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SERVICEMAN'S LOG Big tellys, PCs & car computers Why is it that all large-screen TV sets are located up flights of stairs, making it impractical to move them? I recently came across two such monsters, along with a PC that kept losing its icons. I also got tangled up with a couple of car computers. It was with fear and anticipation that I set off to “look at” a large screen Sanyo TV, as Mrs Collins had begged me to. She was unable to tell me the model number as she could not get to the back of the set but she repeated that the fault was no pic­ture. And because she was recuperating from an operation on her leg, she really needed to watch the telly. When I arrived at the third storey unit, mentally noting that there was no elevator, I was surprised to find that her home was even smaller than I had imagined it to be. You couldn’t miss 30  Silicon Chip the 78cm stereo TV as it took pride of place in the living room. The furniture was spread out in a semicircle around the set, as though it was praying to the craven idol. How they got the set into this matchbox must be a story in itself but one thing was for sure – it was going to have to be fixed on location, as I certainly didn’t have the facilities to take it to the workshop. When I finally managed to move the set out a fraction from the wall and crawl behind it, I established that it was a Sanyo CPP3310TX-01 (A4-A33 chassis). Because of the restricted space, it took some effort to remove the back but this did little to make the access any easier. Secretly, I prayed to the Patron Saint of Television for a merciful and quick conclusion to this repair. I had already established that there was sound but no raster from any source – be it from the TV tuner or the AV inputs. Nor was there any Teletext on Channel 7. I propped up a large mirror in front of the TV so that I could monitor any activity from the rear and then turned up the screen control on the flyback transformer to produce a blank clear raster. Well, the report card on the TV now showed me that almost all the circuits were working. However, the luminance and chromi­nance signals were disappearing somewhere between the video detector and the picture tube. And because there was no picture from the AV inputs, this meant that the problem could be shifted a little further away from the detector – possibly to the analog switching circuitry. Even so, it was looking more and more like a problem with the small-signal decoding stages and for this I needed a circuit diagram. Mercifully, Mrs Collins had kept the one that came with the set and so I started by checking the nine major voltage supply rails (B1-B9) from the switchmode power supply, the flyback transformer and the remote control receiver auxiliary sup­ply. I also checked the ancillary rails which are derived via IC regulators and checked the low voltage rails to the signal pro­cessing circuits but nothing was amiss. By now, I was suspecting IC201 (M513086P) – the jungle IC – but I really couldn’t go any further without an oscilloscope. I left, having made an appointment to call back later in the week. As I drove off to the next job, one point I had noticed with the TV was niggling away at me – there was no on-screen display. I had dismissed this at the time but now I couldn’t help feeling that I had been down this path before. When I got back to the workshop, I phoned Sanyo Technical Support and they also (without prompting) suggested the jungle IC. They also advised me to check crystals X421 and X261. To save time, I ordered the IC just in case this was the problem. I was back at Mrs Collin’s little flat a few days later and this time there was even less room as I had brought an oscillo­scope and a signal generator. Access to the underside of the main PC board was impossible and it wasn’t much better for the top because the AV board got in the way. Because nothing could be seen on the screen, I couldn’t tune the TV into the RF output of the pattern generator so I injected colour bars into the video input phono socket (AV1) instead. It took me quite some time with the circuit to finally work out that the AV module not only selects the various sources but also splits up the video into chrominance and luminance signals. The latter then goes through a “video unit” which is a noise reducer circuit, before finally arriving at pin 18 of IC201. The CRO confirmed this and also that the crystals were oscillating with the remote control. Next, I selected a channel number which had good sound before checking the video output from the IF pack on test point TP-J with the oscilloscope. The signal was there and also at IC201. Reselecting AV1, I then measured the colour difference output signals from IC201 on pins 1, 2 and 3 and was surprised to find them all there. I then followed these signals through the AV board “Super Imposer” (IC1307), which switches through the Tele­text, and onto the CRT socket where the signal died after the emitter input to the output transistors. It was then that the bells began ringing, because I had been here before with a very similar problem in a Palsonic 5138 TV set (see November 1997). Sure enough, here was the exact same component, namely a 120kΩ resistor (R692) feeding the base cir­cuit of the video output transistors from the +200V rail. And once again, the high voltage had been too much for this low-wattage resistor and it had failed, switch­ing off all three transistors. If only I could remember all these faults, although I’m sure I won’t for- Fig.1: the standby switching circuitry in the Sony KV-C2931S. A break in the circuit