Fullscreen HTML5Med Res (??MB)HTML4

SERVICEMAN'S LOG Giving an old companion its voice back I’ve always been interested in loudspeakers. Their electromechanical nature appeals to me, as a good speaker needs to be both mechanically and electrically sound, the two parts working together in harmony. It amazes me that despite modern technology and improved materials, their basic operation hasn’t changed in many decades. Modern speakers tend to be more efficient, and usually offer a wider frequency range in a similar-sized cabinet compared to older speakers. But I don’t think they necessarily sound as good as older models. Of course, this is open to argument. There have been endless flame wars, err, I mean discussions online as to what is the best type of speaker. Some quote specs to prove how much better their speakers are. But like many others, I don’t care about the figures as much as how the speakers actually sound to me. Money for nothing I’ve spent a good deal of time in recording studios and high-end showrooms over the years listening to expensive drivers. They don’t always sound as good to me as the numbers suggest. While it could be that I’m just an audio philistine, I know what I like. Merely throwing money at speakers with fancy-sounding European names doesn’t guarantee pleasing results. I’d also argue that the speaker is merely one of the components in a system; all the components need to be up to scratch. Driving a $5000 set of speakers from a cheap and nasty amplifier (or even an expensive one, if it’s poorly designed) won’t do them any favours. And running a rubbish set of speakers from a $5000 reference amplifier is just a waste of money. The fact is that all speakers are not created equal, and the extensive range of cabinets, enclosure materials, driv86 Silicon Chip er constructions/configurations and crossover designs means there’s a lot of room for experimentation. Anyone who has kept up with hifi magazines will be aware of the trends and fads that have come and gone over the years, with speakers made of everything from concrete tubes to metal drums and even cardboard. I recall back in the 70s, a family friend showing off his expensive (and admittedly cool-looking) electrostatic speakers; the first I’d seen outside of magazines. When he fired them up though, I wasn’t as blown away as I thought I would be. They sounded good, but I reckoned our middle-of-the-road system at home sounded better. And while our speakers didn’t look like a couple of framed antique prints hanging on the wall, at least if I cranked the volume, I could feel the sound as well as hear it. Editor’s note: electrostatic speakers can give excellent mid-high definition but are famously lacking in bass, with some having integrated magnetic woofers to try to overcome this limitation. However, home stereo systems are not the only domain of quality drivers. The live music and sound reinforcement worlds feature some serious, high-wattage hardware. Whether it’s an 18-inch bass driver designed for PA systems or a 10, 12 or 15-inch instrument speaker, the type and quality of driver used will profoundly affect the resulting sound. Australia’s electronics magazine Dave Thompson Items Covered This Month • • • • Guitar speaker re-coning Fixing an inverter arc welder Panasonic AM/FM radio repair Double wall oven repair *Dave Thompson runs PC Anytime in Christchurch, NZ. Website: www.pcanytime.co.nz Email: dave<at>pcanytime.co.nz Let’s say I buy a generic 12-inch, 50W speaker from the local electronics store. It’s likely a lot cheaper than buying a speaker designed specifically for instrument amplification, but it will almost certainly not give me the sound I’m looking for, and given the punishing output of an overdriven guitar amplifier, it may not last very long either. Listen to the music Choosing the right speaker can therefore be a bit of a mission. Only the individual knows what sounds good, and this knowledge is not always transferrable to somebody else. Looking at catalogs doesn’t help much either, with lots of purple prose being used to describe speakers to potential buyers. It’s a bit like trying to describe colour to a blind person; for example, the literature for guitar speakers throws around terms like “crunch”, “throaty”, “warmth”, “vintage”, “punch”, “expressiveness” and “chime”. But what do these terms really mean? Half of them sound more like terms you’d expect to hear when wine tasting, not shopping for a loudspeaker! Most of the musicians I know simply go down to a music shop, plug in their instrument and play various amp and speaker combos until they find one that sounds like what they are after. I’ve only ever purchased one speaker from a catalog, and that was for someone who specifically wanted it to put into an existing cabinet. The siliconchip.com.au result wasn’t to my taste, and certainly didn’t match what I interpreted the catalog’s descriptive terms to mean, but he was rapt with it, which is the key point I guess. This is why musicians become