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SERVICEMAN'S LOG Valve repairs are not for the inexperienced The internet and YouTube are wonderful sources of information for just about any task but the ease of obtaining information does not mean that you can fix the latest Mercedes sedan or an old valve amplifier, for that matter. Just because it looks easy on YouTube does not make you a competent serviceman. Painting is one of those things that most people think is as easy to do as changing a light bulb or hanging a picture; anyone can do it. I don’t mean painting as in creating the Mona Lisa or Girl with a Pearl Earring, which requires a skill set very few people can ever master. I mean painting as in covering the roof or walls of your house with paint. The general consensus seems to be that anyone with at least one working arm and a pulse can paint a house. However, they would be quite wrong. Like anything, painting takes knowledge, experience and skill to siliconchip.com.au pull off properly. To illustrate this, here’s how a recent conversation between me and the paint-shop guy went: Me: I’d like to buy some paint please. PSG: And what paint would Sir be looking to buy today? Me: Duck-Egg Blue please. PSG: Would Sir be requiring oil or water-based acrylic, latex or enamel Duck-Egg Blue paint? Me: Um... And there’s the kicker; while anyone can wield a paint brush, only those with the knowledge and experience to have the right preparation, paint, Australia’s electronics magazine Dave Thompson* Items Covered This Month • • • Repairing valve amplifiers A problem safety switch Neff oven repair *Dave Thompson runs PC Anytime in Christchurch, NZ. Website: www.pcanytime.co.nz Email: dave<at>pcanytime.co.nz methodology and the skill to apply it will get a decent result. The servicing industry is no different; many of us will give fixing anything a go before admitting defeat and calling in a professional. I get the impression that many blokes would do their own brain surgery, as long as there was a video on YouTube showing how to do it and someone to hold the torch and mirror for them. I once had a guy call me, asking whether I could sell him a boot disk. When I pressed him for more information, he said his computer wouldn’t start up. He had an on-screen message telling him to insert a boot disk, hence the call asking if I could sell him one. As politely as I could, I informed him that even if he did have a boot disk, it is doubtful he could get his computer going as he’d have to know what to do once he’d inserted it. His reply was classic and one that I bet a few computer-repair people have heard: “If I come and get the disk, could I call you back when I get home and you can walk me through what to do next?” Well, no. When I first heard someone suggest I walk them through repairing their computer over the phone, I was very polite in my response. Yet as this became a more common occurrence, I began turning it around and asking people if they thought calling a mechanic and asking them to be walked through repairing their car engine or calling an electrician to walk them through fixing a dead circuit was acceptable. July 2018  43 When they inevitably answered no, I then ask them why they think it is OK to ask that of me. Most get all bent out of shape and tell me that all I have to do is push a couple of buttons anyway, so why would they bring their computer in when I could simply tell them what to do instead over the phone. Some offer to pay; most don’t. I point out that this is what I do for a living, and that if I fixed everyone’s computer this way I’d be broke, but this doesn’t seem to wash as a valid argument. Typically, they either hang up or derisively inform me that they could find out on the internet anyway, so I might as well tell them now. At this point I usually wish them good luck, sign off and let them get on with it. I coined a name for this type of person: a WOTAM, for Waste of Time and Money. There’s also WOFTAM, for the really annoying caller. Historically, we ANZACs especially are known for our genetic disposition for DIY culture and I heartily applaud us doing it ourselves, however most of us have the wisdom and good sense to draw the line when we are looking like we are getting out of our depth. Years ago, when my motorbike engine was in bits on the floor of my flat and I wasn’t able to put it back together properly, I hired an engine guy to help me with it. When the power line coming into our just-bought house from the street 44 Silicon Chip started arcing, smoking and stinking of burnt insulation, I was straight on the phone to the power company before you can say CPR. I wonder how many guys would just break out the aluminium ladder and have a go at it. The Darwin awards website is full of stories of people who didn’t have that common sense or if they did, they chose to ignore it, eventually ending up on an ever-growing list of headshaking anecdotes. I mention this because recently I had an old valve amplifier in the workshop that someone had already had a go at repairing and as in a lot of cases like this, instead of helping, it made things worse. Most people seem to know somebody who is “good with computers’ and so naturally they farm all their computer work out to this person. In a similar vein, many of us know somebody who is “good with mechanical or electrical things” in general, so anything broken inevitably gets put this person’s way