between R634 and D624 meant that Q601 was being turned off but the fault was intermittent. get this one the third time around. Anyway, Mrs Collins was delighted and I was left with the challenge of disengaging myself from the tangle of equipment and getting out of that tiny flat in one piece. Another monster In the meantime, yet another house call was booked in for a large screen TV. Why is it that all the biggest sets are located in the most difficult positions – usually up flights of stairs with no lifts? Or is it just me being paranoid? This time, the set was a 1990 Sony KV-C2931S, using an AE-1B chassis made in West Germany. The complaint was intermittent no sound or picture. Mr and Mrs Mowbray, who owned the set, were a retired couple living on the top floor of a duplex and had enough trouble getting up and down the stairs as it was, so they were in no position to assist me with the TV. Once again, it was a case of the workshop going to the TV rather than vice versa. Because I had serviced so many of these sets, I thought that the fault would be easy to fix. It would probably turn out to be a dry joint on the motherboard and I was quite confident of an early and successful victory as I climbed the many stairs. I arranged with the Mowbrays to get a blanket and cushions ready and then gently tilted the set over onto its front to rest on them (the cushions, not the Mowbrays). Removing two screws and a couple of clips gave me access to the underside of the set and I then spent over half an hour re­soldering many suspect joints on the motherboard, especially around resistors R614 and R653 and around Q608. Some of the dry joints were inaccessible under the black plastic frame and the only cheap and quick way to address these is to cut a small part of it away. Anyway, I resoldered all the transformers, transistors, diodes, plugs and sockets and some of the power ICs. Finally, satisfied with a job well done, I reas­sembled it all and made sure all was working well before confi­dently leaving. Unfortunately, my triumph was short-lived – Mr Mowbray phoned about two weeks later and said it was doing the same thing again and could I please call back. Disappointedly, I returned, and sure enough the picture and sound would come and go as you tapped the set. Now I had in fact tested this thoroughly the last time, when tapping it or even thumping it made no difference. This time I left the set in an upright position and gently removed the back, then tapped around the set exploring for more sensitive spots. After a while, I felt sure that the problem lay on tuner IF module A, so I reworked the soldering all over this board, concentrating on the edge connectors, the tuner and any coils. I then unsoldered the IF module (VIF901) and reworked November 1998  31 the soldering inside there, especially over any coils and ceramic filters which, I might add, were almost all dry-jointed. Once again, full of confidence, I reassembled everything and tried tapping around the whole set, especially the tuner IF module. The picture and sound were rock steady and I left feeling somewhat annoyed that having soldered so many bad joints the first time, I had discovered even more the second time. But wait, there’s more. This is where the story should have ended but unfortunately it didn’t. About four weeks later, a rather tetchy Mr Mowbray called again and told me the set still wasn’t fixed. For a while they could tap it and the picture and sound would return but now it was gone completely. The only thing showing any sign of life was the green on-screen display. I returned as soon as possible to the Mowbrays, armed with the full kit again – namely the oscilloscope and pattern genera­tor, which I connected to the AV1 input SCART socket via an adap­tor. I then began tracing the video signal from the SCART socket (pin 20) to the output from the AV board (pin 16, J1-41) and onto the mother­board. From there, I traced the signal from the chroma decoder and finally to the Teletext board (CNV-01 pin 3). But there was no video going into video 32  Silicon Chip amplifier stage Q598 (wave­form 22) which supplies the sync separator (IC501). It was this puzzling part of the problem that wasted half an hour of my time. The circuit clearly shows a direct path from pin 3 to C592 and then onto the emitter of Q598 but in reality, there was a link. This is designated JW105 but had not been installed in this set. Shorting across the link position restored both picture and sound, so you may be forgiven for thinking that just refitting the link would be the answer. However, it does not explain why simply tapping the set used to fix the problem. The answer was that Teletext is an optional accessory and the link is only fitted when this option is left out. If Teletext is included, as it was in this case, the video goes into the Teletext unit on pin 3 and comes back out again on pin 2. This meant that the problem was now narrowed down to the Teletext module, or so I thought. Removing the V board and examining it revealed yet more dry joints, especially around Q01, the 5V regulator stage. Unfortunately, re-soldering the joints didn’t fix the problem – there was still no 5V rail. I soon discovered that not only did I not have 5V but that the 7V rail into the module was also missing. The 7V rail comes in on pin 6 of connector CNV-01 and I traced the line back to the collector of transistor Q601 (the standby switch) on the mother­board. There was 8.4V on its emitter (which is correct) but this same voltage was also present on the base, thus completely switching the PNP transistor off. Q601’s base is controlled by transistors