passionate about their gear. We’ve usually spent years getting our sound, and we don’t want to have to go through all that drama again. Besides, this newfangled stuff generally doesn’t sound as good. So losing an amp or speaker can be like losing an old friend. Having gear nicked by some chancer at a gig, or damaged on-stage by a bandmate who has had one too many falling into it can be tough to take. Losing a ‘vintage’ amp or speaker is even worse, as these can literally be irreplaceable. The sounds of silence Recently, one of my speakers failed; an ancient Celestion G12-65 (12-inch, 65W, 8W) used mainly for workshop testing. I’m not sure why it popped, but given it is at least 35 years old it could have just thrown in the towel. It worked the last time I used it, but when I plugged it in the other day to test a valve amplifier I’d repaired, there was nobody home. I first thought I’d messed up the amp repair, but on fursiliconchip.com.au ther testing, I discovered the speaker’s voice coil was open circuit. I was despondent, as this speaker has been with me through thick and thin. While checking online and reeling at today’s prices for a replacement, I came across some Celestion re-coning kits and thought that this was a perfect solution. Back in the day, proper re-coning kits were hard to come by and expensive; now they’re a dime a dozen. At $65 including shipping, it was a lot cheaper than replacing the whole speaker, which all things considered is actually still in pretty good nick, the blown voice coil aside. The rub with replacing a voice coil (pun intended!) is that to get to the coil, everything has to come out of the basket (the metal frame of the speaker). And those parts are generally glued in pretty well. It might be possible to remove the old cone and spider (the flexible, corrugated disc that covers and protects the coil cavity) intact, using a razorblade or similar. But it simply isn’t necessary as there is a new cone, spider and dust-cap (the bit in the centre of most speakers) provided in the kit. I can already hear the purists wailAustralia’s electronics magazine ing about the fact this will no longer be a vintage speaker. But I’m OK with that; I’d rather have a newish working speaker than a dead vintage one cluttering up the workshop. When the kit arrived, I first tested the new voice coil; I didn’t want to go through replacing everything only to fit a dud. The old speaker had a nominal impedance of 8W, and while the replacement coil read only 6.3W on my multimeter, that’s actually correct. A multimeter measures the DC resistance only, not the complex reactance, which depends on frequency. September 2019  87 You probably wouldn’t get a correct impedance measurement for a coil by itself anyway, as there is a mechanical component to the reactance as well as the fact that the coil is inductive. To measure the impedance of a driver, you need to use a specialised tester. Or you can hook up a sinewave signal generator to the input of an amplifier, connect the driver to the amplifier via a high-power fixed value resistor, then measure the varying voltage across the driver. Some basic calculations using Ohm’s Law then give you the impedance at a given frequency. Don’t fear the repair Re-coning is often seen as a complicated process not worth doing, so many people don’t even consider it. But I’ve done it quite a few times over the years and if I can do it successfully, so can anyone. I’m no magician! Just a guy with some tools and a little bit of knowledge, and I’m not afraid to have a go. I started the job at hand by preparing to remove the old cone. The spider and voice coil are connected to the bottom of it, and the leads from the coil are soldered to the terminal on the 88 Silicon Chip basket. So I first had to release those wire connectors. I could have just cut them off, but I wanted to get an idea of their overall length, so I heated and un-wrapped them instead. I then used a hobby knife to cut around the edge of the spider and removed enough of it to allow me to see and mark the voice coil resting depth in the magnet cavity with a sharpie; I might need this approximate measurement later. Next, I cut the cone away as close to the basket as possible, leaving the gasket (the thick mounting material stuck over and around the outside edge of the cone) behind. The whole assembly was then lifted clear, and I noted the state of the voice coil as it exited the magnet cavity; in this case, it appeared undamaged. This may seem an odd thing to do, but it tells me whether I should check the magnet aperture more closely for blobs of melted wire or other debris. With the cone clear, I quickly sealed the open coil cavity using several strips of masking tape. In a workshop like mine, there are metal off-cuts and dust everywhere, no matter how well I clean it. Murphy’s Law dictates that Australia’s electronics magazine some of this will find its way into that gap otherwise. Given the size and strength of the magnets on guitar speakers, which