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SERVICEMAN'S LOG Odyssey Stratos amplifier voltage conversion Staff member Nicholas Vinen loves big amplifiers. Well, big anything actually – big speakers, big engines, big cars, big monitors, big . . . you name it! Many years ago, he bought a big amplifier in the USA but then had to convert it to operate from 230VAC for use in Australia. The Odyssey Stratos is an impressively large stereo amplifier, with a power output of 150W RMS per channel. I bought this beast around the year 2000 while living in the USA. It was during those heady “Dot Com Boom” days when venture capitalists were just about giving away free money to anybody with a business plan that included the word “Internet” in it. The company I worked for was bought by another company which was then bought by another company which wasn’t quite sure what to do with my division. In the end, I was sent home and I brought the Stratos with me. These amplifiers are still being sold today – see www.odysseyaudio.com/ products-stratos-stereo.html I didn’t know much about amplifiers when I bought it (how things change) but the reviews were good, the “specs” were impressive and it came with a 20-year warranty. I also liked the idea of the whole aluminium chassis being the heatsink, so cooling wouldn’t be a problem no matter how hard I drove it. Anyway, having brought the Stratos home, I had to figure out how to power it. It’s a 115VAC model and for years I used a 2kW step-down transformer. But this arrangement had a few drawbacks. First, the step-down transformer was quite large, as was the amplifier, and the whole shebang took up a lot of space. Second, it was inefficient The Odyssey Stratos stereo amplifier is quite a large beast and is capable of pumping out 150W RMS per channel. Converting it from 115VAC to 230VAC operation proved to be a bit of a challenge. 54  Silicon Chip Items Covered This Month • • • • Converting an Odyssey Stratos amplifier from 110VAC to 230VAC. Faulty home lighting system Dave’s faulty air-compressor Digitor T-1333 sound system and consumed a lot of power at idle, meaning I had to turn it on and off at the wall, which I found inconvenient. And third, if I wanted to transport the amplifier, I had to cart the transformer around too. Before bringing it home I spoke to the bloke I bought it from (at the factory) and he told me that they sold 220VAC versions of the same amplifier for the European market and there were only a few slight differences. So I knew it would be possible to change it to run from 230VAC but, at the time, I didn’t press him for details. Having put up with the step-down transformer for so long, I recently thought that I’d open it up and see how hard it would be to convert it to 230VAC operation. As expected, the mains transformer (a 400VA toroid) has two primary windings and these are connected in parallel for 115VAC operation. This meant that I could change the transformer configuration so that the primaries were connected in series for 230VAC operation. And thanks to the IEC mains input socket, I could easily replace the mains cord after doing this. Before starting, I checked that the socket and mains switch were both rated for 250VAC and this was indeed the case. The mains switch contained a neon lamp with an in-built resistor but I figured since it was 250VAC rated, this wouldn’t need any modification. But how should I reconfigure the transformer? I didn’t want to reconnect it with the wrong phasing and burn it out. The problem was that I didn’t know siliconchip.com.au which colour wire was which and with the windings connected in parallel, there was no obvious way to measure it and find out. After some thought, I realised that if I disconnected one of the primary wires going to Neutral (at the IEC socket) and one going to Active (at the switch), then as long as they weren’t from the same winding, connecting these two together would give the desired result. The primary wires emerge from the transformer in this order: brown, black, red, orange and green/yellow. Green/ yellow is presumably for an earthed screen and I guessed that brown/black and red/orange were the two winding pairs, with brown and orange being connected to Neutral and red and black to Active. Tentatively, I clipped off the black and orange wires as it was difficult to de-solder them (there were multiple wires looped through and then soldered to each terminal). As well as the outer insulation jacket, the wires were enamelled so I placed a plastic tray under them and scraped away the enamel insulation from the ends. I then checked the continuity of the windings. The resistance of the brown/black and red/orange pairs was about 1.5Ω, confirming my suspicion that these were the two primaries. The odd thing was that I was still getting a continuity reading between the two primary pairs which should have now been disconnected. The resistance was much higher though, at around 200Ω. However, I was pretty confident that I had the right wires so I soldered the orange and black wires together and insulated the joint with yellow heatshrink tubing. I was still puzzled as to why I was getting such a low reading between what should have been disconnected pairs though and decided to investigate further. And then it hit me. The front of the amplifier has a glass panel with the Odyssey logo sandblasted into it. This logo is lit by a pair of incandescent lamps. Duh, the black and white pair of wires I noticed earlier running from the mains sockets must be to supply