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SERVICEMAN'S LOG One repair leads to another Dave Thompson There are people out there who obviously love their older radios and stereos. Since word got around that I can repair these devices, quite a few have come through the workshop. While most repairs are simple, there have been some that required a good bit of thinking. Most of these type of repairs don’t warrant much attention due to being relatively simple fixes; replace the odd component here, or reflow dry joints there, and away we go for another 40 years. But there were a couple of recent fixes of which I have been quite proud. While they didn’t require me to do enough research to earn a doctorate, I did have to do some searching and thinking to come up with a solution. The first was an amplifier which is no stranger to my workshop. This is one of those jobs that proves the old engineering maxim: as soon as you mess 64 Silicon Chip with something that has been working well for years, it will develop a heap of problems (perhaps a corollary to “if it ain’t broke, don’t fix it”?) There’s probably a rational explanation for this phenomenon. It often happens that you take the case off an old amplifier just to check it over and huff the cobwebs out, then a month later the caps fail, and the transistors or valves need replacing. Perhaps I disturbed something with my low-pressure air, or the journey to Australia’s electronics magazine the workshop shook up those old solder joints. Or maybe I just displeased the audio gods by intruding on sacred ground! Mechanics often claim that a car engine is never the same once the head has been off, and I’m convinced there are many parallels in electronics. Whatever the cause, there is always the sneaking suspicion that I’ve done something to cause a rift in the space-time continuum, and now I’m paying the price. siliconchip.com.au Welcome back, old friend This lump of a stereo amplifier is one of those old 70s jobs that appear to be milled out of a solid billet of brushed steel, with a couple of polished wooden caps slapped on each end of the case. The power transformer alone is heavier than most modern audio systems, including their speakers! Everything inside is heavy-duty, and the connections are wire-wrapped, a construction method peculiar to that era. While wrapping is an excellent way of connecting individual circuit boards together, and the cabling has obviously stood the test of time, it is a royal pain in the woofer to work on. This is especially true if I need to uninstall and reinstall the board several times for testing purposes – re-wrapping it each time is highly impractical. While I still possess a wire-wrapping tool and a few spools of wrapping wire, purchased circa 1975, I haven’t used them for many years. In cases like this, unless the custom- siliconchip.com.au er specifically wants to retain the vintage authenticity of the device, I replace wrapped joints with soldered connections. While I know my way around this Pioneer SX-series amp, having repaired it before, I think I was the first person to take the covers off. Initially, the problem was that the speaker protection relay was not kicking in at switch-on, and if it did, it would randomly drop out. I documented that repair in the February 2020 issue (siliconchip.com.au/Article/12339). Now I’m wondering if by opening it up I somehow jinxed it, because here it is again less than a year later with a different fault. I knew I shouldn’t have disturbed the gremlins slumbering within its circuitry... The customer reported that, while using it, it made some loud static and clicking and popping noises, then the amp fell silent. The panel lights still glow, but there is no action from the speaker-protection circuit and no other signs of life. At first, I thought my previous repair might have failed, but I removed the protection board and relay and inspected and tested them; all appeared Australia’s electronics magazine Items Covered This Month • • • It’s never as easy as it seems The water-logged electric toothbrush Fixing substandard industrial machinery *Dave Thompson runs PC Anytime in Christchurch, NZ. Website: www.pcanytime.co.nz Email: dave<at>pcanytime.co.nz OK. This is where the now-soldered interconnecting wiring came in handy – if I had re-wrapped the boards back in, it would have made removing them again that much more work. And though there was some slack left in the factory wiring, wrapping uses up a couple more centimetres of wire length each time, so I would have had to replace all the wires. Instead, I could just desolder it, then reconnect everything when I was done. The power supply seemed to be the next place to check. Fortunately, the circuit diagram is freely available online, and I had already downloaded it. November 2020  65 This made things a whole lot easier. The annotation on the schematic is also excellent, with test points and current and voltage values clearly marked. With my trusty (and still working!) analog multimeter – after last month’s shenanigans – I rang out the various outputs on the power supply board and found three of the nine listed were well out of spec. As mentioned earlier, the power supply is a heavy-duty