socket checked OK. I applied 24V to the plug on the light string, and the lamps lit up. So what was happening? I pressed mode until I got two steady green LEDs, then measured the voltage across the output. This gave 3V DC, which was not what I expected, so maybe the output driver stage was faulty. I guessed that the surface-mount transistors marked Q1, Q2, Q4 and Q5 were probably part of the output stage and, thus, the obvious suspects. I measured them with my multimeter set on ohms and, Australia's electronics magazine siliconchip.com.au Ventilation Fans We stock a wide range of DC and AC powered enclosure fans to keep your projects cool. A GREAT RANGE AT GREAT PRICES. 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Jaycar reserves the right to change prices if and when required. Left: the internals of the control unit for the LED ‘party light’ system. Note the object with the white ring around it; its purpose is unclear. Right: a section of the traced circuit for the control unit. after some trial and error, found two NPN and two PNP transistors with standard SMD footprints, all apparently OK. I was starting to get suspicious because something obviously wasn’t right. There had to be some clever device converting the 3.6V from the battery to the 24V required by the lamps, and I assumed this was the smaller IC’s function. So it was time to trace out the circuit. I had to remove the large on/off switch to see where the traces under it went. The board has two ICs with all markings completely removed and numerous passive components. There are also several components not fitted and, curiously, an SMD LED located under the switch with one end not soldered to anything, and the other end soldered to one pad of a component marked C3. One of the components not fitted was marked L1, an inductor I would have expected somewhere in the circuit of a DC/DC converter. In the bottom right-hand corner of the PCB were two manufacturing marks, one for 3V and one for 24V, and the 24V mark was clearly selected. So the control board was supposed to be set up for 24V from the factory. The larger controller IC is powered via the on/off switch. The output from the controller drives a bridge circuit, including transistors Q1, Q2, Q4 and Q5. The supply for this driver is from the battery via R12, a 9.1W SMD resistor. This arrangement clearly cannot ever deliver 24V. The smaller IC marked U1 appears to control the charge current to the lithium-ion cell to stop overcharge and overdischarge. With power on, I measured the power supply to the controller and the output driver at 3.2V, the battery voltage. It was clear to me now that this device was manufactured incorrectly. Despite being marked as 24V, it was actually set up for 3V and could never have worked from the factory, obviously never having been tested before shipping. I quizzed my son further. He confessed he had bought the lights from an internet marketplace, and the seller assured him they worked; they just didn’t need them any more! A likely story... I assumed the missing components must provide the DC/DC converter function. Could I get this working? The adjacent circuit fragment shows the missing components, with L1, C3, Q3 and D2 forming a DC/DC boost converter. The controller feeds a square wave to switch Q3 via R5/ C4. When Q3 switches on, current flows through L1, storing energy in its magnetic field. When Q3 switches off, 98 Silicon Chip the energy stored in the magnetic field causes the current to continue to flow from +3.6V via D2 to C3, increasing the voltage across C3 above +3.6V. The duty cycle sets the voltage across C3. I had no values to go on for any of the components. I fitted an NPN SMD transistor from my recycled components box for Q3, and a through-hole schottky diode in place of D2. I hunted around and found some inductors salvaged from an old TV PCB, fitting a 1μF capacitor for C3. I also removed R12, disconnecting the driver stage supply from the battery. It should now be driven by the voltage across C3. At the same time, I removed drive resistor R14 to the SMD LEDs, which at 2.7kW might not be suitable for 24V output, and replaced it with a 33kW resistor, as well as changing two of the 2.7kW resistors in the driver stage to 15kW. I crossed my fingers and powered up. With my oscilloscope, I could see that Q3 was now fed with a square wave from pin 5 of the controller, and the voltage across C3 was about 5V. I was on the right track. After many hours of trial and error, I found that a 1mH inductor for L1 and a 10μF capacitor for C3 worked quite well. After starting at 10kW for R5 and 100kW for R8, I reduced R5 to 1.5kW. A value of 100nF for C4 squares up the switching waveform of Q3. The result is about 22V across C3. Not quite 24V, but to get any more would need a wider switching pulse width from pin 5 of the controller IC, and there was no control over this that I could see. The W3 and W4 SMD LEDs are pretty bright, which is interesting given that they have less than 0.7mA drive current and are not even visible outside the box. The main LED string now lights up quite brightly and operates as expected. This was not quite the journey I had expected at the start! One mystery remains – the purpose of the sensor with the white collar. It is not to stop the lights from operating during the day; another part of the circuit senses the output from the solar panels and feeds an input to the controller to achieve this. The output of this sensor goes to the controller and is always high. Exposing it to light or completely shading it makes no difference, neither does any movement in front of it. So that is a mystery for another day. SC Australia's electronics magazine siliconchip.com.au