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SERVICEMAN'S LOG Decisions, decisions, decisions... Dave Thompson Ford or Holden? CD or vinyl? Mica or polystyrene? Hybrid or electric? Digital or analog? PC or Mac? Petrol or diesel? Topics like these generate a lot of debate in workshops, pubs and internet forums. While there are no correct answers, this doesn’t usually stop us from holding strong opinions. Which brings me to my love of analog multimeters... I’d like to think I’m not the only one who ponders these important philosophical questions. Servicemen of my generation have had the good fortune to have plied their trade in an era of almost unparalleled technological growth. Believe it or not, there are still people around – they are admittedly getting on now – who cut their servicing teeth on valve-based hardware. They then had to ‘upskill’ to stay current as transistors and integrated circuits became more commonplace, while tubes disappeared into history (and expensive guitar amplifiers!). Many industries lag behind the “bleeding edge” due to the systems involved, meaning there can be considerable overlap in old and new technologies. I started work at our national airline as a know-nothing slip of siliconchip.com.au a lad in early 1980. Even then, there was still some valve-based aircraft hardware being serviced in the various avionics workshops. Don’t get me wrong; it wasn’t a lot, but because at that point there were some relatively old aircraft still being maintained by the airline, I did get to work on a few tube-based components of that era. Remember, this is “only” 40 years ago, and there were many other areas aside from the aircraft industry where vacuum tube technology was still in wide use (broadcasting and music amps to name a few). While it might seem old-fashioned by today’s standards, I was experiencing the Australia’s electronics magazine tail-end of an era of huge technological growth brought about by World War 2. To the people involved at the time, it was just as awe-inspiring as anything we see happening today. The evolution of digital displays Digital displays are one example. Of course, they were around even in my earliest days of dabbling in hobby electronics. In my early days, I saw the exotic (for the time) Nixie tubes, which could spell out numbers and letters, and it seemed like the devil’s magic. Nowadays, Nixie tubes are considered retro-chic and while relatively expensive, such is their popularity that they are still being manufactured. October 2020  61 I still clearly recall seeing my first LED display, a red bubble-style arrangement on a clunky HP calculator my dad bought in the mid-70s. Aside from the mind-boggling capabilities of the device itself (four functions!) with that LED display, the whole machine seemed nothing short of miraculous. I’d seen calculating machines before, and had even built a crude model as part of a team of school pupils for an early science-fair project. However, that device used switches, potentiometers and analog meters to calculate and display basic mathematical functions. While it worked, it was underwhelming, and didn’t win any prizes. It did demonstrate the basic principles that all modern computers run on though (royalty cheques accepted!). The biggest issue was reading those inherently inaccurate analog meters and trying to analyse the results of our calculations. It would have been so much easier if we’d had a digital readout. That is why I was so impressed with that HP calculator; tap in your figures, press a button and there it was; even the dimmest among us could read directly from the display. Shortly after that, I saw my first digital watch. These so-called “moon watches” were unbelievably modern and a much sought-after accessory. At the press of a tiny button, the time (and day and date in some versions) was displayed on a miniature red LED array for around five seconds before going dark. This was a trade-off between functionality and battery life; the tiny ‘watch’ batteries of the day would soon run out, so the time was only displayed briefly at a press of the button. These watches were so über-cool that everyone who saw one immediately desired one, though not many could afford them in the early days. Nobody really wanted to wear those old-timey analog Rolex, Citizen and Seiko watches anymore; all that mattered was having a timepiece with a sleek stainless-steel body and a mysterious LED display! As time went on (LOL!), prices fell, especially with the advent of back-lit liquid crystal displays, whose powersaving properties and increased functionality made moon watches old-hat almost overnight. But those early LED watches are now a sought-after item, with the hipster crowd especially prepared to pay big money for original models. The sad fact is that many of those old-style LED displays are now so weak as to be unreadable because (like me) time has robbed them of their glamour. Analog vs digital: this time it’s personal From an electronics measurement point of view, digital displays were regarded