unit and delivers a range of voltages from 5.4V to ±51.5V DC, as well as 7.5V AC. I measured around ±14V on the nominally ±51.5V lines, and zero on two other points, which both should have been +13.5V. I knew I wouldn’t get any joy without these voltages present and fully accounted for. In the last repair, I replaced all the electrolytic capacitors on this board, and a couple of the power transistors. However, there were still about a dozen smaller transistors I hadn’t tested. Looking through the circuit diagram, it was apparent that I was going to at least have to remove some of those in the part of the circuit responsible for these sub-par readings. Pulling them out is as easy as using a solder-sucker and a hot soldering iron. Unhealthy though it might be, I love the smell of that old solder burning – it reminds me of watching my dad working in his workshop. I found several open-circuit transistors, or more accurately, my Peak Semiconductor Analyser found them. I know that I could have used my multimeter to discover them, but I have an analyser, so I use it. Amplifiers of this era often used proprietary components, or possibly transistors and diodes that were manufactured at the time in relatively small batches and ended up not being used in much else. In this case, the part numbers weren’t familiar, so I hit the web and discovered an abundance of forum posts regarding the same problem. After some research, I discovered that these transistors aren’t overly specialised, and audio purists derided several as being too noisy for use in amplifier circuits anyway. Editor’s note: that probably doesn’t matter if they’re in the power supply, unless the audio circuitry is particularly poorly designed. Luckily, there were recommendations for substitute transistors that 66 Silicon Chip Australia’s electronics magazine would offer significantly quieter performance. Many of these types are still widely available and inexpensive. While I had some on hand, my supply of new old stock (NOS) components is dwindling. So I decided just to buy what I needed new from element14 and Digi-key. After receiving the parts, I replaced all the transistors in that section of the supply. After re-soldering the board in, but without hooking up the outputs yet, I powered it on and measured the output voltages. The numbers were better, but still way off, so something else was clearly wrong. It wasn’t that easy Referring to the circuit diagram, I measured as many of the resistors and caps as I could in-situ, in case one had failed. While not an ideal method, the figures on my LCR meter were within tolerance. That left the diodes. This board has 12, and most are straightforward silicon varieties, with the only difference being their currenthandling characteristics. Two of the diodes are zeners, one rated at 13V and one at 14V, 500mW. I couldn’t measure them properly in-circuit, so I removed them and tested them with my analyser. Both were open-circuit. I replaced them with suitable parts from my own stocks and powered the amp up again; this time, I had voltage outputs that, while a little high, were within 10% of stated values. After connecting the power supply board outputs, I switched the amp on, and after a few seconds the speakerprotection relay kicked in – an excellent sign! I ran the amp on my workbench at half-volume for 24 hours and periodically checked the voltages and component temperatures on the supply board. All remained normal, though as expected, a couple of fibre-sleeved load resistors got warm. I then cycled the power on and off around 20 times within an hour, and the relay kicked in every time. I reassembled everything, re-soldering any connections that were a bit temporary and buttoned it all back up. The customer picked it up and hopefully that’s the last I see of this behemoth for a while! The radio repair The second repair came by way of an enquiry from a reader; he had an siliconchip.com.au older General Electric Superadio 3 radio that had started drifting off-station. The radio was usually used in a setting that once the station was selected, it didn’t change, but lately, he’d turn it on and after a few minutes, the radio de-tuned and was thus unusable. For people of a certain age, modern radios often don’t cut the mustard. While they might have much more sophisticated circuitry, and accurate and stable digital tuner sections, the sound output is often not as good when compared to older models. I’ve found many newer sets sound ‘tinny’, which could be due to smaller speakers and flimsier construction. While perhaps not as portable (in the modern sense), many of us prefer our older radios. So that is why we try to keep them going as long as we can. The Superadio duly arrived at the workshop, and I fired it up to test it. It did sound good, which was likely down to the substantial dual speaker system, consisting of a 165mm woofer and a 50mm tweeter. However, after a short period, the station slowly drifted off, and the audio sounded like any other radio does when slightly off-tune; awful! Fortunately, this model was popular in its day, so it didn’t take me long to find a lot of information about it online. It turns out that the station drift is a known problem, and is