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SERVICEMAN'S LOG Mystery, mystery & frustration This month, I have three short stories, two of which introduce an element of mystery and the third a sense of frustration. It is unlikely that we will ever know the answers to the mysteries but the cures were obvious. The first story concerns a Toshiba 48cm colour TV set, model C-2020. As is often the case, the customer's complaint over the phone was rather vague. His first statement was to the effect that "it keeps going off", but further remarks suggested that the "off" was poor performance rather than total failure. And the "keeps going" suggested it was probably intermittent. Anyway, I suggested he bring it in for closer inspection. So he duly turned up and I set it up while he was there. The picture was faint and lacking contrast. I forced it up to a more recognisable level with the brightness control and was then able to see that it was riddled with hum. The vertical:; were weaving back and forth and it was also displaying hum bars. Well, it looked like a snack. It was obviously a power supply fault and this was a power supply with which I was very familiar. I mentally registered that it was almost certainly the main filter capacitor, C810, a 680µF electrolytic (top of Fig.1). I'd seen the symptoms many times before . At the first opportunity, I pulled the back off and checked the HT rail. This should be about 110V but was only 86V, measured at the emitter of the regulator (Q801). On the other hand, the collector was at 133V which is almost spot on. This raised some doubt about the capacitor but per- haps not as much as it should have. Anyway, I rustled through my stocks for a replacement. As it happened, I didn't have an exact replacement but I did find one that would do, but which would need some extra work to fit. By now I wasn't so sure about the capacitor and, with the need to do this extra work, I was less inclined to rush in. I tacked a couple of leads onto the capacitor, then patched it into circuit across the suspect unit. It had no effect whatsoever. So much for my original "snack" diagnosis; it was time to stop guessing and start working. The circuit is not particularly complex and is quite conventional. There are three transistors: Q803, error detector (bottom of diagram); Q802, regulator driver (above it); and Q801, which is the main regulator (top of diagram). The other major component is the bridge rectifier, consisting of diodes D801, D802, D803 and D804. I went immediately to the voltage adjustment control, R851, a 1.5kQ variable resistor in the base circuit of the error amplifier. This sets the HT rail to the required voltage but I wasn't really surprised that adjusting it had no effect. Next I made a preliminary check of those minor components most likely to have upset the regulator action and, while I couldn't be absolutely certain, all the indications were that these were OK. Which transistor? So that left the transistors. Both the error amplifier (Q803) and the regulator driver (Q802) have given trouble in the past, with the symptoms ranging from the simple to the weird. The regulator transistor can fail just as readily as the other two, but it is normally a catastrophic failure; breakdown between emitter and collector, creating an excessive HT rail voltage (typically around 135V) and causing 40 SILICON CHIP C+)f2y) QBOI 2SCI 195 FA-I POWER REGU, GRN I L1 ~C809 R_~ii(G) USOI D813 - 3,3)1 (160V L901 1S1942 APF SWITCH TSB-2056<2055> DEGAUSSING COIL POWE_R_B_OA_R_D~PW~-~,,~10._. Fig.1: the power supply circuit for the Toshiba C-2020. Q801 is the regulator transistor, Q802 the driver transistor, and Q803 the error amplifier. the set's protective circuits to shut it down. So where should I start testing? Murphy's Law states that whenever a faulty component is one of a string, it will always be at the end opposite to that at which one starts. And if one attempts to minimise this effect by starting in the middle, it will always be at the far end of the last leg checked. But I had to start somewhere and, Murphy aside, I had a funny feeling that this time it was the regulator transistor (don't ask me why). So I disconnected the base and emitter leads and made a simple ohmmeter check in the time honoured fashion. This failed to reveal anything amiss, even when I went looking for leakage. Nevertheless, the funny feeling persisted and, with the leads disconnected, it was just as easy to try a replacement. I didn't have the exact replacement (2SC1195) but I found a BU126 which is an acceptable substitute. It took only a few minutes to fit it and - presto! - that was it; a normal picture and no hum problems. All that was needed was a marginal adjustment of R861 to bring the HT rail down a few volts. I ran the set for the rest of the day, with no sign of trouble, than rang my customer with the good news. As stated earlier, there was a hint of intermittency in the customer's original complaint but there was no sign of it on my bench. In any case, subsequent follow-ups confirmed that all was well. Transistor checks? But I was still puzzled as to what was wrong with the transistor. Granted, the simple checks I made are not always conclusive and, for this reason, I use a curve tracer as a back-up in sticky situations. So I hooked the tracer up to the CRO, set it up for power transistor checking, and tried the transistor. And it traced out a virtually perfect curve. So, was it breaking down - or