for them to repair. It is only if they can’t that the “professional” is commissioned to have a look at it. While this way of going about things can be attributed to our DIY culture, my guess is that it is more a case of economics; getting stuff repaired costs money, and sometimes a lot of money. Call a plumber out on a Saturday night to retrieve a stuffed toy from your overflowing toilet bowl and you can spend a week’s wages on it. Australia’s electronics magazine Calling a white-ware serviceman out to discover why your fancy, justout-of-warranty washing machine is throwing up an E09 error could mean the kids going without new gym shoes this term. I get it; if we can get something fixed cheaper, then we’re all for it. Unfortunately, as the old saying goes, we usually get what we pay for. Repairing a non-working valve amplifier has a few “gotchas” for those who don’t usually deal with tube gear. For one, not many of today’s tinkerers are familiar with valves and how they work and two, these amps can kill you; typically many times over. While we all expect mains-level voltages to be present in that vacuum cleaner or toaster oven we have on the work bench, we can add sometimes 200V or more to that floating around valve amplifiers and that’s a funeral waiting to happen. If one isn’t particularly wary of the potential danger, one will almost certainly get bitten. Then you get the type of hobbyist who gets given an old valve amplifier or mantle radio to “have a look at” and the first thing he does is plug it in and try it out. Even before the fire department has finished dampening down the hot spots, he’s figured out he probably should have checked it over before turning it on. Another problem is spare parts; it isn’t like the “old days” when you could go down to the corner store with a bag of valves, plug them into the tester and choose a new one from the spares on the shelf when the go/ no go gauge told you your valve was “weak” or “gassy”. While good quality tubes are still available, unless you are the member of an antique radio club with access to personal stocks or discounts, or scored yourself a truckload of useful NOS (New, Old Stock) valves cheap on an auction site, you are probably going to get robbed by savvy, tube-selling vendors. Or you will buy a newly-manufactured, Chinese or Eastern Bloc-made valve that may be excellent, mediocre or terrible, depending on the individual tube. Compared to say, the 1950s, practically nobody makes valves any more. Fortunately for tube enthusiasts, the relatively sudden advent of the transistor resulted in literally warehouses stacked full of now-unwanted and unsold valves. siliconchip.com.au Eventually, these found their way into the hands of either recyclers or people with enough vision to realise they needed to be saved for future requirements. Sadly, many were lost to landfills, but enough were saved to keep the likes of amplifier manufacturers and tube enthusiasts in valves for years to come. Obviously, as this limited NOS stockpile dwindles, the harder it becomes to obtain certain types of valves, which pushes up the cost. Matched-pairs of well-known audio output valves can command eye-watering prices these days. You’d think those very few valve manufacturers still churning out tubes today would fill this particular vacuum, ka-boom! But sadly, the handful of factories based in former Soviet republics and China who still make valves don’t make them with the same level of love and attention that the likes of RCA, Sylvania, Mullard or Philips made them with back in the day. Those well-known companies produced valves the old-fashioned way, in huge factories using thousands of skilled workers whose entire careers consisted of making parts for, or assembling, valves. Glassblowers, wire-makers, machinists, engineers, metallurgists, chemists, assemblers, fabricators and a myriad of other professionals depended on the valve market to earn their crust. I’ll bet the advent of the transistor didn’t please everyone! The majority of today’s tubes are made on highly mechanised production lines with minimal human interaction, so modern valves are often viewed with great mistrust and even disdain by tube aficionados. Repairing any valve device means having access to replacement valves. One can usually fudge one’s way through a solid-state device repair using various other transistor or module types; doing this in an equivalent valve-powered device can be a bit trickier. And many of the peripheral components, such as bypass capacitors and plate resistors, were chosen for a very specific valve; simply plugging in another one that happens to fit the empty socket is a recipe for disaster. In the same way we used transistor substitution books to find an equivalent transistor for one that has a weird number (or no number at all), techsiliconchip.com.au nicians of yesteryear relied on telephone-book-thick manuals for valve substitutions. You could cross-refer different valves to see if a 12AX7 could be used instead of the ECC83 specified in the schematic (in this case you can; they’re the same valve). Most books also offered possible equivalents, along with tweaks you’d have to make to the circuitry in order to use the suggested alternative. If all else failed, you could look up the tube’s specs and match it with another candidate; as long as power curves, plate and grid voltages and current, amplification factor, mutual conductance and a