usually down to the tuning potentiometer wearing out. The job was made slightly more difficult due to there being two different circuits (and circuit boards) employed in this model, so determining which one I had was the first hurdle. Luckily, the online ‘fan pages’ I found enabled me to quickly determine that it was an earlier board. This information also documented several other inherent ‘flaws’ with the original design, and offered fixes for these issues. Older radios are typically tuned using a variable capacitor, a so-called “tuning gang”. I have a drawer full of these sometimes-substantial components, salvaged from radios over the years, and they are a marvel of engineering. Essentially, they are just a set of rotating metal plates that intertwine. The degree they overlay determines the overall capacitance. One of the marvels of modernisation (and circuit design) was to shrink the size of these variable capacitors down to a small mostly-plastic version which siliconchip.com.au was used in the majority of ‘pocket’ transistor radios. These ‘miniature’ tuning gangs are still being manufactured, and are available from the usual suppliers. In this radio, though, varactor-diode tuning was employed. While this is usually a robust system, it relies on the integrity of the potentiometer used to tune the radio. When the carbon track inside the pot inevitably wears out, tuning becomes increasingly erratic. And to make matters worse, the value of that potentiometer is 300kW, a rather oddball figure and (for me) very difficult to source. It is also an unusual size, 16.5mm in diameter, and I couldn’t find any new versions to replace it with, regardless of electrical value. While I could squeeze a modern pot in there with modifications, it would be preferable to use a similar-sized replacement. Back when this radio was designed, there was no doubt a good supply of different potentiometer values and physical sizes; but over time, manufacturers pared down their product lines to supply only standard sizes and values. So replacing pots in older equipment is increasingly problematic. I went through my pots bins and trawled the usual supplier suspects, but nobody had a 300kW pot of any size. Needing to compromise Fortunately, one of the websites included a ‘mod’ where a 500kW pot could be used instead. However, even if I could find one to fit on the board, Australia’s electronics magazine this would have the effect of shifting stations down the scale and making tuning in the upper regions of the band very finicky. I went back to the customer and asked if this would matter; his original communication stated he tuned the radio to one station and left it there. Assured this wouldn’t be an issue, I proceeded to disassemble the unit. Like most of these jobs, it was merely a matter of removing the external knobs, taking out some standard screws, desoldering a couple of flying leads and removing the back half of the case. If only modern manufacturers would use these methods, instead of those pesky security fasteners and breakaway clips; life would be so much easier for us servicemen! Once exposed, I removed the old tuning pot by the usual methods and replaced it with a similarly-sized 500kW model sourced from an online supplier. I didn’t bother with matters like choosing a logarithmic or linear taper; I found a 500kW pot the right size, so it would have to do! After all, tuning wasn’t going to be the same after the fix anyway, and the customer would simply ‘set and forget’. I considered making the suggested mods for the first-revision board that aimed to improve performance. While they might not be pertinent to the owner, I figured that as the thing was already dissected on my workbench, I might as well do them. The antenna circuit Q can be increased by changing one resistor on the November 2020  67 board. The original is 100kW; changing it to around 50kW apparently helps, so I just soldered another 100kW resistor across the original. There is another similar mod that significantly lowers the AM noise floor. The fix is again to parallel a 100kW resistor across an existing 100kW on the board, halving the resistance. The radio can apparently also benefit from a narrower ceramic filter, and as I already had a suitable replacement in my parts boxes, I removed the original 280kHz component and replaced it with a 120kHz version. Later revisions of the radio had these mods already implemented at the factory. Another mod is to improve bass response by increasing the size of certain off-board capacitors. However, as the customer already liked the sound, and the case would require modification to cram in bigger capacitors, I didn’t bother. Once reassembled, I ran the radio for three days on the bench, and it didn’t drift at all. So that was my jobs done, and thank goodness for resources like the internet and decent documentation. Perhaps I should also thank the electronic spirits inside these devices, lest I incur their wrath once again! Do you have any good servicing stories that you would like to share in The Serviceman column? If so, why not send those stories in to us? We pay for all contributions published but please note that your material must be original. Send your contribution