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SERVICEMAN'S LOG It was a dark & stormy night Yes, it was; very dark and very stormy. The storm had blacked out several Sydney suburbs and, in the process, created a line surge which damaged the set featured in this month’s notes. And it was a dark and stormy exercise correcting the damage. The set was a National Panasonic model TC-68A61, fitted with an M16M chassis. It is a 68cm set, featuring remote control plus all the latest bells and whistles, and retails for around $1800. It was quite new, being only about 14 months old. I discovered later that this was not the only TV set to be damaged in this 40  Silicon Chip and subsequent storms a few days later. There were many more from all over the suburbs, one of which was described as a complete write off. The set came in with the simple description of being com­pletely dead, which it was from the customer’s point of view. A quick bench check produced a violent squeal from the switchmode power supply, suggesting a short on one of the supply rails. Unfortunately, this model set was a complete stranger to me. I had never even seen one before and had absolutely no data of any kind. Nevertheless, I decided to at least take the back off the cabinet and check for any visual clues. This operation produced its own shocks. Firstly, everything was jam-packed in – a real servicing nightmare. Secondly, the cabinet was of rela­tively flimsy plastic so that, when the back was removed, it distorted noticeably under the weight of the large tube. There were no obvious signs of damage, so I decided to pull the chassis for a closer look. This was a difficult operation, due in part to the distortion of the cabinet, although I realised later that there were some tricks which made it easier. Anyway, with the chassis out, my main aim was to try to find whatever it was that was obviously overloading the power supply, as suggested by the squealing. I went first to the hori­zontal output transistor, Q551, and checked for voltage on the collector. There was none so I pulled this transistor out, ex­pecting it to be shorted, but it was intact. So it looked as though the fault was closer to the power supply but, without a circuit, it was impossible to identify the various rails or even to know how many there were. My best effort was to find that there was a dead short to chassis from a test point labelled TPD1, which appeared to be one of the rails. At this point, I realised that it was hopeless to proceed without a manual or at least a circuit. Fortunately, I was able to find a colleague who did have a circuit and he was quite happy to lend it to me. It amounted to a total of six A3 pages! These cover a swag of boards or modules, designated alphabetically. I ran out of fingers trying to count them but I make it about 16. The accompa­nying illustration is part of the D board. Just as importantly, my colleague was able to pass on a lot of valuable information based on his own experience with this model set. Of particular value was a warning about power­ing up the set after a repair. It appears that the set is very easily damaged if other faults are overlooked. This was a kind of “good news/ bad news” situation; I was extremely grateful for the warning but not very happy about the need for it. Circuit details Anyway, now that I had a circuit I could at least begin to sort things out. The set has two switchmode supplies: (1) a main one supplying the high voltage rails; and (2) a subsidiary one supplying a 5V rail for the remote control functions, plus a 12V rail. This 12V rail is very important because, among other things, it powers standby and protection circuits. And it func­ tions continuously. The main supply centres around transformer T801 and the short I had Fig.1: portion of the D board on the National Panasonic TX-68R71. The subsidiary supply, involving T881 and its associat­ed parts, is at top left, while the main switchmode supply invol­ves T801 and transistors Q801-Q805. IC801 is at bottom centre, IC802 to the right and SCR Q821 above it. found was in fact on the main HT rail, normally operating at 139V. It involves transformer pin S2, diode D808, filter capacitor C828 and IC801. I connected the ohmmeter between TPD1 and chassis and pro­gressively removed components from this line, including IC802, C828 and some other minor components, until I came to SCR Q821. I pulled it out and the short cleared, which meant that the SCR had broken down. But what was the SCR’s function and, most importantly, why had it failed? Once again I am indebted to my colleague for saving me from having to try to work this out for myself. SCR Q821 is part of an over-voltage protection circuit, particularly guarding Q551 and the horizontal output stage in general. And it had done a good job, to the point of sacrificing itself. But the implication from such a drastic reaction could only be that it must have been a very severe voltage overload. So how could I fire up the set safely to make further tests? Normally, I would use a Variac for this job, possibly with a series lamp in the mains line as a current limiting device. Unfortunately, another colleague had passed on some hearsay advice that a Variac could not be used on these sets, although the explanation was hopelessly garbled. As it turned out, this was a furphy. It appears to have arisen from a warning in the manual, which I saw later, against depending solely on a Variac