something - at the higher voltages in the set? It is possible to vary the operating conditions of this equipment to a limited extent, so I adjusted the settings to increase the base current and also to increase the collector sweep voltage, within these limits. But there was still no sign of a fault. I left it running like that for an hour or so, at the end of which time the transistor was quite hot; I could touch it briefly but it was too hot to hold. And still a perfect curve. However, this involves only relatively low voltages; not more than 30V on the collector. And that's about as far as I can go. I still think the most likely explanation involves the higher operating voltages which I cannot duplicate. But now, having made these checks, I would dearly like the chance to put it back in the set, or another one like it. This is out of the question at the moment but I'm keeping it handy, should the opportunity arise. So for now, the mystery remains. A record for rarity The next story also poses a mystery but of a completely different kind. And it must surely set some kind of a record for rarity; I'd be prepared to bet long odds that no one has ever encountered it before. Rash words? Well, see what you think. The culprit was a Mitsubishi model CT-2963 68cm colour TV set. It was brand new, straight out of the carton at my local retailer colleague's shop. It is an upmarket model which, in addition to the large screen, boasts a flat, square picture tube. An intriguing aspect of the set was that it was made in Scotland. A Mitsubishi TV set made in Scotland! it's almost a contradiction in terms. Perhaps these sets should be renamed Macsubishi? All right, so I digress. Now to business. My colleague had unpacked the set, to put it on display, when a preliminary check indicated that all was not well. So he brought it around to me, complaining that "it's got hum on the picture". The model was new to me, apart from showroom acquaintance, and I APRIL 1991 41 SERVICEMAN'S LOG - CTD .,~9,... _ ~ ..... A. N\\-r"SOS\SHl -rv se..-r ff. .. N'A"DE- IN ~-n.ANT)f had no data on it. So my first reaction was that I would have to obtain a manual. But I thought I might as well have a look at it anyway, just to confirm my colleague's symptoms. So I unpacked it and set it up. And as luck would have it, it was switched to SBS which was running a test pattern; I couldn't have wished for a better demonstration of the fault . On the other hand, describing the fault is not easy, so please bear with me. Those familiar with SBS will know that, superimposed on the test card itself, are two running displays, one near the top , carrying program data and channel data, and one near the bottom of the picture, carrying the title and other data on the music being played. And it was the bottom display which exhibited the fault. The message comes in from the right and this was quite normal for the first few centimetres, until it reached the first vertical colour bar; green at the top and blue at the bottom. Here it kicked upwards for about 15cm, then ran downhill towards the centre of the screen, where there is a short red colour bar. From here it ran uphill and traced what was virtually a mirror image to 42 SILICON CHIP the left hand side of the screen; up to the magenta bar, a short downward kick, then straight out the left. Any ideas? Forget it; you'd never guess it in a month of Sundays . Shock, horror My own reaction was one of bewildered shock. I could understand why my colleague had described it as hum. He is familiar with the classic hula girl pattern caused by hum in the horizontal system and, with his limited technical knowledge, assumed something similar in the vertical system. And, while I ruled out hum, I went through the mental exercise of trying to visualise what kind of vertical waveform distortion could cause such an effect. After a couple of tries, I gave up; the brain simply refused to cope. I let the set run while I went on with a couple of other jobs, hoping for inspiration. None was forthcoming - at least not directly. But while working on other sets, I had been made more aware of the difference in picture tube faceplate curvature, between the old tubes and the new. When I went back to the Mitsubishi, I idly ran my hand over the tube, mentally assessing the curvature - just to reassure myself that it was not truly flat. It was not of course - it was significantly convex, just as I knew it would be. At least most of it was. When I ran my hand across the bottom of the tube I suddenly realised that a particular area (need I say where) was not convex but concave, with about the same order of curvature as 'the convex area. So that was it; it wasn't an electronic fault at all, it was an optical effect caused by a distorted picture tube faceplate. And that was the end of the exercise as far as I was concerned; the set was going straight back to Mitsubishi. The warranty payment for changing a picture tube is not exactly generous and their service department can do the job cheaper than I can. But my main reaction was that I wanted the thing out of my workshop as quickly as possible. Maybe I was over reacting but I felt sure that any tube with a distorted faceplate - and certainly one as bad as this one - was a prime candidate for spontaneous implosion. (I've seen the result of an implosion and it isn't funny). I don't know much about the glass technology of picture tube design but I do know that the curvature is a vital factor in tube