these lamps with 115VAC! Had I plugged it into 230VAC, the lamps would have immediately blown and I’d have had buckley’s chance of finding exact replacements locally. Any­way, I dodged that bullet and since there were two lamps, I reasoned that I could connect them in series and they would happily run off 230VAC, at least siliconchip.com.au until one eventually blew, when they would both go out. This wasn’t difficult to arrange. All I had to do was clip the white wire going into the first lamp and the black wire running between the two, strip the ends back and connect the incoming white wire to the black wire from the second lamp. The two would then be in series. I did this, then insulated the solder joint and the two wire ends which now connected to nothing. Finally, I changed the mains fuse from 6.3A to 3.15A 250VAC, stood back and powered it up. You beauty, it worked! It did blow the fuse the second time I turned it on though, obviously due to the transformer inrush current. Replacing the fuse with a 3A slow-blow fuse fixed that problem. At last, I could say goodbye to that clunky step-down transformer. Home lighting system Sometimes, a puzzling fault can have a very simple explanation as G. B. of Ararat, Victoria discovered when he was recently called out to service a large diesel-powered generator. Here’s what happened . . . Many years ago (more than I care to remember, in fact), I learnt the art of servicing Cooper engines from my father. Very popular from the 1930s though to the 1950s, these engines could be found on practically every farm in Victoria that hadn’t yet been connected to the SEC, as the elec- tricity grid was then known. Basically, they were single-cylinder petrol engines that were used to drive shearing machines and 32V home lighting plants. And it was not uncommon for some farms to have several such machines. One day back then, we got a call from the father of a lad I went to school with. “Rodney says you’re pretty smart. Will you come and see if you can fix our lighting plant? It won’t run properly and every electrician we can find hasn’t been able to fix it.” I should have woken up then: electricians don’t fix engines and I don’t fix electrical problems. I know better now but then I said “OK” and collected the tools to give the engine the usual required valve grind. A faulty valve seat was about the only thing that could make one of those things hard to start. The trouble was I either hadn’t listened or hadn’t been told the full story. I arrived at the farm and was pointed to the engine shed with the comment “It starts OK but won’t keep going.” Well, this was going to be easy, I thought. If an engine has fuel, air, compression, ignition and exhaust, it must run. The first shock came as I walked into the engine shed. Instead of a Cooper engine happily turning a generator to charge a bank of 32V batteries, I was faced with a huge Lister diesel rigged as a “Startamatic” plant. The principle of the Startamatic was that if somebody turned on a April 2016  55 Serviceman’s Log – continued One thing leads to another with DIY It’s marvellous how one thing can lead to another. All I wanted to do was paint a room but I ended up stripping down an air-compressor. By Dave Thompson One of the tasks I undertook during my recent workshop clean-out was to improve accessibility to my aircompressor. Until then, the compressor had been sitting beneath my drill press-bench. Whenever I needed it, I had to drag the thing into the middle of the workshop floor and clear away all the dust and swarf from it before rolling out its retractable hose. For the amount of times I used the compressor, it was a tolerable workaround. However, as I was moving stuff around during the clean-up, I finally decided to find a better position for it. Recently, I took delivery of a heavyduty transformer-winding machine and it came with its own solid worktable that fitted perfectly into a corner of my workshop. There was an area under the table, beside the treadleoperated clutch, that begged to have something stored in it and I soon discovered that my air-compressor fitted into that space as if it was specifically designed for it. I then mounted the retractable air-hose reel onto one of the 10 x 75mm legs of the table and from there I can now roll the hose out anywhere into the workshop or into an adjacent garage. That’s the way things remained until I needed the compressor recently. Nina and I had decided to redo one of our bedrooms because the previous owner of our house had done a slapup job of throwing up some wall­ paper without properly preparing the wall. As a result, several patches of unsightly and unhealthy-looking mildew had begun peeking through the wall covering. Stripping away the old paper fell to Nina. The theory was that once she’d removed it all, we’d throw an anti-mildew undercoat onto the now-bare wall, followed by a couple of top-coats to finish it. But, as we 56  Silicon Chip all know, DIY doesn’t always follow one’s nicely laid-out plans. First, we discovered that instead of standard plasterboard (or drywall), whoever built the house in 1959 had decided to go for fibrous plaster. It’s an absolute <at>!