host of other values corresponded, or these could be achieved with circuit tweaks, you were good to go. Generally speaking, if the book said it would work, it would. This level of certainty was down to the consistency of valves produced back then. The same doesn’t hold true today, where automated manufacturing creates differences between valves even from the same production lot. As these differences became more of a problem, circuit design evolved to cope, with the likes of variable biasing and adjustable feedback loops incorporated in an effort to ‘balance’ performance. A novice serviceman might get caught out after replacing valves and neglect to adjust biasing, which could at best result in a bad-sounding amAustralia’s electronics magazine plifier and at worst, result in output transformer or tube failure. There are a few traps for young players then, but by far the biggest trap is the lethal voltages present in most valve amplifiers and radios. Even battery-powered devices have the potential to hurt the unwary. Informal workshop rules were imposed to keep people safe; putting one hand in a pocket while working on a live chassis meant it was less likely the serviceman would get a shock through the chest and across the heart, which is potentially fatal. One sore hand or arm from a highvoltage belt is God’s way of telling you to be more careful! Another rule is to avoid wearing rings or other jewellery that could short out something inside the case. Gold chains around the neck are a really big no-no! Yet another rule advises no fiddling with live hardware while chemically altered, on the phone or otherwise distracted. My own mantra is that I suspect that every valve amp is constantly trying to kill me. This usually keeps me alert. This amp in question is a 1970s Fountain; a 10W, push-pull stereo amplifier made in New Zealand using common valves. However, it had been sitting unused for a long time and the “repair guy” had simply plugged it in and turned it on to see if it would work. It did, as a smoke generator! He pulled the power as soon as he saw smoke July 2018  45 but the damage had been done. Time to call the professionals! After removing the case and giving the chassis a puff with compressed air, I noted several power-supply capacitors were blackened - possibly the source of the smoke. Capacitors “drying out” or otherwise degrading when not being used are a major cause of hardware failure, valve or solid-state. While old caps can sometimes be electrically recovered (see the Capacitor Reformer project in the August & September 2010 issues; www. siliconchip.com.au/Series/10), I make a practice of changing them on older amps as a matter of course, especially the power smoothing and output coupling capacitors. They are (usually) relatively cheap, readily available and easy to replace, so it makes sense to do it. With the caps replaced, I removed the valves and plugged the amplifier in to my light-load and auto-transformer, gradually increasing the AC voltage. No bright lights or smoke, so with supply input at 230VAC, I measured voltages at the usual points. While I had a typical 6-ish volts AC for filaments, I had only a fraction of the hundreds of volts I expected on the plate pins of the valve sockets. Tracking back, I could see the power supply’s transformer fed several carbon composition series dropping resistors adjusting voltage for each stage of the amplifier, with the resistors bypassed to ground by now-replaced dead capacitors. Though the resistors looked OK, I suspected some might have gone open-circuit. I shut everything down and replaced them all with new, 2W alternatives from my parts boxes. The valves, an ECC83, two EF86s and four ECL86s, tested OK on my Valve Heaven DIY tube tester (siliconchip. com.au/link/aak5), and after plugging them all back in and powering up, a faint but gratifying hum gradually sounded from my test speakers. A signal injector clipped to each input now confirmed everything else worked as expected. The pots and switches required a squirt of cleaner and a bit of working to settle them down but once done we were up and running and sounding great. Job done. Tripping the RCD A. L. S., of Turramurra, NSW, previously wrote of a problem with one of the safety switches (RCDs) in his home. He had to tackle another similar fault about a year ago but this time, it had a different cause... There are two safety switches in my household wiring and the repair of the first one, which protected half the power outlets in my house, was described in Serviceman’s Log in the October 2016 issue. Imagine my surprise when the second one, which protects the front part of my house, started to do the same thing and cut out intermittently for no apparent reason! After the previous adventure, the first thing I checked was the RCD itself but it seemed perfectly fine and tripped exactly as it was supposed to with a leakage of 30mA (as confirmed with an RCD tester). It was also cold to the touch so I discounted it as being the culprit for the time being. I would have to think a little harder to figure out this one. The RCD seemed to trip and cut the power when I was in the downstairs workshop which has many mainspowered test instruments, chargers, powerpacks, computers, an air conditioner and even a freezer. The first time it cut out I didn’t take too much notice what was actually plugged in and switched on but as a precaution, I turned off all the power outlets except for the freezer. Returning to the workshop, I began to turn things on one by one hoping it would isolate the offending item but it didn’t trip again. So I thought maybe the freezer was the problem and it would only trip the RCD when the thermostat cut its compressor back in. So I waited patiently for its motor to come on but when it did, still nothing happened. 