are typically welded or otherwise permanently bonded to the basket, the potential for contamination is high. Removing anything magnetic that gets stuck in there can be very difficult. I know techs who don’t bother with this masking-off ritual, especially if they are going to re-cone the speaker immediately, but I neglected to do this on one of the first speakers I re-coned and some foreign objects got in there. It was a right-royal pain clearing them out. For the sake of a minute or two of time and a few strips of tape, I avoided much potential misery. I proceeded to strip the basket of the remaining gasket, cone and spider material. It depends on how this has been attached as to how much work it will be to remove it. In this case, they used some kind of cement. I used a hobby knife and razor-scraper to very carefully cut the remains as close to the basket rim as I could. The possibility for slipping and carving up one’s own hands at this point is very high, and as I’d done exactly that a few times as a youngster building model aircraft, I was particularly averse to having it happen now. Nothing teaches sensible tool skills better than the memory of a painful injury (and boy, do I have a few of those, as regular readers will know)! Even though I cut very close and removed almost all of the remaining bits, I couldn’t get it all with a knife. While I could possibly have glued the new bits onto this and had no further problems, it only takes a slight amount of asymmetry at the wide edge of the cone to stress it. That can result in the voice coil rubbing on the side of the magnet cavity or non-optimal sound reproduction, even if the voice coil does clear the sides. It is well worth the extra effort to clear the old glue and cone from the basket completely. In such cases, I break out my trusty rotary tool and use it with a brass wire-wheel attachment to clean up the rest. This tool is speedadjustable and perfect for the job, but it does make a real mess, so I made sure to do this job outside (see above on sealing the magnet gap!). A quick brush and vacuum afterwards had the basket completely free of any debris. Compressed air can also siliconchip.com.au be used, but I’m wary of blowing more rubbish about, so prefer the vacuum cleaner option. Before going any further, I checked that the basket was still reasonably flat by sitting it face-down on a saw-bench table. These speakers can get knocked about a lot on tour, and over-worked (and possibly over-enthusiastic) roadies can sometimes distort the basket when securing the driver to a cabinet using power screwdrivers. A twisted basket can result in a trickier set-up during the re-coning process, and baskets that are out-ofround or too far gone usually need to be replaced. This one was still fine, despite its long history. Meet the new cone, same as the old cone Speaker coning kits come in several forms; some are pre-assembled, which means the voice coil, spider and cone are all glued together at the factory. This makes the job considerably easier, as the new assembly can simply be dropped into the basket, aligned and glued down. However, many kits come as separate components, and while this makes things a little trickier, the process is still relatively straightforward. My kit came unassembled. After double-checking there was nothing loose that could foul things up, I removed the masking tape covering the voice coil gap. Despite having taken this measure, I decided to check that the gap was clean by wrapping some double-sided tape onto an old ice-cream stick and inserting it into the coil gap, probing it around inside the cavity to pick up anything that might have gotten in there. The first tape came away slightly grubby, so I repeated the process with fresh tape a couple of times until it came out completely clean. I compared the new voice coil to the old one, making sure they were the same physical size; they were. I positioned the new coil into the gap, using the supplied Mylar shims packed around the inside edge to centre everything properly over the magnet. I then adjusted the coil to sit at roughly the same height as the mark I’d made on the old one; I’d fine-tune it later once the spider and cone had been fitted. I used a spare shim to check there was enough clearance between the outside of the coil and the magnet and siliconchip.com.au ensured the coil’s flat, tab-like connecting leads were positioned directly adjacent to the basket terminals, where they’d eventually be connected. I then dry-fitted the rest of the components; though my kit was unassembled, at least the cone and spider had been pre-cut to the right size. Two small holes for the voice coil connections hadn’t been made in the new cone though, so after referring to the old cone, I used a scriber to punch new holes in the same locations on the new one. Satisfied everything fitted, I glued the spider to the basket using the supplied adhesive, with the voice coil’s tabs protruding through the centre of the spider. After clearing