Q606 and Q609 via R634, D624 and R618 – see Fig.1. There should have been 0V on Q609’s collector and this proved to be the case. There was also 0V present on the cathode of D624 but R634, which is connected directly to it, measured 8.4V. This meant that there had to be a break in the board track between D624 and R634, although the track looked perfectly OK. This track, by the way, is only about 25mm long and passes next to a hole drilled through the board. This might have been a screw hole but there was no screw fitted – maybe it was only used in the course of manufacture and discarded later. I scratched away the lacquer covering the track and, using an ohmmeter, eventually found a hairline fracture in the copper. Resoldering the crack fixed the problem and I carefully reassem­bled the set. A check some three months later revealed that everything was still OK, so I think I’ve finally fixed the mon­ster. If I’m wrong, I’m returning with my best fine-tuning tool – my baseball bat. John Cleese will have nothing on me after I’ve given that set a stern talking to. Missing icons In addition to the usual VCRs and TV sets that come my way, I’m also often confronted with computer problems. Some of these can be quite unusual and I have one such story this month. The first was a 1994 ICL ErgoLite computer, which is actu­ally an Acer 80486 DX-50 in disguise. Its main problem was that it was unable to save any setting made within Windows 3.11 on exit, resulting in a blank Program Manager with no icons. At first, I thought that this would be a case of simply reloading Windows over the top of the existing installation but this didn’t help. I then spent some time reloading DOS 6.22 and customising its config.sys and auto­exec. bat files to optimise memory but that didn’t help either. I then ran Scandisk, a virus checker and defragg­ ed the drive but I wasn’t getting anywhere. The resources were fine and the computer hardware was fine too. My next step was to examine sys­ tem.ini and win.ini but again I could find nothing untoward. What finally put me onto the right track was thinking more carefully about what was actually happening and especially why the icons were disappearing (very Agatha Christie). This part of the operating system is controlled by Progman.exe and customised with Progman.ini. Using my trusty XTree Gold (the one and only), I examined Progman.ini and com­pared it with the Progman.ini from another computer that worked. Normally, this text file has two headings, [Settings] and [Groups], the former giving details as to how the groups are arranged and the latter describing what the groups actually are. However, on the faulty computer there were two or three lines of machine code and then the settings and codes were repeated about half a dozen times, often with conflicting information. At last I was getting somewhere. Hopefully, all I would have to do is edit the Progman.ini file but first I went to the Windows directory and printed out a list of all the group files (*.grp) for use as a reference. After that, I backed up Progman.ini and then edited the file by removing all the machine code and then listing all the Group files under the [Group] heading. Finally, I edited the lines under the [Setting] heading, removing any duplicates in the process, and resaved the file. This simple measure completely fixed the problem. The next time I went back into Windows, all the icons were there – indeed there were even some duplicates. A little extra housework made everything shipshape and the program groups now remained in place each time I exited and rebooted Windows. My guess is that the problem had been caused by the comput­er being turned off (or crashing) while Windows was still run­ ning. This had created lost clusters and chains which had been repaired as best as possible by Scandisk. Unfortunately, the Progman.ini file had been corrupted and could only be manually repaired using the method just described. The moral of the story is to always exit Windows properly before turning the computer off. Car computers My final two stories for this month are also about comput­ ers but this time, they involve car computers. So how did I come to get involved in car computers? Simple – a mate of mine runs an automotive workshop. The first story is about a Eunos that wouldn’t start, although we were pretty sure that it wasn’t the computer itself that was at fault because a substi­ tute didn’t fix the problem. The car would crank and there was spark but no injector pulses. Based on this, we deduced that the problem probably lay somewhere between the computer and the fuel injectors but there were a few other possibilities. With amazing luck, we actually managed to obtain a photocopy of the wiring diagram for the car but not of the computer itself, unfortunate­ly. We began by using a multimeter to establish that 12V was getting to the computer and to the injectors – and this proved to be the case. However, it was prophetically pointed out at the time that it isn’t a good idea to use a multimeter in auto elec­trics, as the internal resistance is so high that it can give very misleading results. Instead, it is far better to use a 12V lamp probe but we couldn’t find the workshop unit at first. After these initial checks, I decided to use a CRO to check the input pulses from the two crank sensors (why two?) and the output pulses from the computer to the fuel injectors – this while someone else cranked the engine. Everything seemed to be working perfectly and so it definitely looked like a wiring loom problem. Next, I moved the CRO to the engine bay and checked the pulses going