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SERVICEMAN'S LOG Well, we all make mistakes sometimes If there is a common theme in this month’s notes, it is the significance of the phrase “if only” – once described as the most tragic phrase in the English language. While not tragic in this context, it does emphasise that there are always lessons to be learned. Dealing with irate – and unreasonable – customers seems to be on the increase, at least in my neck of the woods. In my December notes, I told the story of one such character who called down the curses of the damned on both the set manufacturer and yours truly – simply because the set failed under warranty. This month’s story is almost an exact replica, at least as far as the abuse is concerned; almost word-for-word in some cases. As with the first incident, it involved another Samsung set but there the similarity ends. They were quite different model sets and the technical problem was quite different. Some of the preliminary events occurred before I was in­volved, so I am only assuming some aspects of these. 50  Silicon Chip It concerns a Samsung CB-515F colour set and, as some readers may recall, I related a story about this model some time ago. This involved, among other things, a modification to the horizontal output circuitry around the pincushion transformer (T402). More specifically, inductor L401, capacitor C414 and a diode/resistance package designated RH01, needed to be replaced, the soldering around them carefully checked, and the board checked for possible burn damage. As well as circulating their various service personnel, Samsung published a recall notice, advising owners of this model to contact their nearest Samsung dealer or service centre to have these modifications carried out under warranty. Apparently, this is where it all started; the customer re­ sponded to the recall notice by ringing Samsung’s headquarters and asking where the set should be taken for this to be done. As it happens, the customer lives some distance from me and so was directed to Radio Rentals in an adjacent suburb. Radio Rentals duly made the modifications and returned the set to the customer. Unfortunately, a couple of days later, the set developed a fault; the picture was rolling downwards very slowly. At this point the customer “did his lolly” as the saying goes. Instead of contacting Radio Rentals, he rang Samsung and demanded that his set be repaired immediately or that he be given a new set. And he refused to have anything more to do with Radio Rentals. So, whoever it was he spoke to at Samsung sooled him on to me as being the next nearest service centre (you’ll keep mate)! Thus it was that he turned up on my doorstep. He started off by complaining to me about the lousy service he had received from Radio Rentals. Well, I wasn’t going to become involved in that kind of argument, even by default. I suggested he calm down and stop making wild accusations. And I added that it was most unlikely that the present fault was in any way due to Radio Rentals’ work but was almost certainly a different problem. In any case, I needed time to look at the set before making a pronouncement of any kind and suggested he go off and do some shopping for an hour or so while I checked things out. So off he went, muttering witch’s curses (at least, that’s what they sound­ed like). With the back off the set I went straight to the modifica­tion site. As I fully expected, Radio Rentals had carried out the modifications exactly according to the modification sheet, neatly and professionally; there was no way that the job could be criticised, or that they could be blamed for the present problem. When the customer returned, I confirmed that the problem was nothing to do with the Radio Rentals modification, that it was a quite separate problem, that it would be fixed under war­ranty, but that it might take a couple of days to sort it out. Well, he wasn’t happy at this but then, I doubt that he has ever been happy about anything. That said, he had little option but to accept the situation and so he went off in high dudgeon. Rolling picture His description of the fault was quite accurate; the pic­ture was rolling slowly downwards and, more importantly, adjust­ ing the vertical hold control in the chip. Anyway, I tried increasing the value of R306 by adding another 22kΩ. That done, the system looked good. I let it run for a couple of days, which it did without so much as a blink, then phoned the customer and told him it was ready. So he duly picked it up. I reminded him that the set was still under warranty and that, if it gave any further trouble, he should contact me imme­diately. He didn’t say much, choosing instead to maintain a rather surly silence – if that makes sense. Anyway, I hoped I had seen the last of both him and the set. The phone explodes (VR301), had only a marginal ef­fect. In any case, it was insufficient to correct the problem. The vertical hold control is a 250kΩ pot, wired as a