for protection before all the recom­ mended tests had been performed. But that was later and, right now, with various warnings ringing in my ears, the best I could do was settle for a 200W series lamp in the mains lead. I also took the precaution of disabling the horizontal output stage by shorting the base and emitter of Q551. Then, with a meter monitoring the main HT rail, I switched on. The reaction was quite dramatic – the meter shot up to over 200V, clearly indicating something seriously wrong with the power supply regulation system. And, as if to confirm this, in the few seconds I took to absorb the reading, there was a loud bang. The excessive voltage had proved too much for C760, a 0.47µF electrolytic rated at 160V, which had exploded. And when they explode they don’t muck about. Fortunately, this was easily fixed and there appeared to be no other damage. At this stage, I encountered another colleague who was able to loan me a copy of the service manual. This includes a section entitled “Service Hints for M16M Power Supply Repair”. And almost immediately, it begins listing “possible causes for a power supply primary shutdown”. Among other symptoms, it mentions the mains fuse, F801, being obviously O/C, and transistors Q803 and/or Q805 being physically blown apart! It also suggests checking IC801, with a low ohmmeter, in anticipation of it being “absolutely S/C between all three terminals!” The manual goes on to list all the components which should be checked in the event of a “primary shutdown”. And it includes instructions as to how components should be tested, strict warn­ings about the critical nature of many components, and the risks of using substitutes. All told, it lists no less than 16 components which should be tested before applying power. The risk appears to be that a serviceman may follow the usual practice of progressive testing; ie, find and replace a faulty component, then reapply power, check performance, and search for further faults if necessary. The manual warns that this approach could likely result in fur­ther severe damage. It’s not the most encouraging introduction to a strange set! Voltage regulation But at least I had been warned. And my attention was now directed to the voltage regulation system; to find out May 1996  41 Serviceman’s Log – continued how it worked and why it didn’t. Once worked out and explained, it is not hard to follow but it wasn’t easy coming to it cold. It all hinges around IC801 and D812, the latter an opto-coupler IC801 is a 3-terminal device. Pin 1 connects to the 139V rail, pin 3 connects to chassis, and pin 2 connects to pin 2 of the optocoupler, which is the cathode of its internal LED. Pin 1 is the anode of this LED and goes to a 12V rail from IC802. The other half of 42  Silicon Chip the optocoupler is a transistor, with the collector connected to pin 4 and the emitter to pin 3. The base is activat­ed by light from the LED. In operation, IC801 conducts between pins 2 and 3 when the voltage on its pin 1 terminal reaches 139V. This completes the circuit between the 12V rail and chassis via the LED in the associated optocoupler. The LED now glows and turns on the tran­sistor between pins 3 & 4 of this device. Pin 3, in turn, drives a transistor network consisting of Q802, Q803 and Q801. The latter is at the heart of the switchmode supply and switches the primary of transformer T801. By control­ling the oscillator activity when the main rail reaches 139V, that voltage is maintained. At this point, I decided that the best approach would be to order all the components listed as likely needing to be changed and put the set aside until these arrived. This would save time and any components not needed could go into stock. I had an idea that this would not be the last of these sets I would see. The only snag was that I was quoted up to three weeks delay on some parts. This was an irritating setback but I decided to make the most of the time by trying to pinpoint the most obvious fault – the failure of the IC801/ optocoupler combination to regulate. Testing IC801 As already mentioned, the manual suggests that IC801 is a prime suspect, most likely going short circuit. Well, I’d already cleared it of short circuits but it could still be faulty. How to test it? Well, not in situ, since power could not be applied. A preliminary resistance check revealed no continuity bet­ween any of the terminals but that didn’t really mean much. Once again my colleague came to the rescue. He had already made up a simple test jig and gave me the details. It was a simple enough arrangement to knock up and I soon had it working. And it worked very well; so well that it clearly indicated that IC801 had carked it, which was one good reason why the HT rail was not being regulated. What about the optocoupler? The manual had made the point that if the optocoupler proved to be faulty, then IC801 should be replaced automatically. Would the reverse be true? The manual suggests testing the optocoupler using an ohm­meter and I have no doubt that it is technically accurate. Howev­er, the optocouplers are very small devices and trying to test them in this manner is fiddly, at best. So I added to my collea­gue’s jig, making it a combined tester. It was all very nice in theory but I needed a known good IC801 to make it work. This was one of the components on three weeks delay, so I cheated by connecting the prods of an analog multimeter (low ohms range) across