directly to the injector connectors. These were all present, so why wasn’t it working? It was at this point that the lamp probe was found and we checked the 12V rail to the injectors again. And whereas the meter had told us there was a full 12V, the lamp told us differ­ ently. The 125mA globe (24V 3W) hardly lit, which indicated a high resistance in the wiring loom and/or the relays between this point and the battery. Even though we had the wiring diagram, we still had diffi­culty following the leads and identifying the various plugs and sockets. To begin with, in order to confirm our diagnosis, we connected a jumper from the fusebox to the injectors and cranked the engine. To our delight, it immediately fired up but the engine stopped as soon as we removed the jumper It was while we were tracing the 12V rail with the lamp probe that we almost had a disaster. The lamp probe consists of a festoon globe inside a hollow uninsulated metal cylinder with a sharp point on one end. The other end has a lead with a crocodile clip going through an insulator, a spring and another insulator to the other end of the globe. Unfortunately, I pulled too hard on the lead while trying to reach one of the sockets and broke the internal insulator, thereby shorting the lamp completely out. Immediately, buckets of amps start­ ed flowing and the lead started to melt. Fortunately, I was able to break the circuit within a second or two of seeing the smoke – but what damage had November 1998  33 Serviceman’s Log – continued I caused? Well, as it happened, none because when we cranked the engine, it started immediately. The high current had “cleaned out the cobwebs” and improved the bad connection we were trying so hard to find. This was maddening because this was now a classic Clayton’s fix – ie, the repair you have when you are not having a repair! Luckily for us, we had already narrowed it down to a plug and socket near the fusebox marked X07 (white and red wire) and actually wiggling it with a x1 ohmmeter across it saw the needle tremble slightly. So rather than leave it with a “she’ll be right, mate” attitude, we soldered a jumper lead across the plug and socket. The car was then soak tested(!) but it gave us no more grief. Airing a Subaru The final story in this computer trilogy of bravely going where no-one else will go concerned a 1990 Subaru, which had an electronically-controlled suspension height system. This consist­ed of four air-bags in lieu of coil springs that were individually pump­ed up and down with a compressor and controlled by solenoids. These were in turn controlled by a 34  Silicon Chip microprocessor, either au­tomatically or when commanded by the driver. Of course, the problem was that nothing was happening when the buttons were pushed – the suspension remained in its lowered position. Various other people had had a go at trying to fix the problem and the bone was being pointed at the computer as the likely source. Once again, we had a wiring diagram for the vehicle, which was a relief, but not for the computer. Initially, I was given the computer box only and told “go fix”. I must confess that I wasn’t all that optimistic at first but what did we have to lose? The computer consisted of a large microprocessor chip and a lot of peripherals, plus seven 2SD1392 transistors whose collec­ tors went directly to the connection sockets. Initially, I spent some time checking for all the obvious things (ie, dry solder joints, dry electros and short/open circuits) before turning my attention to the transistors. These all measured good except for one which was badly cooked. You beauty! I reported back that I had found a fault in the unit but pointed out that some external fault must have caused the damage. There was nothing for it but to check it out in circuit with a lamp probe. First, we established that there was 12V going into the computer on pins 2 & 3 and that there was also 12V on the collec­tors of all the transistors. We then identified which transistor did what. Four controlled the airbags at each wheel, one the compressor relay and the other two the charge and discharge valves. Our next step was to short each of the collector connec­tions on the socket to ground in turn (with the computer un­plugged). Each solenoid operated correctly except for the dis­ charge valve which is controlled via pin 1. And guess which transis­ tor controlled pin 1. Temporarily, we took the discharge valve out of circuit and refitted the computer. This confirmed that it was actually doing its job because we could hear the relays working. So why wasn’t the suspension being raised and lowered? The answer was that the discharge valve was not only short circuit but was also stuck open. Unfortunately, it is mounted on the compressor and this can only be accessed by removing lots of other parts and putting the car up on the hoist (the compressor and valves sit just above and in front of the passenger’s front wheel). At first we thought that we would be scuppered by the hoist because the wheels would drop and the computer would think that the system was fully discharged so we couldn’t test it. However, we managed to confirm that the air-switch solenoids were operat­ing but that there was no air to pump the airbags. We checked the reservoir and pipes for leaks but found none, which meant the problem was definitely the discharge valve. Unfortunately, this is an integral part of the compressor assembly and is a totally unserviceable unit. It wasn’t easy removing and replacing the compressor but a new one at – wait for it, almost $2000 – finally fixed it. After all that, give me a telly SC any time.