vari­ a ble resistor, which connects to pin 29 of jungle chip IC501 (TA7698P) via a 39kΩ resistor (R305). The other end of VR301 connects to the 12V rail via a 240kΩ resistor (R306). The only other component in this circuit is C352, a 0.22µF electrolytic capacitor. So it is all very simple without, seemingly, very much to go wrong. I checked the 12V rail, which was correct, as was R305 and R306. VR301 was then checked and the value of resistance in circuit seemed to make sense for the setting of the shaft. That left only C352 which, being an electrolytic, was a suspect. I pulled it out and checked it, and it measured OK. But I replaced it any- way; no point in taking chances with these devic­es. Unfortunately, this had no effect. So I had checked and cleared all the external components and found nothing wrong. That left only the chip as the main suspect. I don’t like changing chips without good reason but it seemed the only thing left. And I had one on hand, so I made the swap. And that seemed to be the answer. The picture immediately locked up quite firmly with the vertical control in its existing setting and this seemed about right, although it was somewhat towards one end. I let the set run for a few hours and it re­ mained rock steady. But I was a little concerned that the control was not as well centred as I thought it should be, although I couldn’t think of any reason for this apart from possible tolerance spread I thought I had too, because several weeks went by and I had almost forgotten about it. Then one morning the phone rang and when I picked it up it exploded – verbally, that is. Yes, it was Moaning Mick and he really turned it on; the so-and-so set had broken down again, it was no so-and-so good, I was no so-and-so good, I was dishonest, everyone else was dishonest, and on it went. I let him rave until he paused for breath, then quietly asked him what was wrong. That didn’t go down too well because apparently I was supposed to know. But, between splutters, I gathered that it was the same fault as before. So I simply said, “Bring it in and I’ll check it again. And it won’t cost you anything”. He muttered something which I took to be an acknowledgement and hung up. And so the set finished up back on my bench. Naturally, I wasn’t any happier than the customer, After all, I had to find the fault; all he had to do was complain. More to the point, I realised I had a tricky problem on my hands. Every likely compon­ent had been either checked or changed, yet the fault persisted. It is a truism that when this situation occurs, the most likely explanation is that something hasn’t been checked proper­ ly. We think we’ve checked it, but we’ve overlooked something. So what was it? Thinking about it like that, I realised there could only be one likely answer – VR301, the 250kΩ vertical hold control itself. Yes, I’d measured its resistance in circuit but had more or less simply March 1994  51 The vertical hold control circuit in the Samsung CB-515 TV set comes off pin 29 of IC501 & connects to the 12V rail (bottom of diagram). Changing the IC was not the answer. accepted the reading as being “rea­ sonable”. And that wasn’t good enough in this case. So I pulled the pot out and measured the resistance between the wiper and each of the outer contacts in turn, while rotating the wiper through its full range. And this told a very different story; different, and quite strange. The range of resistance from the moving arm to either terminal was only about 50kΩ. But as strange as this was, it did provide an immediate explanation for the fault. There was simply not enough resistance range to cope with all the variables in the circuit, including the tolerances in the IC chip. And it was now clear that it must have been just such a tolerance that tricked me into believing that changing the IC had cured the fault. As to why this provided a temporary “cure” which didn’t last – well, more on that later. Right then I was in no mood to speculate on any of the finer details. I had found the fault, the pot was crook, and I needed to fit a new one. Unfortunately, I had nothing in stock which would suit (at least, physically) and a new one had to be ordered from Sam­sung. This arrived in a couple of days, 52  Silicon Chip I fitted it, removed the extra 22kΩ I had added previously, and put the set through its paces. It worked perfectly but I ran it for a couple of days before calling the customer to tell him it was ready. He called a couple of days later, still grumbling and threatening everybody concerned with the most dire consequences if anything further went wrong. And that’s one of the most difficult aspects of this job – to remain civil in the face of such rude­ness and ignorance. I did my best to explain to him that everyone concerned had acted in good faith but it didn’t seem to work. Anyway, he went on his way and that was the end of the matter. It all happened several months ago and I have not heard from him since. And I doubt whether