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SERVICEMAN'S LOG Lightning didn’t strike this time If a TV set stops working while there’s a storm around, many people assume that it has been struck by lightning. They automatically link the two phenomena together simply because they happen at the same time but it ain’t necessarily so. My first story this month concerns a 1990 Grundig 68cm TV set. According to the customer, it had gone off during a storm and so she immediately jumped to the conclusion that lightning was responsible. I suppose it’s a natural enough conclusion under the cir­cumstances but in this case, it turned out to be quite wrong. The set was fitted with a West German 2-pin plug (with an Australian adaptor added) and the lady had obviously brought it with her from Germany. As I very rarely see German television sets these days, I was intrigued to see what might be involved. The set modestly proclaimed itself to be a “Grundig Monolith Blackline Multisystem”. A European set of this age could present problems. For one thing, their power grid is 220V and the increase in heater vol­tage, without a modification for our 240V, could be a tube kill­er; the picture tube could very well be low in emission. Another problem concerns model identification. The number of models in the Grundig lineup is bewildering. In fact, identi­ fying this model set requires several numbers: model M70575/90, chassis series CUC5836, and main chassis part number 29701-057. Despite all this, when a service manual which I ordered arrived, it was only a supplement which covered the differences between this and the CUC 5820, 5835, 5860 and 5880 models. So it was very much incomplete and among other things, lacked all the information for the plug-in modules. Later, I acquired the main manual but even then, this was incomplete. The PC board component layouts were not included and there are many differences and modifications between part numbers and the component reference numbers, which aren’t marked on the board. The back was easily removed but access to the horizontal main chassis underside was not as easy as on earlier models. The service manual suggests it should be pulled out and lifted onto its righthand side on the bench. This didn’t work because the speakers are directly below the chassis and the leads weren’t long enough. There wasn’t much life in the set except a low level motor-boating, which suggested the power supply was functioning – at least in a fashion. What was more immediately obvious was rust and corrosion everywhere – definitely not something caused May 1998  27 Fig.1: part of the deflection module circuitry in the Grundig M70-575/90. IC7010 (TDA8146) had blown apart and had to be replaced along with IC7020 (TEA8170A), zener diode D7012 and resistor R7033. by lightning and storm damage! The set had obviously had a hard life in a damp environment or near the sea. A glance at the tripler revealed several telltale carbon track deposits leading from holes in its insulation where it had been sparking and burning. For the time being, I disconnected it and went to the horizontal output transistor T541, a BU508A. It measured short circuit and so I replaced it, hoping that might be all the damage. With power applied, the motor boating disappeared but not much more was happening. And then suddenly, I heard a slight click and it started motor boating again. Not only had transistor T541 become red hot but so had diode D546, a BY228, and both had failed. The overall damage looked pretty severe – two horizontal output transistors, two diodes (D546 and D547), the tripler, almost certainly the horizontal output transformer, and probably the horizontal drive IC (IC500 – TDA8140). I would have to give an estimate on these before proceed­ing; if there 28  Silicon Chip were any other problems they would probably be minor. The lady had no problem with this and so I ordered the parts which arrived promptly. I decided to replace them all at once, to eliminate the possibility of further damage due to faulty parts. On switch-on, I was disappointed to find only limited im­ provement. Granted, the EHT was now working, the HT rail measured 165V and there were no signs of distress or overheating in T541 and D546. However, there was still no sign of a raster or sound, apart from a few noises in the speakers. I checked all seven voltage rails and this quickly revealed that there was no voltage on the 29.5V rail. This was quickly traced to R525 (0.33Ω) which was open circuit. Replacing this produced some sound but it was garbled, although this could have been due to mistuning. And there was still no picture; just a blurred blob which was uncontrollable. The EHT meter confirmed 25kV on the picture tube final anode and there was a healthy spark from the CRT socket focus pin. There was also plenty of G2 volts but shorting the tube cathodes to chassis produced no intelligent raster or picture, other than the unfocussed blob (the focus control had no effect). I prayed the tube wasn’t U/S; shorting any cathode to chas­ sis should give an intense bright raster. The only thing left to check was the deflection circuit. The CRO showed that there was no vertical deflection on the yoke, while there was too much signal at the chassis end of the horizontal coils. It was time to unplug and examine the deflection module. Once the deflection module was on the bench, it was obvious it had sustained major damage – IC7010 (TDA­8146) had literally blown apart, leaving a blackened hole in its lower half. A lot of power would have been required to do this, which meant that there was bound to be collateral damage. And sure enough, a quick check soon revealed that D7012, a 36V zener diode, was short circuit, while R7033 (100Ω) had gone high. In addition, R546 (4.7Ω) was open circuit. Until now, I had managed to obtain all the parts easily and cheaply from my local supplier and so I ordered a replacement TDA8146 (IC7010) from them. But IC7020 (TEA8170A), which I also strongly suspected, had to be ordered from the Grundig agents. While waiting for these parts, I had the opportunity to clean up the corrosion and to check the other components around these circuits. Nothing else appeared to be