as revolutionary. Way back when, I only used analog meters because that was all that was widely available. Those of a certain age will recall those large, heavy Bakelite Avo-style multimeters (and their clones) that cornered the market in the 60s, 70s and 80s, before the likes of Fluke and others popularised the digital multimeter, driving analog meters increasingly out of fashion. On the face of it, having a digital meter made sense. For one, you could read the exact value on the display, so there was no misinterpretation of the results, or pesky parallax errors. And you could see it in the dark, which alone was bordering on voodoo to many servicemen. Many digital meters also featured a ‘hold’ function, meaning you could measure in cramped quarters and extricate yourself before checking the results on the meter. This was something just not possible with the analog meters of the time. This is what they call progress. However, there were problems. Digital displays require actual reading. Pilots, for example, don’t need to know their exact exhaust gas temperature; they just need to know the needle is in the right place and a quick glance tells them all they need to know. Reading EGT on a digital readout takes time and breaks concentration. It takes me longer to note my car’s speed on a digital speedo than an analog dial, requiring me to take my eyes off the road for longer. Editor’s note: I find the exact opposite to be true, despite using analog speedometers exclusively for almost 20 years before getting a car with a digital readout. These days, I often find myself using my analog multimeters, but it also depends on the task in-hand. I’m lucky to have options, because I’ve built up a collection of both analog and digital types over the years. And given that I can buy a digital multimeter for just a few bucks that (on paper at least) matches the specs of any multi-hundred dollar analog model of just a decade ago, there is no excuse not to own more than one. Items Covered This Month • • • • • Decisions and hard choices Yamaha amplifier and Simmons subwoofer repair Battery replacement for tablet Vox valve guitar amp repair Mitsubishi aircon repair *Dave Thompson runs PC Anytime in Christchurch, NZ. Website: www.pcanytime.co.nz Email: dave<at>pcanytime.co.nz 62 Silicon Chip Australia’s electronics magazine siliconchip.com.au For one project, it was cheaper for me to buy two digital multimeters from the local electronics store than to buy dedicated volt and ammeters. And if I am only reading the battery voltage under the bonnet of the car, or using the continuity beep function to ring out a cable loom, I don’t need anything as fancy or as inconvenient as those bulky, olde-worlde movingcoil models anyway. However, like many service people, I still love my analog multimeters! Disaster strikes I only bring this up because the other day I was moving an amplifier chassis around my workshop and as I picked it up from my (admittedly overcrowded) workbench, I accidentally snagged one of the leads of my oldest and dearest multimeter. Not realising I was caught up, I dragged the meter off the bench and it fell and hit the barely-carpeted concrete floor of my shop with a sickening crunch. Yikes! This particular meter has a nice leather case, and while you’d think that this might help save it, alas, no. While not appearing physically damaged, the meter’s needle now sat fixed at a weird angle at around 30% of the scale, and wouldn’t move when the meter was lightly shaken from side to side. This is a classic sign the meter’s armature had either been shocked free of the pivots it usually sits in, or was just broken. It wasn’t looking good. Many people would just shrug their shoulders, throw the meter in the bin and get another one out of the drawer, but you know me; if this wasn’t repairable I might consider chucking it, but until I ran out of options, binning it wasn’t going to be one of them. The first thing I did was undress it. The leather case might look authentically vintage, but it didn’t do much to prevent the meter from getting clobbered. Then again, perhaps things might have been worse if it didn’t have any protection at all. Ah, yet another philosophical question to ponder! I quickly removed the few screws that held the back on. No pointless anti-tamper fasteners here, just good, old-fashioned self-tapping screws. There were two batteries inside, one PP3 and one AA. This slightly surprised me; while I am obviously siliconchip.com.au aware these things have batteries, this meter is at least 30 years old and I can’t recall ever replacing them! I’d take the opportunity to do that now, but first I’d need to get the meter working again. The PCB was held in with another couple of screws, and with those out, only the front panel knobs prevented it from being lifted straight out. The knobs popped off without too much effort. The four main connectors, what I call banana sockets (but are also known as 4mm connectors) were hardwired to the PCB and came out with it as an assembly. I de-soldered