excess glue, I left it overnight to cure. The next morning, I used epoxy resin to ring the centre of the spider, the voice coil former and the rim of the basket. I then set the cone into place and gently twisted it side-to-side to bed it into the glue. I also let this set overnight. The following day, I carefully slid the plastic shims out and gently moved the cone back and forth to check that the voice coil was clear; all was well. After soldering two lengths of the supplied ‘tinsel’ wire to the voice coil tabs, which now sat at the base of the new cone, I fed them through the holes I’d made in the cone and soldered them to the basket terminals. Finally, I centred and glued the dust cap to the cone, using some of the glue to run over the short lengths of exposed tinsel wire. I used the same glue to stick the four-part, hard-cardboard gasket ring around the top edge of the cone, and the job was done. I tested the speaker. It sounds as good to my ears as it ever did. Quite “crunchy”, “throaty” and “punchy” in fact. I’d go as far as maybe even calling it “gravelly”! Inverter welder repair Don’t you hate it when you buy something, check that it works and then put it aside, and when you go to use it again, it doesn’t work at all? Especially if it’s no longer under warranty! Well, that’s almost what happened to B. P., with his wonderful new welder. He managed to get a replacement unit but also got to keep the faulty one. So of course, he had a go at fixing it... I bought my first arc welder some 45 years ago. It was an Abel 110A AC unit Australia’s electronics magazine and over the years, I’ve done a lot of welding with it. It’s massively heavy with wheels and a handle to make moving it easier. The transformer has a copper secondary winding, which was the standard back then, but is unheard of these days. I’ve done very few repairs on this unit during its life but I did replace the electrode holder a long while back and around five years ago, I replaced the old dilapidated welding cables with some 250A cables that were salvaged from a defunct mobile diesel welder. I still have this Abel welder and it still works well after all this time. Around five years ago, I bought a smaller 100A AC arc welder from Aldi when it was on special. This unit is much smaller and considerably lighter than the Abel welder but it still has a transformer, although it has an aluminium secondary. I’ve made a few modifications and improvements to this unit over time and it’s still working well today also. This year, I decided to buy one of the newer models of DC inverter welders. I chose a 250A unit, which was the most powerful that was available at the time. It weighs around 5kg and is smaller than the 100A Aldi unit while delivering 2.5 times the amperage. That just goes to show how fast technology improves. After the welder arrived, I unpacked it and connected the cables and did some test welds. I was impressed with the high current and the ease of striking the arc. I’ve only ever previously used AC arc welders and it’s a lot harder to strike an arc with an AC welder than a DC welder. After the successful test, I put the welder away, as I didn’t have an immediate need to do any welding. Around six months later, I got the welder out to do a small welding job but I was amazed and disappointed to find that it no longer worked. All I got was a tiny spark which looked like it was about 10A worth. I suspect this was from the power source intended to initiate the arc. But where was the main welding current? Fortunately, the welder was still under warranty, being less than 12 months old, so I contacted the seller, who requested a video of the fault, which I supplied. The seller then promptly sent me an identical replacement welder but they didn’t want the old welder back. With the high cost September 2019  89 This photo shows the repaired arc welder PCB, with the problem diode circled. All six diodes were re-soldered. Note the generally poor soldering quality. Many of the SMD pads have lumpy-looking joints, indicating a lack of flux activation. of repairs these days, it must have been cheaper for them to just replace the faulty unit with a new one, rather than having it sent in for inspection and possible repair. As soon as the replacement welder arrived, I tested it and confirmed that it was working correctly and I put it away. Then I contacted the seller again and thanked them for their excellent service and very fast replacement of the defective welder. The repair Now that the original welder was officially scrapped, I could take it apart and have a look at it. It would have voided my warranty but the warranty now applied to the new unit, so there was no reason not to open up this nonworking welder to see if I could fix it. I removed the eight screws securing the top cover, lifted it off and put it aside. I could now see the inner workings, which consisted of a couple of circuit boards and a lot of aluminium heat sinks. It still looked brand new inside, which was not