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. I put it down to a possible surge or overvoltage and carried on regardless. Then one day, I switched on my Audio Precision ATS-1 audio analyser to do some tests on a subwoofer amplifier and the safety switch cut out about five minutes later. Repeating the exercise, the same thing happened, so the instrument was quarantined for later investigation! All went well for about a week but then, 10 minutes after I switched on my oscilloscope, the circuit cut out again. I couldn’t believe it! Surely two instruments which were normally very reliable couldn’t go south at the same time. I checked them both for possible earth leakage which could trip the RCD but they seemed OK. What I did notice that both instruments were plugged into the same power board. This is a supposedly good quality Jackson model PT8888 made in China and boasts EMI/RFI filtering, surge protection and overload cut-out. It has eight outlets, two of which are widely separated to fit large plugpacks. It is made of very strong metal and was expensive when purchased from a reputable electronics retailer. Fearing the culprit may be one of the eight devices plugged in, I powered them on one-by-one to see if the safety switch would cut out but again, nothing happened. Then one day, switching on another instrument, it tripped the RCD off once again. This was another different ATS-1 analyser which at first made me think there may be a design flaw with them but it was also plugged into the Australia’s electronics magazine siliconchip.com.au Servicing Stories Wanted 46 Silicon Chip Jackson power board. So I switched off everything connected to the power board and I also switched the power board off, both at the wall and via its onboard power switch. Having quarantined everything on that power board, the circuit was fine for the next week. I then decided to have another look at why those items connected to that power board were triggering the RCD. I had to do it during the day when everyone was at work because the TV was on the same circuit and my family members complained loudly and bitterly every time I cut the power during their favourite programs. I started by plugging the Jackson board back in and then plugging each of the eight items in, one by one. After a while, the safety switch cut out. Making this more difficult to diagnose was the 5-15 minute delay between adding a device and the power cut. At this point, I tried switching the board on and off with its own onboard switch. At one stage, I had nothing plugged in at all and upon switching the board on, the RCD politely cut out and therefore identified the Jackson board to be the culprit at last! The board looked very professional but on closer inspections, had dubious approval markings and even had a strange warning which read “AS/ NZS TESTING NOTE This device contains voltage limiting devices, test at 250V only”. A continuity check indicated 409kW between Active and Earth which was suspect because any varistors it used for surge protection would have a much lower resistance at full mains voltage and this would be enough to leak more than 30mA to Earth. Normally I would throw such a board away but it was so expensive and its metal case was really handy in the workshop because it was so rugged. Not only that but I had another one exactly the same so I wanted to find out the reason for the failure. I decided that a repair may be possible and that the result may assist anyone else who has a problem with this model or similar models. My first rather optimistic theory was that a spider or insect had crawled into a small gap between the metal panels and was cooked and carbonized, creating a residual current between Active and Earth. So I tried to undo the two small siliconchip.com.au Neff oven repair J. B., of Melbourne, Vic, recently had to delve into the innards of his oven. What seemed like a simple light bulb replacement turned into a complex and technical repair... I’m a self-taught radio technician from the 60s and later became a black and white TV valve jockey, eventually getting into colour TV and tape recorder servicing. For a day job, I am an aeronautical and mechanical design engineer and I’ve retired as an airworthiness regulator. I now repair aviation headsets and represent a US/UK Company using neural network synthesis to find intermittent faults in cables, connectors and chassis. One day, as my wife and I were preparing for the evening meal, we turned on the light in our Neff oven and it blew with a blinding flash. I replaced the bulb but the new one failed almost instantly. On inspection, the filament support wires inside the bulb had touched. This had caused some kind of internal damage to the oven so after removing what was left of the failed bulb, I traced its wiring to the Operations Module. This module receives DC power from a switchmode supply and signals from a switchboard. The Operations Module contains the microprocessor, driver integrated circuits and numerous relays. The light circuit had an SMD NTC thermistor