strengtp.. And I understand that the so-called '"flat" tubes have only been made possible by modern computer design techniques, which have been able reduce the curvature significantly while retaining an acceptable safety margin. That, in my language, means that they have pushed the design about as far as they dare. Not that there is anything wrong with that in itself, but does it allow for the kind of distortion I found in this tube? More to the point, how did this tube ever get into the field? How did it pass the tube manufacturer's inspection and how did it pass the set manufacturer's inspection? The drifting Tandy My next story concerns a Tandy 48cm colour TV set, a CTV-2001. It's not a very profound story technically but it does emphasise some of the day-to-day problems and traps one encounters. One problem was to get a clear history of the fault, which came in dribs and drabs. The set came to me from a local Tandy dealer, who had sold it to one of his customers. It was a new set and still under warranty. The customer's complaint was that it "drifted" off channel after running for some time. Fortunately, I have a colleague - a fellow serviceman - who works for another Tandy outlet several suburbs away. The local Tandy dealer had approached him first , since he had the service facilities, but had been redirected to m e. The main reason for this was the distance involved. Everyone is anxious to avoid transport costs these days, so my colleague suggested th at I could handle the job. The warranty situation did not present any problem, even though I do not normally handle these sets on this basis. The plan was for the Tandy dealer to pay me normal rates, then straighten this out between the two . Tandy outlets. That was about as much information as I had when the set came to me and so I immediately rang my colleague, first to confirm the arrangement and second to find out what I could about this particular model. I Mitsubishi/AWA AV10 Symptom: no sound or picture. All mechanical functions are normal and the machine will record normally for playback on another VCR. Playback audio and video are present at the NV output sockets but not via the RF out. Cure: RF modulator unserviceable. The usual procedure is to replace the modulator at considerable cost. However, replacing the TA7673 IC in the modulator is worth a try. The chip is cheap and has cured the problem on at least one occasion . TETIA TV Tip is supplied by the Tasmanian branch of The Electronic Technicians' Institute of Australia. Contact Jim Lawler, 16 Adina St, Geilston Bay, 7015. didn't even have a circuit. The business side of things was quickly confirmed and we moved on to techicalities. 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The ranges are from 0 to 5kV Negative and from 0 to 10kV Positive. Double insulated for safety. $94 .oo $ 99.00 • Systems built to your personal requirements. • We stock a full range of Roctec external disk drives. • We also carry a full range of monitors, disk drives, modems, printers and other computer accessories. • Enquire about our comprehensive range of Australian made lighting surge arrestors, power and telephone line filters as well as standby and uninteruptable power supplies. • Phone today for details and competitive pricing. liiiiMiil * Mail and phone orders welcome. _ _ _ _ - ~ RAYTEC DATA SYSTEMS Phone (043} 23 1966 Fax _ (043) 23 3760 _ _ _ symptom:,, my colleague suggested that I look immediately at the tuner control board. Though these sets are very well made in most respects - a fact that I confirmed later - they appear to have one weakness. To understand this it is necessary to jump ahead a little and describe the layout of the set. The main board is about 25cm square and sits in the bottom of the cabinet, mounted in a reasonably substantial moulded plastic frame to give it additional strength. The whole assembly is held by four screws, one at each corner. This plastic frame also serves to support the tuner control board which is a narrow board, about as long as the main board is wide. This is mounted vertically on the main board at the fron_t of the cabinet, in a vertical section of the moulded plastic frame. The top edge of the control board sits just below the bottom of the picture tube. This board carries the tuner programming ICs and all the front panel controls. The tuner itself is mounted on the rear of the main board, allowing the antenna coax to connect directly to it. So what was the weakness? Accord- TETIA TV TIP PO Box 1174 Gosford Sth, 2250 • Freight chartge on normal size Items $10.00. Monttors etc. $15.00. • lnsurnce charge is 1% of total order value. PRICES VALID FOR THE MONTH OF PUBLICATION OR WHILE STOCKS LAST + $5 .oo p&p DEGAUSSING WAND 7,700 apere turns. Strong magnetic field, larger than usual coil with multicore centre. Double insulated for safety with momentary on/off switch. 240V A/C 2.2 amps. $75. This one is just about as important as having a soldering iron in your toolbox' . 