surprising, because it had barely been used. The first thing I did was to remove the main circuit board and inspect the soldering on the back for any faulty joints. On a previous occasion, my gasless MIG welder had developed a fault whereby the wire speed control no longer worked and the wire ran at full speed. It turned out to be noth90 Silicon Chip ing more than dry joints on the PCB, which was an easy fix, so I wondered if a similar fault might be at work here. But the PCB soldering was all good, so I turned my attention to the component side of the board, where I looked for any obvious signs of blown-up components. I found nothing, so I took a closer look with a magnifying glass, but there was still nothing obvious. I was beginning to think that this fault was beyond my ability, due to the obvious complexity of the circuitry involved in the unit, when I noticed something that was not quite right. There were six of what appeared to be surface-mount diodes next to a small daughter-board and the middle diode (D15) on the right-hand side just didn’t look right. This diode was sitting at an angle and one of its legs appeared to be raised slightly above its solder pad. It looked like a manufacturing defect, where the component had not adhered to the PCB and therefore was not soldered properly, but it must have been initially touching the pad for the welder to have worked in the first place. I was sceptical that this could be the fault that had stopped the welder from working, but I also wondered if it might be the culprit. As this was the only obvious thing I could see at this time, I decided to resolder the leg of diode D15 and while I was at it, I also re-soldered both ends Australia’s electronics magazine of all the diodes, as they all looked to be lacking a good amount of solder at their joints. This isn’t the first time I’ve seen something like this (see the photo of the repair). I then connected the welder up again and got out what I needed to do a quick test, just in case it was now working again. I wasn’t really expecting it to work, as I didn’t think that something as simple as this minor manufacturing defect could have caused the failure. However, as I didn’t know just how this circuit worked, it was a possibility. To my astonishment, the welder was now working properly again. I was amazed that just one bad solder joint in this part of the circuit had been the cause of the failure. I almost couldn’t believe that I’d found the fault and fixed it so easily. I’d been expecting to find something major to be wrong with the welder that would have either been impossible to find or impossible to fix. I put the welder back together and put it away. Since then, I’ve used it several times for small welding jobs and it has been working faultlessly. One of the great things I’ve discovered about it is that since it’s so much easier to strike the arc, I can use old welding rods that I’ve had stored for some time. Over time, the flux absorbs moisture, which causes difficulty in striking an arc with an AC welder. But not so with this one! It works with rods that I reckon I’ve had for around 25 years. Anyway, it just goes to show that it’s worth having a look at a faulty device, even if you only have limited experience with repairing these devices, as sometimes the fault is easily found and fixed. Panasonic RF-P50 AM/FM radio fix G. C., of Wellington, New Zealand rescued a Panasonic RF-P50 AM/FM radio from the rubbish bin. As is so often the case in equipment that is about to be discarded, the fault was a simple one, easily fixed by someone with some repair skills... The Panasonic RF-P50 in question is powered by two AA cells and it had a very loud crackle in the audio output when the volume control was operated. Also, the audio would “drop out” entirely at some positions on the volume control, making the set virtually unusable. I thought the fix would be trivial: all I would have to do is dissiliconchip.com.au mantle the plastic case and clean the volume control pot. The previous owner had taken the set to an electronics repair shop (there can’t be many of those left these days, surely) and was told the repair was uneconomic (which it probably was!). The set was given to me and I quickly had the case apart and cleaned the volume control pot with a cotton bud and some isopropyl alcohol. After re-assembly, the set worked fine for a day or two but then the crackle and blank spots progressively returned until again it was unusable. Oh well, perhaps a further clean was required, so I did so again. However, the fault reappeared after each clean and the radio was eventually put aside again as unusable. That was a shame as it worked well, with a clear, undistorted audio output for a period after each clean. Finally, after some time I had a “lightbulb moment”; I thought maybe this perhaps this was the classic case of a DC current flowing through the volume control potentiometer. That would likely be due to a faulty (leaky) series capacitor or, lacking such a