the few flying leads connecting the batteries and meter, but I was careful to leave the rotary switch components sitting in the top cover, as there are lots of small bits and bobs that make up the switch and these are easily lost. I’d made this mistake before years ago, creating a lot of extra work for myself, so I was prepared for it this time. I took a digital photograph of the positions of all those bits before removing them and putting them aside in a parts tray. I was now left with the plastic top ‘half’ of the body and the meter assembly. The clear plastic meter facia simply pried off with some gentle persuasion applied to the slots provided. From this point, I had to be ultracareful, as the meter movement was now totally exposed and I didn’t want to damage it further. The white meter face was also vulnerable to contamination with dirt, fingerprints and wayward tools, and any mistakes at this point would really decrease my chances of a good outcome. Analog meter 101 This movement is what is known as a moving-coil meter, and they are used in many indicator and measurement roles. Their operational theory is simple; a soft iron armature, with a needle attached and a coil of wire wrapped around it, is suspended within the magnetic fields of a permanent magnet. When a current is applied to the coil through two tiny counter-wound hairsprings (which also assist with meter damping), a rotational force is created proportional to that current. The amount the needle deflects is then apparent against a scaled meter face. Australia’s electronics magazine October 2020  63 The trick is making the armature as friction-free as possible, increasing sensitivity, and in really goodquality meters this means mounting the pointed ends of the armature into jewels embedded in the meter frame. Mechanical wristwatches also commonly use this technique. Cheaper devices use either bronze or steel bushings to do the same thing, and usually work just as well; the only downside being they may not last as long without adjustment. In this case, one end of the armature’s mounts is adjustable via a setscrew, which also fine-tunes the ‘endfloat’ of the assembly, and correct adjustment means an almost friction-free movement. End-float is usually set at the factory, then sealed with a dab of paint. This adjustment lasts (hopefully) a lifetime. Most meters also have a zeroadjustment screw accessible from the outside that engages and alters the armature spring tension, allowing a null balance point to be set. I thought the armature might have simply been jolted out of its mounts by the fall, which would explain the cock-eyed angle the needle was sitting at, and the lack of free movement. Visual inspection under magnification confirmed this was the case. The only way to replace the armature into its mounts was to back off the top adjustment screw until I had enough room to reposition the pointed ends back into the bearings. I could then re-set the end-float for maximum free movement without any play. I backed off the set-screw by breaking the paint seal and gently coaxed the armature back into place with tweezers, a job made trickier by the springs, as they tried to pull the armature in different directions. I managed to position it, tightened the screw and carefully adjusted it. However, the needle still read half-way up the scale, and the zero-adjuster had minimal effect. With the armature in place, the movement should have been free to find its natural balance against the spring tension, and would be adjustable with the mechanical zeroset screw. However, the needle was still ‘sticky’ and moved erratically, so something else was going on. Another inspection showed the outer coil of the bottom hair-spring was wrapped around one of the tiny, sol64 Silicon Chip dered coil connections. It wasn’t going to move freely if that spring was impaired, so with two pairs of tweezers, I gingerly extracted the spring from the obstruction. This is trickier than you might think, as these coil springs are extremely thin and fragile, and any kink or other anomaly would alter its tension and would prevent the meter from ever being accurate. Luckily, I managed to unhook it, and the needle immediately fell back into place. With a sigh of relief, I reassembled everything and checked calibration against some known values. I used a 50W reference resistor and a regulated 5V output from a power supply, and after some adjustment, both results were close enough. Disaster averted, and my favourite analog meter lives for another day! Yamaha AX-300 amplifier and Simmons S-10W subwoofer repair R. W., of Lismore, NSW took a punt on buying some cheap old audio gear in the hope that he could fix any problems that might crop up due to its age. As it turns out, his confidence was not misplaced… Motivated by letters in Silicon Chip on repairing older hifi amplifiers, I Australia’s electronics magazine kept an eye on an internet auction site with a view to obtaining a decent amplifier and a small subwoofer. Eventually, I bought a 1980s Yamaha 30W per