amiss. The ICs duly arrived and, when fitted, restored the pic­ ture. After some tuning and setting the CCIR B/G system standard (which is actually a story in itself), I had a watchable picture with good colour, sound and focus. The only problem remaining was severe pincushion distortion. Adjusting the pincushion control (R7011) made no dif­ ference. Ditto for the width control (R7002) and the trapezoid control (R7007). In fact, no east-west correction controls were working around IC7010 (TDA8146). I tried two more TDA8146s for IC7010 but to no avail – a matter of some significance, as it turned out. After a lot of CRO measurements, the only two waveforms I could fault were waveform 10, which is the vertical input to pin 2 of IC7010, and waveform 9, which was slightly different on output pin J7 to the deflection coils. I was really stuck now, as I couldn’t determine whether the pincushion fault lay in the motherboard or in the deflection module. panied by a whistling noise. Close inspection revealed that though both sets were similar, the Blaupunkt mod­ ules had major differences. Because the Grundig was a multi-system, with picture-in-picture, the boards were larger and contained a lot more components. I was mainly interested in the deflection module and the only major difference between the two was that IC7010 in the Blaupunkt deflection module was a smaller 8-pin TDA8145 device instead of the 14-pin TDA8146 in the Grundig. Despite this, I decided it was worth the risk and swapped the modules over. Surprisingly, this fixed the Grundig’s pincushion distor­tion problem completely. I was somewhat taken aback at this because I was sure I had checked every component in the module. Conversely, the Grundig’s deflection module (29504-107.80) did down to the IF module, of all things. This was established by temporarily fitting the IF module from the Grundig set. Unfortunately, I ground to a com­ plete stop here after replacing IC2340 (TDA2579) in the IF modu­le, which made no difference. I now had two problem sets: the Grundig with pincushion distortion and the Blaupunkt with lack of height. It was then that my friendly leprechaun came to my rescue for the second time. By chance, I was talking to a colleague and happened to mention my problems with these two sets. And as luck would have it, he knew the answer to the lack of height in the Blaupunkt, as he had once spent a lot of time tracking down this very fault. In his case, it turned out to be diode D2334 (TD190) in the IF module. And he was spot on. I subsequently discovered that the diode in my set was slightly leaky and replacing it A friendly leprechaun not fix the height or width problems in the Blaupunkt, nor did it correct its east-west foldover problems. Well, at least I had localised the Grundig’s problem. However, I decided to leave this set for the time being and concentrate on the Blaupunkt instead. The first step, of course, was to reinstall its deflection module. The lack of width and the horizontal foldover turned out to be due to IC526 (TDA8140), C527 and C574, which were causing transistor T572 (BU508A) and resistor R574 (18Ω) to overheat. The exact sequence of events involved here is not clear; I could not determine which had failed first and what was damaged as a result. All I can tell you is that all these parts had failed and had to be replaced. The lack of height was tracked completely cured the problem. It was at this time that I was blessed with an surprising coincidence (and for this, I imagine I should thank some friendly leprechaun – I’m writing this on March 17 which is St Patrick’s Day). As stated before, I very rarely see German TV sets and yet, incredibly, it was just then that another customer brought in his Blaupunkt IS 70-39 VT (FM 500.40 chassis 7669 800) which is manufactured by – yes, you guessed it – Grundig. And the main chassis Grundig part number was 29701-056 – only one digit dif­ferent from the Monolith. Luck doesn’t often come like this. The problem with the Blaupunkt was a shrunken picture (both vertically and horizontally) which became folded after about 10 minutes, accom- Back to the Grundig My next problem was what to do about the faulty Grundig deflection module. One option was to send it back to the Grundig agents for servicing, a process that would take two or three weeks. Another option was to buy a new one but this was not readily available and, in any case, is rather expensive. In the end, I played a hunch. I had begun to suspect that the TDA8146s (IC7010) which I had purchased from my supplier might be the problem. And I became suspicious because of their price. They were only about $7.00 each from my supplier whereas they were closer to $20.00 each from the Grundig agents. May 1998  29 Fig.2: part of the IF module in the Grundig M70-575/90. Diode D2334 (TD190) at lower right had gone leaky. OK, that’s fine; I’ll buy in the cheapest market, all else being equal. But were these parts really equal? To test my theo­ry, I ordered a TDA8146 (part no 8305-358-146) directly from the Grundig agents and when it arrived, I noticed one obvious dif­ference in the batch number. The original Grundig number was W994A9409 but the ones I had fitted during testing were marked W994A9422. And that was the answer; it was all that was required to cure the problem completely and the east-west correction circuits now worked perfectly. Both customers were pleased that their sets were now work­ing properly but neither was nearly as pleased as I was. And what was the final verdict? Was the set struck by lightning? I dunno, please. I have no doubt that the Grundig set failed while a storm was in progress but that doesn’t necessarily mean that the two events are directly related. From the evidence before me, I would have to say that lightning was not the probable cause. However, the customer is firmly convinced that lightning caused the problem and the matter is still before her insurance company. Well, if she’s happy, who 30  Silicon Chip am I to disillusion her? For my part, I returned the three cheaper ICs to my local supplier with a please explain note and am awaiting their re­ sponse. Unfortunately, this wouldn’t be the first time that bodgie, off-tolerance