capacitor, I could add one into the circuit. On a rainy afternoon and with little else to do, I once again pulled open the Panasonic radio’s case. The set uses a 30-pin Sony CXA1619 FM-AM receiver IC centrally placed on the PCB, so I googled the IC number and studied a number of typical broadcast radio receiver circuits based on that IC, one which showed a volume control connected via capacitors between pin 24 (the detector output) and pin 25 (the AF input). Although the PCB was tightly packed with small components, the volume control arrangement could be seen and the voltage divider circuit easily discerned, but no capacitors were located in this area of the PCB. Maybe I was on the right track after all. I had to figure out how to fit a small capacitor in series with the volume control pot. I thought about cutting the very fine track, which was going to be quite difficult if I was to avoid damaging the PCB. Then I realised that the top end of the voltage divider was connected via a miniature 2.2kW 0.25W series resistor. All that needed to be done was to cut one leg of the resistor, lift that end of the resistor off the board, remove the siliconchip.com.au remainder of the lead from the pad and then solder a capacitor between the pad and the remaining lead on the end of that resistor. And that is what I did. I added a 100nF Mylar capacitor in this fashion, then re-fitted the AA battery to test it out. What a difference it made! Even though the volume control pot had not been cleaned this time, a few operations of the pot cleaned up its operation and it then gave perfect audio output. After re-assembly, this little radio was restored to pride of place in the household. It seems obvious now that it was a design fault all along. One has to wonder how many other examples of this little radio have been tossed into the rubbish bin because of this annoying fault. St George DEO-6 Double Wall Oven repair R. L., of Oatley, NSW knew that he would be on his own when it came to fixing a 25-year-old appliance. He used a methodical approach, and it paid off in the end… Approximately 25 years ago, when we renovated our kitchen, we bought a St George Double Wall oven, with digital control. It performed flawlessly until about two years ago. Since then, occasionally while in use, it would emit a beep and shut down. Resetting the circuit breaker would restore its operation. It would then work perfectly for several months until the same thing would happen again. It got to the point where the fault would occur every other time that we used it. My wife was not happy. As I knew there would be no service support for a 25-year-old oven, I would need to fix it or replace it. The oven was in otherwise perfect condition and to replace it would probably involve modification to the surrounding cupboards, so I decided to try to fix it first. I found a replacement controller on eBay, except that it was for the single oven model. But I decided to buy it and become familiar with the circuit before I disassembled my oven. The module consists of two circuit boards. One is a power supply/relay board, and the other, a display/controller board. From the connection diagram supplied with the oven, I figured out how to connect the purchased module to mains power and the other connections; it turns out that the single oven module is identical to the dual oven version, but with a few parts (eg, relays) missing. I plugged it in and ran through all the functions, and it worked fine. So, the big moment came, and I disassembled my oven, removed the controller connections (about 30 wires) and connected it to my bench set-up. I applied power and went through all the functions; it worked as expected. I decided to heat it up a bit and tried again; it still worked. So, I disconnected the module, got out my magnifying headset and carefully scanned the boards looking for dry joints, as the problem was obviously heat-sensitive. I found a couple of suspect joints on the digital board and re-soldered them, but they were not drastic and unlikely to be the cause of the fault. I then checked the power supply board. All was fine until I got to the filter capacitor joints. They looked very strange. The capacitor looked perfectly OK from the top, but I decided to remove it because of the strange-looking solder joints. And that was it; the 2200µF capacitor had leaked (and probably dried out), but because it had been sealed to the board, no electrolyte had spread out from the base. I checked the other three 1000µF electrolytic filter capacitors in the supply circuit; they looked OK, but I replaced them all, as it was reasonable to expect that they would be on the way out as well. I replaced the capacitors with 105°C-rated units, cleaned up the board, reassembled the module and installed it. The oven now runs like new. I saved us the cost of a new oven and my wife is happy. SC Servicing Stories Wanted 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. Australia’s electronics magazine September 2019  91