%$# to work with! The molecule-thin top-coat of plaster on fibrous plasterboard breaks away with just the slightest provocation to reveal the true make-up of the board, which appears to be plaster mixed with horse-hair bristles. This meant that wherever the wallpaper had been stuck on with a bit more glue than usual, the plaster had pulled away and there were now decent-sized patches of rough, hairy bristles showing through. Being a serviceman, I did what anyone else in my position would do and retrieved the biggest randomorbital sander I could find in my workshop. However, all the sander did was remove more of the plaster holding the bristles together. As a result, the patch I was working on grew slowly larger until I had to chuck in the towel and admit that that particular strategy wasn’t helping. I then decided that what I needed to do was trim the hairs off the wall altogether and since there weren’t that many patches, I could simply use a sharp blade to “shave” the walls. After that, a bit of filler added here and there would be all I’d have to do to prep the walls for painting. Except, of course, it didn’t work out like that (but you probably knew that already). It turns out that the fibres used in fibrous plaster are made of the same stuff used to sew Superman’s cape and underpants together. Nothing short of a freshly-stropped straight razor would cut them and then only by direct perpendicular pressure from blade to bristle. This also left more marks on the wall. At the rate I could trim hairs back, it would take until Christmas 2019 before I’d done half the room so I called in my builder friend Dave for some advice. He told me that the best solution was to put what he called a “skim coat” of plaster on the wall. Once we had that, we could then prime, paint and be done with it Anyway, as usual I digress. One of the other reasons I stopped my initial sanding was that the dust being generated choked everything. While the sander itself has a dustextraction nozzle, which I married up to a backpack-style vacuum cleaner, even this set-up couldn’t cope with the sheer amount and texture of the plaster dust. It was as fine as talcum powder and it spread out every­ where. What’s more, the vacuum cleaner’s bag choked after just a few minutes, rendering it useless. So what’s all this got to do with my air-compressor? Well, another problem I encountered was that the sander got so hot I couldn’t hold onto it and that was only after 10 minute’s sanding. So it was out to the workshop to clean those tools and filters with my trusty air-compressor, only to have it run out of puff after a just few seconds of cleaning. Thinking that I must have a loose power lead, I checked the connection but that seemed to be in order. I then tried switching the compressor off and on again, only for it to start and then immediately stop. Just what I needed – a faulty air-compressor. siliconchip.com.au Mildly annoyed because my plans for that afternoon hadn’t included stripping down the air-compressor, I pulled it out from its new hidey-hole and lifted it onto the bench. This machine is one of Dad’s old compressors and isn’t one of those cheap units that are available these days. On the contrary, it’s a high-quality Italian-made unit with impressive specifications for such a small unit. Usually, with a compressor, it’s the seals and gaskets that wear out and you simply replace them to get things huffing and puffing again. As a result, I stripped it down with the aim of doing just that. The head of the pump was held on by four long bolts and they were soon out and the head lifted clear. This revealed what looked like stainless-steel spring valves, all embedded into the head itself. Inside the pump body was a piston and con-rod assembly. However, instead of a gudgeon pin joining the two parts together, the piston and con-rod were a single forged item, with a hard-rubber and plastic ring arrangement built into the crown of the piston. An electric motor ran the crankshaft directly, driving the piston up and down in the aluminium (yes, aluminium!) sleeved bore. Due to the very short stroke, the crown of the piston simply pivoted from side to side as it moved up and down inside the bore, relying on the seal to make and keep contact with the bore all through the stroke. It was a nice, simple system but one that’s prone to wearing out pretty quickly I’d imagine. I was going to need a new piston and bore, plus a new head assembly, and that sounded expensive to me. There was a service sticker on the compressor so I dialled the number only to discover that it was now disconnected. I then hit the Internet and discovered that the company in question had shut its doors in 1997, after more than 50 years operation. I then found the manufacturer’s website and sent off an email asking if they had an NZ agent. A week later, I received a response which recommended I contact a Hamilton company, which I did but they dealt mainly in $10,000 plus air systems and weren’t overly interested in one small compressor. After a bit more searching, I found another company who had the compressor manufacturer’s logo on their website, so I emailed them and received a prompt response asking me what model I had. I’d included all that detail in the first email, so they obviously hadn’t read it carefully. I repeated what I’d previously told them and after some email to-and-fro, they finally sent me some explodedview diagrams of the pump unit. I went through them and supplied the relevant part numbers plus photos and details of my worn-out parts as well. That was weeks ago and despite sending off two emails in the interim, I’ve heard nothing. In the meantime, I still needed an air-compressor and so I ended up shelling out for a budget one which did the job nicely. If I ever hear back from the parts company, I expect that the replacement parts for my Italian job will cost far more than this new