channel amplifier and a 100W subwoofer with an odd 10-inch driver for a bargain-basement price. The vendor of the amplifier stated that one should not expect it to perform as it did when new, but I was not perturbed as it cost less than $50 and could use the case if it was a write-off. After performing some safety checks, I powered up the amplifier and sampled its performance. What a disappointment! It had quite a lot of hum in both channels, the volume pot was noisy, and the sound could best be described as “thin” – lacking in fidelity and dynamic range. On removing the cover, the innards were relatively clean but 6000µF power supply filter capacitors had noticeable bulging. I was unable to obtain a schematic for the amp but decided to take a punt and replace the caps and see how things went from there. As it happens, Altronics sell similarly sized electrolytics rated at 10,000µF, so I decided to mail-order some. The cost was well below their minimum for mail order, so I decided to change every electrolytic capacitor in the amp. siliconchip.com.au On powering up with the new caps, the hum had completely disappeared, and the performance was probably as good as new. Some contact cleaner for the pot and a new coat of satin black paint on the cover, and it was like new. I remember being impressed with the Yamaha amps in the 80s and this one was no different now. A few days later, the subwoofer arrived. It had clearly been stored somewhere damp, as there was mould residue on the driver, the case and the grille. This was easily cleaned off, and the enclosure was then immaculate. Powering the speaker with nothing connected and I was greeted with – yes, you guessed it – hum! Removing the integral amplifier again showed bulging filter electros in the power supply with a value of 10,000µF that were the same physical size as the ones from Altronics. Two more of these fixed the hum issue, so I set the system up with my TV. On switch-on, after the speaker protection relay engaged on the subwoofer, I was yet again greeted by hum! This had to be an Earth loop as both the sub and the Yamaha amp were quiet when separated. The AX-300 has a shielded power transformer but is supplied with a two-pin mains plug and had an Earth binding post for use with a turntable. The sub has a 3-wire IEC power input socket. I made a 3-pin mains plug with just a green wire connected to the Earth pin, and I connected the other end to the binding post on the amplifier. The hum disappeared, so it was definitely an Earthing problem! So I replaced the amplifier’s power cord with a 3-wire mains lead, properly secured and Earthed. For less than the cost of a soundbar, I had obtained a sound system that would blow any of them out of the water, and saved some old but still useful pieces of equipment from the scrap heap. I’ve been using a Samsung Galaxy S 10.5 tablet for a few years now. Lately, I noticed that its screen was bulging in the middle and it had come loose from the frame. I immediately realised that the battery must be failing. I’ve seen this happen to other devices, so I knew it was time to replace the battery. Before I started working on my tablet, I found a good video on replacing the battery in this particular tablet on YouTube. Watching that, I picked up some useful tips. The first thing to do was to remove the back of the tablet to access the battery. This proved to be somewhat tricky, as the back had not been removed previously, but with some effort and a couple of phone repair tools, I was able to remove the back and gain access to the battery. With the back removed, I could see just how badly the battery was bulging. I was also able to press the screen back into place. I was hoping that the screen had survived being bent; I later determined that it had not suffered any damage from being bent away from the frame. Removing the battery is fairly easy; it just entails disconnecting two ribbon cables and the battery connector, then undoing the four screws that hold the battery in place. I was going to or- der a new battery via eBay; but first I thought I would try the battery from another identical tablet I had, to see if it was still usable. The other tablet’s battery charging circuitry had failed, and I couldn’t fix it, so it was a suitable donor. Because this battery had been flat for months, I was concerned that it might not charge, but decided to install it anyway and give it a go. After moving it from one tablet to the other, I reconnected the ribbon cables, plugged in the charger and left it for several hours. I occasionally checked it to see whether it was charging. The charge indicator sat on 0% for quite some time, leading me to think that it wasn’t going to charge. Eventually, it went up to 2%, which was a good sign. It took a long time to charge the battery fully, but it did reach 100%. The next thing would be to see if it retained its charge, after being flat for months. I checked it the next morning, and it was still 100% charged, so it looked like the battery was still viable. I was a bit