0 p&p r'_:. . . . . ., REMOTE CONTROL TESTER ~ , r'\'\ " ,. '.J;y $85.00 + $4.00 p&p TUNER REPAIRS (INFARED OR ULTRASONIC) Designed to test any lnfared or Ultrasonic control units. With the extension lead you can also test infared units which cannot be placed in front of the testing unit. Requires a 9V battery . Output is via the LED diode and piezo speaker. from only $11.00 + p&p Chekque,dMonMey Order,d 8 an ea~ or asterca~ TIJNERS[\216 Canterbury Road, Revesby, ~ NSW, 2212, Australia Telephone (02) 774 1154 We also repair most tuners on an exchange basis. TV 1'• • APRIL 1991 43 SERVICEMAN'S LOG - CTD ing to my colleague, it was the "just below" which wasn't "below" enough - if you follow me. In other words, the tuner board was too close to the bottom of the picture tube. And the end result was that, if the set received a moderately severe jolt, as in transit, the board and picture tube could come into contact with enough force to crack the board or, at least, some of the tracks on the board. It had happened on a number of occasions and was now the first thing to be checked whenever any tuning problems were ,encountered. Hence his advice to check this board. Which brings us to the point where I had the set on the bench and was ready to turn it on and see what happened. The first surprise was that , there appeared to be no channels programmed into the tuner. Initially, I wondered whethe:r this was a manifestation of the drift problem but, fortunately, I had occasion to talk to my colleague again before I acted on this thought. It was then that I learned that the set had only ever been used with a video recorder. In fact, it was used in a coffee shop/milk bar to play video clips for the entertainment of its teenage patrons. So it had only been programmed for channel 1. Since it was easier to use off-air signals, I set to and programmed the tuner for the local VHF channels, plus SES on the UHF channel. Then I switched it to channel 2 and let it run. It ran like this for some hours, with no sign of trouble, which didn't seem to tally with the description of the fault - at least in terms of time. ·So I turned to the pattern generator, set it to channel 1, and fed the signal into the set. And that did get results; in a very short time, the signal started to drift. This was a bit puzzling, so I repeated the test on some of the high band VHF channels and also on the UHF channel. They were all rock steady. By now I was having definite second thoughts about the cracked board theory; the selective nature of the fault suggested an internal tuner fault. But, in any case, the board had to come out for inspection; if there was a "funny" due to damage and I missed it after being warned, I'd never hear the end of it. I pulled the back off the set and then, on impulse, tried another tack. I ran the set on all channels in turn, at the same time applying pressure and a twisting action to the tuner. None of this had any effect until I reached channel 1, whereupon it responded immediately; by applying the twisting action in the right direction and with the right pressure, I could virtually tune the signal in or out. This was even more evidence to support the faulty tuner theory. But I still had to clear up any suspicion of a cracked board. I pulled the board assembly out, unscrewed the control board from its frame - to get it in a good light - and went over it carefully, using a jeweller's loupe. After much searching, I was forced to the conclusion that there were no cracked or damaged tracks. While it was thoughtful of my colleague to warn me - and I'm passing the warning on - it was not the problem this time. .1.m.,.;. r.:.1;i:.11: RCS Radio Pty Ltd is the only company which manufactures and sells every PCB & front panel published in SILICON CHIP, ETI and EA. 651 Forest Road, Bexley, NSW 2207. Phone (02) 587 3491. 44 SILICON CHIP So back to the tuner. I turned the main board over and examined the various soldered connections to the tuner pins, again with the jeweller's loupe. I could find nothing wrong. I then set up a meter to monitor the tuning voltage but again drew a blank; it was perfectly steady. Finally, more or less in deperation, I went over the soldered connections and remade each one. It was more to be able to say that I had done it than any real hope that it would effect a cure. Imagine my surprise, therefore , when I set everything going again and found that the fault had vanished. None ofmy previous tricks to bring it on had any effect. So what had I done? Had there been a faulty joint which was not visible? Perhaps, but I was not convinced. The whole setup just didn't jell. I couldn't reconcile the channel selective nature of the fault with a faulty joint. Nevertheless, the set ran for the rest of that day without missing a beat, in spite of all the twisting, pressing and bashing I could dish out. Murphy's little joke So I finished up with a conflict of ideas. The more emotional line of thought wanted to believe that the fault was fixed; the commonsense line told me that such a cure just wasn't on. It was all one of Murphy's horrible jokes. But the set ran all the next day and all the following day. By this stage, I was about to send it back to the customer but then, on the fourth day, there was the fault, exactly as before. It was a narrow squeak. More to the point, I had no doubts now that it was an internal tuner fault. And that meant a new tuner, which seemed simple enough until we tried - and by "we" I mean the Tandy personnel as well as myself. The final answer was: "On back order; delivery date uncertain." I didn't go into the finer details of this situation, or who or what was to blame. Suffice it to say that there seemed to have been a miscalculation somewhere. But that's not unusual. So the set is sitting in a corner of my bench, waiting until a tuner becomes available. In the meantime, I have left it to the Tandy dealer to make peace with his customer. It's all very frustrating. SC