I will now – I hope! It comes in twos But that is not the end of the faulty pot story. A couple of weeks ago, one of my regular lady customers brought in an Akai model CT-K115, and it was rolling in exactly the same manner as the previously mentioned Samsung. More to the point, this model Akai uses the same chassis as the Sam­sung. Naturally, I went straight to the vertical hold pot, pulled it out, and made the same measurements as before. Sure enough, it was the same fault. It was not quite as bad as in the previous case but was still obviously bad enough to cause problems. At this point, I made a rather fortuitous decision. In the normal course of events I would have simply discarded the pot and fitted a new one. But I didn’t have one in stock and the customer had asked whether I could possibly get the set back to her in time to see a special program that night. And I had said I would do my best to oblige. The upshot was that I decided, for a couple of reasons, to pull the pot apart. One reason was an attempt to satisfy my curiosity as to why it was faulty; resistors do not normally go low, although rare cases have been reported. The other reason was a faint hope that, if I could find the reason, I might be able to do something about it and avoid the delay in obtaining a replacement. Granted, it was a long shot but what did I have to loose? The pot is in two parts, held together with five little clips, which were easily prised back, releasing the punched bakelite plate carrying the carbon element and the three termi­nals. And this was most revealing. From the three terminals there are three short, parallel, carbon tracks; two outer ones to the ends of the resistor ele­ment, and a centre one to a circular carbon pad which makes contact with the wiper mechanism. Mechanically, it was all per­ fectly conventional. What wasn’t so conventional was a strip of reddish-brown paint that had been applied across these three parallel tracks. Or at least it looked like paint; its real composition, or its purpose, remains a mystery. But its location aroused my suspi­cions immediately; if it had developed any kind of leakage bet­ween the carbon tracks, it would have produced exactly the be­haviour I was observing. More to the point, if this was the case and I could remove this coating, I would have achieved both my aims; proved the cause of the fault and salvaged the pot for immediate use. I decided to try to scrape away the paint – or whatever it was – from be- 12 MONTHS WARRANTY ON PARTS & LABOUR THAT MAKE LIFE EASIER PRODUCTS YOU NEED AUSTRALIAN MADE TEST EQUIPMENT SHORTED TURNS TESTER Built-in meter to check EHT transformers, in­ clud­­­ing split diode type, yokes and drive trans­ formers. $95.00 + $3.00 p&p DEGAUSSING WAND Great for comput er mon­­­i t­o rs. Strong magnetic field. Double insulated, momentary switch operation. Demagnetises colour picture tubes, colour computer monitors, poker machines video and audio tapes. 240V AC 2.2 amps, 7700AT. $85.00 + $10.00 p&p HIGH VOLTAGE PROBE tween the tracks using a sharp needle. It was a fiddling job, due to the small dimensions involved, but it came away fairly easily. And my suspicions were justified; the pot values immediate­ly returned to normal. I lost no time in fitting it back into the set, confirmed that the fault had been cured, let it run for a couple of hours, and had it ready for the lady in time for her evening program. So that’s the saga of the dicey pots. I know it poses as many questions as it answers but at least one can now be on the alert for similar situations. One thing seems obvious; the paint apparently deteriorates slowly over time so that, initially, the fault could be corrected by readjustment of the control. Thus, in some cases, service help would only be sought when the adjustment ran out. And this is also the possible explanation for the second failure of the Samsung set after the first apparent cure. But what is the paint and what is its purpose? I have no idea but it is significant that it is not common to all these pots. All those that I have encountered with the paint – and there have been some since – have been in original equipment, while the replacement units appear to be free of it. So your guess is as good as mine. And yes, “if only” I had taken more care with that initial pot measurement. Something different And now, from my colleague J. L., south of Bass Strait, something a little different. This is how he tells it. The dear old lady arrived at my door bearing her much-loved radio cassette player. Her problem was that it had blown up after the power supply authority crossed its wires and put 6kV on one of the local phases! Her neighbours had lost refrigerators, TV sets, microwave ovens, video recorders, and washing machines but, as far as she could tell, the only thing she had lost was her radio. However, after six months of trying to get compensation from the authori­ty, she decided to see if it could be repaired at a price she could afford. I