components had found their way onto the local scene and been marketed quite innocently by local dealers. Yes, it’s a rough world out there. The JVC video JVC videos don’t seem to like me. This week, a lady brought in her JVC HR-D600 VCR and complained that the picture was “sort of distorted but only with some tapes some of the time”. Obvious­ ly, she imagined, I must know what the trouble was, so could I please tell her what it was, how much it would cost and how long would it take to get it fixed. Ironically, I had a sneaking suspicion as to what the trou­ble might be but I certainly wasn’t going to commit myself to a guess. Instead, I suggested she leave it with me and I would make an assessment after I had seen the fault. I put it on the soak bench with an E240 cassette in standard play and left it produc­ing an excellent picture. Every so often, I would check that it was going OK and restart the tape. After three days of this, I was beginning to despair and so, when she phoned to enquire about progress, I had to confess that there was none; it hadn’t missed a beat. Perhaps it was her tapes? Her answer to this question was an emphatic “no”, because it also happened with hire tapes. Not wishing to start an argument on that basis, I told her she might as well pick it up and try it again later. She didn’t call for two more days and when I took her to meet the monster, the tape had stopped. So I pressed play again to show her it was still going OK and would you believe it, the wretched machine started to do its trick (I told you they don’t like me). The picture had three or four sets of noise bars across the screen – permanently. Somewhat embarrassed, I did my best to assure her that this was the first time that the fault had ap­peared. I don’t know whether she believed this or not but, in any case, I could only mentally shrug my shoulders; after all, that was the true situation. “Leave it with me”, I said. “Now that the fault has shown up, I should be able to get at the problem”. The customer readily agreed to this, so my explanation must have been at least partially accepted. Despite being busy, I decided I would tackle it straight away while the fault persisted. The symptoms were typical of a misaligned tape path and the tape guides used in this deck do give a lot of trouble. The top cover comes off conveniently but the bottom is not only screwed in via the fancy feet but also clips in on the sides and centre. And it was while I was removing this cover that the so-and-so bit me, a sharp edge cutting one of my fingers and causing it to bleed. The next step is to move the top PC board into its service position. This involves removing a screw at the rear, then using a smaller Phillips screwdriver to remove five self-tapping screws that hold it. The board was then parked in a vertical position along the rear of the video. Next, three more screws had to be removed from the metal cover over the heads. With the tape in the play mode, everything looked OK except that the entry and exit tape guides hadn’t engaged properly at the end stops. By wiggling them, I could restore the picture. It was while I was wiggling these guides that the machine bit me again; it cut another finger, this time on the sharp edge of the cassette ejector housing. Things were bad and getting worse. I removed the tape and put in a dummy cassette (one with no insides, just the outside framework). I then pressed the play button and pulled the mains plug when the two arms were only half-way along their action. Now I could see how loose things really were. On the underside of the deck, the guides are held in place with a brass plug/collar assembly and a plastic pin. Both of these are just a snug push fit and the brass ones were not all the way home. I removed them and applied some superglue before pushing them all the way home while holding the guides in on the other side. This done, I cleaned the heads and tape path and checked that everything was shipshape before trying another tape in the play mode. Everything now worked OK, so I refitted everything and gave it one more final test. I couldn’t believe it – not only was the original problem still in evidence but there was now another even heavier noise bar at the top of the picture. I stopped and started it several times but it wouldn’t go away. There was nothing for it but to go back in again and access the tape guides. Well, the cause of the problem was immediately obvious – the pin in the input guide had fallen out, presumably when I turned the machine upside down to fit the bottom screws. The annoying thing was that I had originally checked it and it had appeared to be tight. I decided to superglue it in and put a few drops on the end of the pin before pushing it into its hole. Big mistake – the pin set hard in the socket but wouldn’t go all the way in. There was nothing for it but to remove it and start again. It took a lot of aggro to remove and clean it with acetone. Before starting again, I made sure the pin could slide all the way home in its socket and that it did so easily. Another drop of glue and once again it seized half way in. I was on the verge of chucking the whole thing out the window when common sense told me to try some lateral thinking. After removing and cleaning the pin, I refitted it in the hole and applied the glue around the edges afterwards. Sure enough, it finally locked in place and capillary action made the glue sink in. I did the other guide too and then after cleaning every­ thing, tested it again. I was dismayed to find that the original symptoms were still there, although they were now quite inter­ mit­tent. In desperation, I went in for the third time and after a very careful examination of the guide assembly, I found I had inadvertently pulled the plastic pin connecting it to the loading arm. Consequently, this arm was also loose. I glued this and its partner on the other side, and anything else that might possibly come loose. Fortunately, that was the end of the story because it now worked like a steam train and was still working when the lady called to pick it up. I just hope that I managed to wipe off all those bloodstains and that she won’t notice the dents in the sides! SC May 1998  31