by email to: editor<at>siliconchip.com.au Please be sure to include your full name and address details. way through before slowing to a crawl as the formed product exited the machine, with the motor protesting, and finally the variable speed drive shutting down to protect itself. The machine was supposed to roll a 96mm top hat from up to 1.2mm steel, yet plainly did not have the torque to run smaller gauge. The motor was rated at 5.5kW; we replaced it with an 11kW unit with a chunky gearbox, along with a very much larger VSD. Suddenly, these machines didn’t look so cheap. We fired it up again and found the current drain was precisely the rated current of the new motor, which was sort of good but it needed to roll thicker steel, so it would probably need more torque. Furthermore, the section emerging from the machine was quite warm to the touch, which indicated it was being rolled up too abruptly as the machine was too short, and the roller stations were too close together. This was resulting in the product being forced into shape instead of being guided. So, hoping for the best, we loaded the thicker material and off it went for a few metres, until the drive chain shattered. An examination revealed that the strain was so great the links had stretched until one gave up. A good quality chain was fitted and tried again; the motor was protesting, and disturbing noises were coming from the front roller set, but the piece was emerging from the machine. After the machine had measured and cut off the section, one of the workers picked it up and promptly dropped it with a yelp, saying something along the lines of “it was rather hot’. He was right! It was dangerously hot, with zinc flaking off the steel, so a tremendous amount of friction was being generated in the last roller set. Checking the gap between the roller pairs that sandwich the steel, we measured 0.8mm but it should have been 1.5mm, to allow for the full range of steel used. So the machine had become a roller mill trying to compress the steel thinner. Goodness knows how the bearings coped! The rollers now needed to be turned down slightly in a lathe, as there was no gap adjustment. This is not an easy thing as they usually are extremely hard steel. Fortunately(?) the manufacturer hadn’t bothered with that Australia’s electronics magazine siliconchip.com.au Fixing substandard industrial machinery G. S. of Montrose, Tasmania, has sent in the following saga both as a servicing story and also to remind our readers that if the price seems too good to be true, it probably is. That includes industrial equipment! In previous submissions to Serviceman’s Log, I reported on work I do for a long-term client that manufactures steel building products. He has made a habit of purchasing old, worn-out machines, and as he is blessed by having a very skilled fitter in his employ, we have been largely successful in bringing them back to life. Recently, he strayed from this policy and elected to purchase two new “top hat” roll formers from a manufacturer in Asia. He paid around 30% of what locally built machines cost, which should have rung alarm bells, but he saw it as a great deal. Roll formers are essentially a long heavy steel frame with ‘stations’ spaced along its length that are fitted with rollers. They are progressively shaped to slowly form the required product profile from a steel strip. There are generally top and bottom rollers that sandwich the material between them and either the top or bottom row are driven by heavy chains and sprockets, in turn, driven by an electric or hydraulic motor. There are plenty of examples of such machines on YouTube if you are interested. I received a call saying the machines had shown up, but the electrician refused to connect them, stating they were substandard and wasn’t risking his license to do so. As I was eyeing retirement, we were trying to get a new electrician up to speed, and he was on a steep learning curve. His background was commercial, so he had a bit of a hard road ahead learning to be an industrial electrician. I went to the factory and found two nicely painted machines, which looked very short for the task at about eight metres (more about this later). Looking them over, I discovered all the problems we get with a lot of Asian machines: no Earthing on the motors, green Active conductors, no emergency stop system, no guards, no motor overload protection on the hydraulic pump and so on. So refusing to connect them as they stood was valid. I got the electrician onto replacing the switchgear and rewiring it while I sorted out the safety circuitry. This required the installation of a safety relay and the addition of three emergency stop buttons, low voltage control circuity and circuit breakers. Meanwhile, the fitter fabricated guards for the roller stations and guillotine, finally resulting in something you could use with a reasonable chance of survival. So all should be well, but of course, it wasn’t. We loaded a strip of 1mm steel and set it going. It got most of the Servicing Stories Wanted 68 Silicon Chip and had made them from mild steel, so it was easy to run them down to the proper size. Just to be sure, we added an oil feed so that lubricant was sprayed on the