concerned when I noticed that there was a “no go” symbol next to the battery charge indicator on the screen. But I thought this might be because the back was off, so I refitted it. Now that the back was on, the “no go” symbol was no longer present, so The bulging case of the Samsung tablet is shown above, with the battery shown below. Samsung tablet battery replacement B. P., of Dundathu, Qld has become quite adept at keeping old electronics going. This time, he noticed a quite worrying symptom in his tablet and luckily, had a ‘donor’ device which provided the parts he needed to fix it... siliconchip.com.au Australia’s electronics magazine October 2020  65 that confirmed that the assumption was correct. Often when a lithium-ion battery is flat for some time, it will no longer accept a charge. Whether this battery’s bad start to life will come back to haunt it in the future remains to be seen, but for the moment at least, it seems that it’s still good. I tried fitting the damaged battery into the other tablet with the faulty charging circuitry, but it wouldn’t switch on, and I noticed some parts of it getting very hot, so that tablet is only useful for spare parts now. Regardless, this was another repair that ended up not costing me anything, thanks to having access to the defunct tablet for spares. These batteries are around $25-30 on eBay, so they aren’t that expensive, but it was nice to be able to complete the repair at no cost. I hope to get a lot more use from this tablet before it becomes obsolete. Vox valve guitar amplifier repair S. W., of Fulham Gardens, SA had to guess at the values of some burnt components to repair a guitar amplifier. While it turns out that the values he chose weren’t the same as the originals, they must have been close enough as the repaired amplifier worked well enough... Some time ago, I was asked by a family member to repair a Vox AC4TVH 4W valve guitar amplifier that was inadvertently operated without the speaker connected. Apparently, the user was unaware that this amplifier does not have a built-in speaker, and should only be used with an external speaker plugged in. After removing the amplifier module from the case, I observed that heat stress on several resistors had caused their values to become unreadable. High-voltage supply fuse FS2 was also blown. I checked the audio output transformer and found it to be OK. I then powered up the amp and found that the mains transformer secondary voltage was acceptable, and the filaments of the two valves were alight. Hence the two major components appeared to be undamaged. Great! Further testing revealed that three of the heat-stressed resistors were opencircuit. I was unable to find a circuit on the web to identify their values. Therefore, I saw no alternative except to trace out the circuit of the module using some valve circuit theory from decades past. This resulted in the circuit diagram shown here where R5, R17 and R24 (highlighted in red) were the open-circuit resistors. Using basic circuit theory, I selected a value of 180W for R5 to allow the EL84 to self-bias at about 8V with an anode current of around 50mA, and 820W for R17 and R24, to keep the anode and screen voltages for the EL84 below 300V when operating. Once the resistors were replaced, I soon discovered that the major problem was an internal short between the screen and grid of the EL84. With a new tube in place, the amplifier fired up as expected. The only problem was that the power output was only about 3.8W across a resistive 15W load before amplitude limiting set in. I contemplated lowering the value of R17 and R24 to increase the available power to 4W. However, the owner was not concerned about the lower power level and was just happy to get the working amplifier back. As I was working on it, I noticed signs that the amplifier had some problems in the past. For example, blown fuse FS2 was labelled 200mA, while the value written on the circuit board was 125mA. This may indicate that sometime in the past FS2 was replaced with the higher-rated fuse. Possibly, this was The full circuit diagram for the Vox AC4TVH valve-based guitar amplifier, which was found online. 66 Silicon Chip Australia’s electronics magazine siliconchip.com.au done as a quick fix for an intermittent short in the EL84. One can only speculate! Some time later, while I was searching the web for information on singleended valve amplifiers, I came across a full circuit diagram of the AC4TVH. In this circuit, R17 and R24 are shown as 220W while R5 is 180W. So I got R5 right, but went a bit high on R17 and R24, hence the slightly reduced output power. Mitsubishi aircon repair D. S., of Maryborough, Qld took a look at an aircon that had been deemed unrepairable by a professional. And guess what, he managed to fix it with just a bit of effort and a less than a dollar’s worth of parts. Sadly, this is far from an uncommon story... Air conditioners, in both vehicles and homes, usually come up for repairs and service during summer. So, I wasn’t surprised when I got a phone call