compressor but I’m hoping I’ll be pleasantly surprised. I’ll let you know what happens. Last week, a chap brought in two rather large, powered PA speakers, complaining they had blown woofers. They’d been using the speakers at a function and according to the owner, they hadn’t been playing them “that loudly” when suddenly the sound level dropped dramatically and became “tinny” and “screechy”. After connecting a signal injector to each amplifier’s input, I quickly confirmed that the two big 15-inch woofers had indeed stopped working. A bit of research then revealed that these speakers are supposed to be able to cope with 150W RMS and I soon found suitable replacements on an online auction site. The owner subsequently purchas­ ed the required units and brought them around for me to fit. They were about twice the weight of the old ones and the voice coils half as big again. Given the difference, I’d estimate the original speakers to be 100W maximum, though the stickers on the plastic-moulded speaker cases claimed they delivered up to 800W! In my opinion, they might get to 800W peak music power output (PMPO) but certainly not 800W RMS. Swapping the speakers over was a doddle; I simply removed the 10 large screws holding each woofer in place, pulled the old speakers out and slipped the new ones in. Pushon terminals had been utilised for the speaker connections but as the terminals seemed rather flimsy, I cut them off and soldered the output wiring directly to the speaker terminals. Once both were done, I paired the speakers with my phone using Bluetooth and gave the neighbours a short demonstration of my favourite music. light (or something else) in the house, the engine came to life and supplied 240VAC power. It then automatically shut down again when the last appliance was turned off. I was informed that every electrician they could get had looked at the thing and none of them could fix it. The problem was that the engine would start OK, the voltage would start to rise and then the engine would go into its shut-down routine. Ah-ha, I thought. No wonder the electricians couldn’t fix it; what would they know about air, fuel, compression etc? This was going to be easy. And so I began my checks. The air-cleaner was OK, the fuel tank was full (and it was diesel, as it should have been) and compression could be felt when turning the engine over by hand. Of course, ignition in a diesel has to be taken on faith unless there is an obvious mechanical failure. siliconchip.com.au PA speakers April 2016  57 Serviceman’s Log – continued This photo shows the burnt-out windings in the toroidal transformer, caused by a hotspot created by the insulation under the thermal sensor. There was only one thing left; the engine muffler must be blocked. My theory was that as the exhaust pressure built up, there was nowhere for the exhaust to go and the engine was choking itself. At this stage, I needed to go back to my car to see if I had enough tools to strip down the exhaust system. As I walked back down the yard, I noticed that the junior members of the household were having great fun kicking a football over the open power lines going to the house. And then I saw it; they had managed to twist the two wires around each other. This meant that as the voltage came up, the control circuit sensed an excess load and rather than burn out the alternator, it shut down the engine. I grabbed a long stick (quite safe with the engine not running), sorted out the twist, then went back to the engine shed and flicked the light switch. The engine sprang into life, the lights came up and the owner came tearing out of the house. “What, you’ve fixed it already? You’re as smart as Rodney said you were!” After that praise, it was indeed hard to tell him that it would be better if his offspring kicked the football around the other side of the house. Digitor T-1333 sound system Protection devices can be added to transformers and electronic circuits with the best of intentions but they themselves can also be the cause of puzzling faults. A. L. S. of Turramurra, NSW recently encountered one such puzzling situation . . . My daughter recently complained that her Digitor T-1333 Sound System would not turn on. She used it when watching movies in her bedroom; it 58  Silicon Chip was small enough to fit around her dressing table and she also liked the sound. I had previously repaired this unit about four years ago and this had involved replacing a blown toroidal power transformer. As a quick check, I removed the 1A fuse and it was completely black. This indicated a catastrophic short circuit, as per the previous fault. So it was the same symptom but how could it be the transformer again? I told her that it may be the same problem and suggested that the repair may not be worth it. “No no!”, she said, “it is just the right size for my room and the tiny speakers fit in all the right places”. Then with a tilt of the head and a smile she pleaded: “Please, can you fix it for me?” How could Dad say no? The unit itself has a switch for “aux/5.1 surround” and a ganged volume control for five amplifiers which are basically five LM1875 power amplifier chips for front left & right, centre and rear left & right. Another LM1875 feeds a subwoofer and all of this is mounted inside the subwoofer speaker box which fits neatly into a bookshelf. There are five external speakers included with the unit, each of which is only marginally bigger than a can of baked beans and has a single 75mm driver unit (ie, no tweeter). Power for the unit is