strip. This resulted in a motor current about 20% less than the rating, so finally, all was well. Well, almost; the bearings also needed replacing, as they just weren’t up to the job. So several thousand dollars later, we got to a machine that would do what it was supposed to do, without the operator risking life and limb. The final cost was perilously close to what a locally-built machine would have cost (which presumably would have worked off the bat). The second machine purchased has an even deeper profile, and the feeling is we will need to change its sprockets to slow it down and get enough torque to do the job. We haven’t started on that one yet, as the client still needs to get over the shock of the first unit. There are some very good Asian machines on the market, but it is an expensive process to discover which ones they are, so caveat emptor! The water-logged electric toothbrush G. C. of Nelson Bay, NSW, was getting ready to use his electric toothbrush when, as he lifted it off the charger, it started all by itself. Unfortunately, the toothbrush then decided to switch off after 30 seconds and then wouldn’t start again, so he decided to see if it was repairable... My toothbrush cost less than $30, so it was uneconomical to repair on a commercial basis, but that’s irrelevant in this case as it was my own toothbrush and it’s never useful to charge yourself. Upon inspection, it appeared that the inner bottom (charging) end was removable, so I used the tiny knife from my Swiss Army Card to pry this bottom base part out of the handle (this blade is great for opening iPhone screens too). When the bottom popped out, unfortunately, so did some gungy looking water, so the reason for the toothbrush malfunctioning was obvious. I kept going and eventually found that the complete motor, battery and charger electronics assembly could be pushed out by pressing (very) firmly on the brush end. I always found it a challenge the first time I have to open something, as I have to figure out how siliconchip.com.au hard each part can be pushed before it either opens or breaks. The PCB has a wireless charging coil at the bottom end, diodes to rectify it and quite a few SMD components, presumably to make the regulated charger for the NiMH 2.4V/500mAH battery. The 8-pin SMD IC has to be a microcontroller of some sort as it would have to control the charging, monitor the tiny pushbutton and control the transistor that switches the DC motor on and off. The final part is, of course, the little DC motor that moves the brush. Everything was slipped into, clipped or soldered to a cunningly designed moulded plastic part which holds it all in place. The electronics was wet and had some slight corrosion; toothpaste and water is not a recommended environment for electronics, so I unsoldered the PCB and followed my usual routine for wet electronics. I got out my trusty Jaycar ultrasonic cleaner, waited 90 seconds and voilà – no more visible contamination. After a quick rinse in clean water, out came the hot air gun until everything was dry. I decided to set up a simple test jig before trusting the NiMH battery. I just soldered wires to the + and - battery pads on the PCB and reconnected the motor leads, then set the voltage to 2.4V with a low current limit on my bench supply. Switching it on, nothing happened and the motor stayed off when the tiny pushbutton was depressed, but at least no smoke escaped. After years of experience, I’ve found that mechanical parts fail much more often than solid-state parts, so next, I checked the miniature switch. Press- Australia’s electronics magazine ing it produced quite inconsistent resistance readings, varying between 1W and 10W. I’ve found this frequently happens with these little switches, especially when they have been wet, so the switch had to go. I had ordered 100 of these switches when I had to repair several car remote controls (all love jobs too, and all had been wet) and I still had a great many of them left, so it was just a matter of out with the old switch and in with the new. I also decided, as the solder joints didn’t look quite ‘right’ to me, to apply flux and redo every solder joint, which only took a few minutes for this little PCB. This time, when I applied power, the toothbrush worked correctly, with the motor turning on and off as usual. It was then just a matter of resoldering the connections to the PCB. I did make two changes – I replaced the very thin motor wires with some stripped out of Cat6 cable, and also added some 1mm Teflon insulation to one lead of the charging coil where it came very close to other components. I even remembered to finally check that the wireless charging light turned on when the toothbrush was very close to the charger base station. All that remained was to reassemble it, but as I wasn’t impressed with the original sealing method, I made new seals at the motor end and to the bottom base part with neutral-cure silicone sealant. Over many years I’ve found that silicone seals 100%, but can usually be removed, even if requiring a bit more force than the original sealing method. At least it will never leak and kill the insides again, so it shouldn’t need to be disassembled again! SC November 2020  69