from a friend in January asking if I could take a look at his home aircon. I was told that a service agent had already looked at the system but had told the owner that the mainboard had been destroyed by gecko urine, and that replacement mainboards were not available, so the whole system would have to be replaced. I thought there must be a cheaper way to fix it, so I switched off the power and took off the covers. Many covers... I have to admit, the chances of a gecko getting into the enclosure which held the circuit boards seemed slim with all the covers in place. Upon initial visual inspection, I found no physical damage, so I grabbed my drivers again. After rechecking to ensure the power was off, I disconnected the mainboard, marking all the various connectors and taking photos on my phone to ensure that I could reassemble it later. 15 minutes later, I had the mainboard out. A much closer inspection showed nothing of interest, no damage, no swollen electros, no burnt-out transistors, nothing! I searched the rear of the board, checking for dry or broken solder joints. The board did have a few heat marks and discolouration on it from thermal stress, but removing and checking the affected components, I found no problems. It’s a Mitsubishi split system aircon, and these units are very well made. siliconchip.com.au The boards are fully marked with all components types, values and test voltages silkscreened to the board, so it was a pleasure to find everything as it should be. So I refitted the board, plugged everything back in and turned it on. The head unit lit up like it should for about five seconds, then shut down again. The condenser section was doing the same. The main condenser fan started up and ran for about two minutes before shutting down, but the compressor did nothing. I searched the internet hoping to find that this unit had on-board diagnostics and it did. However, we could not locate the remote to press the appropriate buttons to get any fault codes. After further checking, I noticed that the mainboard switched mains to the various mains components, but the control circuitry ran from 24V DC, 12V DC and 9V DC rails. The mainboard did not have any form of power step-down circuitry, so where were the logic supplies coming from? I removed the mainboard again and discovered another board hidden beneath it, inside its own plastic enclosure. Another 15 minutes of fiddling finally got it out. Close inspection revealed several leaking electros and quite a bit of corrosion across this smaller board. Replacing the leaky electros and cleaning up the mess was easy. All the caps were common value, high-temperature through-hole components. I also found the remains of a very small signal diode that had virtually rotted away. I had to guess its type as the corrosion had also destroyed that part of the silkscreen. I used a 1N4007 type rated at 1000V, 1A. This power board supplied the various DC rails to the mainboard via a small relay, so I reasoned that this was a good choice. I also found a tiny ceramic capacitor fitted across two wires of a five-wire connector. I backtraced this small harness back to the mainboard where it connected to the processor chip. The power board had several ICs controlling the various switchmode sections, along with the usual opto-isolators, SMD transistors and various other surface mount components. This harness carried control data to and from the mainboard to the power board, although I could not locate any other information on its function online. I cleaned up the corrosion, re-drilled the through-holes and concentrated on the remains of this small capacitor. It’s difficult to clean off corrosion without removing the markings, but after some very gentle wiping, I found the value 104 marked. Thus it was a 100nF capacitor, which I had in my parts box. After replacing that capacitor, I began to reassemble the various boards. That’s when I found that the corrosion on the small harness had eaten through one of the conductors. I have heard many times that “it just fell off in my hand”. Well, this time it really did! I cut off the damaged section of cable, reattached the connector and refitted the board into its hiding place. The mainboard went back without a hitch, and I plugged all the wiring back into their clips. On went the power; no bangs or puffs of smoke occurred so I flicked the manual off/on switch on the head unit and it sprang to life. After 30 minutes, I refitted the external covers and gave my friends the good news. There was some bad news; he would need a new remote control, but at $65, that was going to be a lot cheaper than a whole new system! Another air conditioner saved from landfill; it was almost chucked out because a service technician could not be bothered to diagnose it fully. I have no idea which fault or faults were actually stopping it from working, but I have always been a firm believer in fixing that which can be fixed! SC 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. Australia’s electronics magazine October 2020  67