derived via a 120VA toroidal transformer, which I had previously upgraded to a 150VA unit. This has a 12VAC secondary which is fed to rectifier diodes and two 4700µF electrolytic capacitors for the power amplifier. At the time, I felt that the larger transformer would offer more protection. It was also labelled “heat protected” and 120VA units were no longer readily available. Because of the new transformer’s higher rating, I was very confident back then that the unit would be more reliable than before. I certainly never expected it to return with what appeared to be the same fault! I dismantled the amplifier unit and checked out the primary resistance of the transformer. And just as I suspected, it was open circuit! I then proceeded to replace it, which was a fairly easy job as I had done it all before. As soon as power was applied, the unit sprang to life and all the channels and the subwoofer worked perfectly! I was, however, rather worried because I had already upgraded the transformer and it should have been virtually bullet-proof. So why had two transformers now burnt out and would the latest replacement be reliable? After all, it was identical to the previous transformer and there was no room for a beefier unit, rated at say 300VA. What’s more the unit only drew a maximum of 70W, which is well within the transformer’s rating. But what if something peculiar was going on inside the unit and it got some “killer” stress from another part of the circuit – would this destroy the transformer again? My thoughts were that a thorough investigation was necessary to see if there was a hidden fault and that a long soak period would not be enough to prove it one way or the other. After fitting the new 150VA toroidal transformer, I decided to check out the rest of the circuitry for possible problems. The first stop was to check each of the five LM1875 ICs and all the voltages were fine. There are two 3.15A fuses between the transformer secondaries and the rectifier diodes. These were OK, indicating that there was little or no stress from the amplifier chips. LM1875s have very good protection against heat and short circuits and any faulty loudspeakers couldn’t hurt the unit unduly. So, what could the underlying problem be? In the past, I have replaced many toroidal power transformers from 25VA up to 300VA and all of them had blown primary windings, probably due to the higher voltage and the small diameter wire on that side. This is nearly always due to manufacturing problems such as pinholes in the insulation, or damaged or faulty wire. In fact, one year, I replaced over 100 siliconchip.com.au power transformers in just one model of TV (all under guarantee) due to faulty manufacture! If a customer rang up and said that they had smoke and a nasty smell, I could invariably get the repair done in minutes once I had arrived at the scene! However, in the case of this Digitor, I was not entirely convinced it was due to faulty manufacture of the transformers. After all, there were two different manufacturers involved, so surely I wasn’t that unlucky? The next step in my investigation was to check each of the six LM875s for total harmonic distortion and noise (THD+N). If there were any damaged chips, or voltage supply problems, or a lack of cooling, the distortion and noise levels would be very high. This only took a few minutes on my Audio Precision ATS-1 test set and they all came through with flying colours. Next, I decided to have closer look at the burnt-out transformer. This had the words “130deg Thermal Cutoff” printed clearly on the side. What if it siliconchip.com.au had failed simply because the thermal device had cut out and either hadn’t reset or wasn’t a resetting type? There was only one way to find out: unwind the transformer and examine the thermal cut-out. After removing all the exterior insulation and the heavy secondary windings, I began to question my own sanity for doing all this. However, I managed to justify it because the secondary wire is over 1.5mm diameter and I find it very handy to keep for those repairs which need heavy-duty wiring. When the primary winding was exposed I could clearly see the burntout wires. Amazingly, they were only about 20mm away from the thermal device, an “AUPO A4-3AN3 130°C 250VAC” made by Xiamin Electronics Ltd. This device is about the size of a small transistor and it goes open circuit at about 128-132°C and stays that way until the temperature drops back to about 100°C. As usual, the thermal cut-out had been soldered in series with the pri- mary windings. However, the manufacturer had also sandwiched it between some rather thick cardboard to insulate it from the secondary windings but, in doing this, had inadvertently insulated it thermally as well! So what happened is that a hot spot developed under the cardboard and this had eventually caused the primary windings to short circuit and burn out! So it would seem that the transformer manufacturer had actually created the problem. By trying to protect the transformer using a thermal cut-out device, they created a “hot spot” due to the arrangement used, which caused the windings and insulation to fail. Now that I felt sure that I knew what the problem was, I introduced some extra ventilation around the transformer to try to keep it cool. We’ll have to wait a few years to see if works though. In the meantime, I’ve examined the two dozen or so toroidal transformers in my stock and I’m happy to report that none of them have “heat proSC tected” printed on the side. April 2016  59