had to tell her that I didn’t like her chances. The set was a Sanyo AM/FM stereo cassette player, about 10 or 12 years old. Although nominally a portable battery-operated unit, it was fitted with an internal AC powerpack to make it a generally more versatile model. And as is so often the case, the owner had never had a set of batteries in it – it had always been used in the kitchen as an AC-powered radio. Built-in meter reads positive or negative 0-50kV. For checking EHT & focus as well as many other high tension voltages. $120.00 + $5.00 p&p REMOTE CONTROL TESTER Designed to test infrared or ultrasonic remote con­ trol hand­pieces; eg, for TVs, VCRs, house alarms and car alarms. Supplied with extension infrared detector lead. Output is via a LED and piezo speaker. $97.00 + $4.00 p&p. SILICON CHIP COLOUR TV PATTERN GENERATOR Built-up kit comes with power plugpack, RF lead. $250.00 + $9.00 p&p. TV & VCR (new) tuners – $47.00 each VCR converters – $49.00 each TV, VCR TUNER REPAIRS FROM $22 REPAIR OR EXCHANGE Phone for free product list 216 Canterbury Rd, Revesby, NSW 2212, Australia. Phone (02) 774 1154 Fax (02) 774 1154 Cheque, Money Order, Visa, Bankcard or Mastercard March 1994  53 My first test was to check the continuity of the AC lead and the primary of the internal power transformer. It was no surprise to find the lead OK but the transformer open circuit. Next, I fitted a set of seven C cells and tried the radio switch on the front panel. The volume control had been left full on and I was nearly deafened by the roar of one of the local rock stations. Given the history of the defect, I would have wagered nothing on the survival of the internal electronics, yet it seemed that the transformer primary was the only casualty. As is usual with so many of these types of jobs, the prob­lem was not so much replacing the transformer but in knowing what value of transformer to replace it with. The battery voltage (10.5V) gives some clue as to what the transformer voltage might be but one can never be sure. One way is to fit a transformer with, say, a 25V secondary and then activate it slowly with a Variac. As the secondary voltage comes up the set will come to life and the art is decid­ing when it seems to be operating normally. At that point, the secondary of the substitute transformer should be delivering around the correct output for that particular set. Having decided what voltage is needed, the next problem is to check if the best available transformer will actually fit into the space vacated by the defunct unit. However, when I opened the cabinet, I was faced with a transformer the likes of which I have never seen before. It was a very thin, flat package, rather like an ordinary transformer after an attack by a steamroller. There was no way that I could get a transformer of conventional con­ 54  Silicon Chip struction to fit in the space available. It was quite obvious that I was going to have to get an original replacement if the set was ever going to be restored. So I enquired of my local Sanyo agent to see if the transformer was still available and what the price might be. It was a case of good news and bad news. Yes, the trans­former is still in stock but it was going to cost me $40. With freight and a small retail margin added, plus my labour charges, the total cost was going to be more than the old radio was worth. Then I had the bright idea of using an AC plugpack to replace the internal transformer. The low voltage AC could then be fed into the set through the existing AC socket. It would need to be modified in some way to avoid the risk that someone might try to inject 240V but that didn’t look like an insurmountable problem. The more immediate worry was to find out what value of low-voltage AC was needed to operate the set. So I asked my friendly Sanyo agent if I could look at his service manual for this model. In fact, we took a photocopy of the circuit so I had all the information I might need to get the radio working again. It was while I was pondering over the diagram, trying to work out what the AC input to the power block might be, that I noticed the words “Ext DC in”. As it turned out, all my worrying had been for nought. The set had provision for 3-way power – 240V AC, internal batteries, or an external 9-10.5V DC supply. The external supply was via a conventional DC socket so all I had to do was to get a 9V DC plugpack and the DOL’s set was going again, without any need to cut, drill or modify! As it happened, I had a suitable old plugpack in stock and was able to press that into service, at minimal cost to the customer. If only I had looked more closely at the set when I first fitted the batteries, I would have seen the DC socket and could have saved myself an hour or more of angst. I need not have even taken the back off. That’s what comes of spending so much time inside TV sets and VCRs. All jobs look like big ones until you find out that they aren’t! Thanks J. L. Yes, “if only” – I know SC how you feel.