in series with the relay coil (just a guess as the device was a charred blob). The PCB tracks had also been burnt beyond recognition. Whilst there was some separation between the 230VAC light track and adjacent low voltage control tracks, the separation was insufficient for this failure. The torching thermistor burnt a PCB track which powered a suite of relay coils and conducted 230VAC to the driver ICs. I repaired the PCB and the burnt tracks and soldered a 10W resistor where the thermistor used to be. On reassembly and power application, the oven went berserk; the door lock mechanism continued to cycle closed then open, the replacement oven light would go on but not turn off and the circulating fan in the heating space accelerated to take off power and stayed there. So I decided Australia’s electronics magazine to pop the module back out and take some voltage readings. I found that when the microprocessor outputs went high, a hex Darlington switch with diode protection turned on but not off. Four days had passed since the lamp blew and my wife was getting impatient. Plan B was a replacement Operations Module; none in Australia, a few in Germany, an estimate of a one month lead time and some $500. Upon further investigation, I became more certain that there was something wrong with the Darlington IC. Replacing one SMD in the middle of a ‘farm’ was a challenge; I’m not practised at this but the internet was very helpful. I removed the immediately adjacent relay and wrapped the remainder of the components and tracks first in paper and then in aluminium foil, leaving exposed only the IC to be removed, along with a few adjacent components. I cautiously applied a heat gun to the IC and in seconds, the parts were removed whilst the masked components remained in place. This was actually my second attempt as I first practised the technique on a disused board. I then tidied the PCB tracks and fitted the replacement IC and other parts. The original relay had a 9V DC coil but I only had a 12V DC replacement on hand, so I fitted that and ordered some 9V relays to swap in later if necessary. The oven was reassembled, power applied and all functions tested serviceable. The roast lamb which followed was delicious and I’ve banked some brownie points for the future; I focussed on the time difference between repair and overseas supply, definitely not the money saved! Carefully measuring and recording voltage readings in the unserviceable state then comparing these with values expected from first principles was the key to figuring this one out. Impatience and a hasty dismissal of what seemed to be zero voltage when in fact there was some small residual led me down a wrong path. But most importantly, when replacing the lamp in your oven, make sure the new one is designed for high-temperature use. July 2018  47 DID YOU MSS OUT? Is there a particular project in S ILICON C HIP that you wanted to read – but missed that issue? Or perhaps a feature that really interests you? Grab a back issue . . . while they last! The SILICON CHIP Online Shop carries back issues for all months (with some exceptions!) from 1997 to date. 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Full details are at siliconchip.com.au/shop/subscriptions 48 Silicon Chip Australia’s electronics magazine screws underneath to have a quick look. However, these proved to be of a triangular design and none of the hundreds of bits in my toolkit would fit. They were also countersunk so impossible to butcher with a grinder or hacksaw. The hardware shop didn’t have anything either but fortunately, the local Turramurra cobbler (yep, the cobbler!) was able to grind one up in about two minutes and we had the back open just a few minutes later. Back in the workshop, I took a good look at the innards. There was a small PCB which had three varistors, type 14D147K (rated at 275VAC) across each of the rails and one 100nF mainsrated capacitor. That was about it; so much for the extravagant claims about RFI/EMI protection! It also had an integral 10A circuit breaker and an onboard neon-lit mains switch. It was perfectly clean and no cremated insect or spider could be seen! So the PCB had to be removed because all the suspect components were underneath. This was a real pain because it was held in place by a heap of big soldered connections to the long brass outlet rails and to the chassis mounted switch. After desoldering everything, the chassis also had to be bent back 90° to release the PCB-mounted circuit breaker. Then I was able to remove the components one-by-one and check them for continuity. As luck would have it, the last varistor I removed proved to be the faulty one. While the PCB was bare, I replaced all three varistors with similar types and put it all back together then stood back and switched it on. It has given no trouble since. It does bring home the fact that components used for safety can sometimes fail in strange ways and maybe cause other problems like arcing or fire. I am not sure why this one failed; it may have been shock, vibration or humidity or a mains spike but I can never be certain. What is certain is that just about all “surge protected” powerboards use exactly the same components so it’s something to look out for and it is a good idea to switch power boards with integral protection or filtering off and/or unplug them when not in use, to protect them from lightning damage and so on. SC siliconchip.com.au