pins 1 and 2 of the optocou­pler to energise the internal LED. I could get no response from the original device but the new one, which arrived early, pro­duced an immediate response from the external green LED. Eventually, the remaining parts arrived and I replaced the SCR (Q821), IC801 and the optocoupler. I had already replaced C760 which I had blown up earlier and had spent some time checking and double checking all the other components listed – as well as some that weren’t. In theory, I should have been able to switch on safely. However, the manual suggests a proper routine for switch-on at this stage and I wasn’t prepared to take any chances. What this amounts to, in essence, is to disable the horizontal output stage, replace it with a dummy load, then wind up the supply voltage via a Variac. Talk about a belt and braces approach! In greater detail, the procedure involves lifting a 1.2Ω resistor (R561) on the X board, which is in the 139V rail to pin 9 of the horizontal output transformer (T501). At the same time, a dummy load, consisting of a 60W globe, is connected from this supply rail to chassis, most conveniently from pin 1 of the X10 plug on the D board to pin 1 of the X11 plug, which is chassis. These are not shown on the accompanying circuit. The manual also suggests lifting D560, which I did. This is to disable a protection circuit involving transistors Q553, Q554 and Q555. If this circuit had been activated by a fault, it would shut the set down and inhibit further testing. Having done all this, I connected the set to the Variac but left my 200W globe in series. I must admit that I was extremely nervous about the whole situation and felt that another belt added to the belt and braces wouldn’t do any harm. I also con­ nected the CRO to the collector of the chopper transistor (Q801). I switched on and wound the Variac up slowly. And, with only about 30V in, the CRO indicated oscillation around Q801. Beyond this level, it abruptly stopped oscillating. I gradually increased the voltage, eventually reaching 150V, which was as high as I was game to go – still no oscillation. I backed the voltage off and moved to the subsidiary power supply. I checked the 5V rail out of IC803 and, at about 100V in, it came good, as did the 12V rail at zener diode D883. Well, that was good news; very good news in fact, because according to the manual, this supply is vital for the remote control and protec­tion systems. Remote control switching But it didn’t help much with the main power supply problem. In order to follow what happened next, it is necessary to look at the remote control ON/OFF switching function. Working backwards from the switch­ mode section, involving transistors Q802, Q803, Q804 and Q805, we trace the circuit up to pin 3 of D841, the second optocoupler. And pin 4 of D841 connects to the 12V rail which we had just checked. So the role of the D841 is to switch the 12V supply to the transistors in the switch­mode supply. D841 is controlled by transistor Q841 between pin 2 and chassis. This transistor is controlled, in turn, by the remote control system on board E, involving microprocessor IC1213 and transistors Q1231, Q1207 and Q1209. I won’t bore the reader with all the details of this circuit operation – just that it finishes coming in on pin D5 on the D board and goes to the base of Q841. So the remote control system switches Q841 on or off, switching May 1996  43 So what was wrong now? All kinds of weird and complex possibilities raced through my mind, without making much sense. Then I suddenly looked up and caught sight of the 200W lamp in series with the mains; it was glowing a dull red. I had complete­ ly forgotten that the lamp was still in circuit. I disconnected it and tried again. And this time everything came good –correct HT rail voltage, no signs of distress any­where, and the set actually functioning. And functioning very well, too. Insurance D841 on or off, and turning the switchmode system on or off. It’s simple when you say it quickly. Having worked out what should be happening, I was able to trace the circuit through and establish that every stage was functioning up to the base of Q841. But Q841 wasn’t doing any­thing about it. I pulled it out and found that the base-emitter junction was open circuit. This presented something of a puzzle. As far as I can work it out, this transistor must have been working when I first turned the set on, otherwise there could have been no HT rail voltage (the excessive voltage which blew up capacitor C768). So, was Q841 damaged by a kickback from this misadventure. We’ll probably never know. Anyway, that problem was easily fixed. I didn’t have a 2SD1010 and, conscious of the dire warnings about substituting alternative components, I hesitated initially. But it didn’t appear that this was anything more than a general purpose tran­sistor so I took a punt and fitted a BC547. That started things working. As I advanced the Variac the CRO indicated that the system was oscillating and it kept on oscillating. And there was voltage on the main HT rail at test point D1 which, according to the manual, should reach its normal 139V operating voltage with an input as low as 120V. Unfortunately, it didn’t. At 120V on the Variac the best I could get was about 117V. I wound the Variac up to around 150V, which the manual warns is the limit if a normal HT value is not reached. There was no significant improvement. But there was one more job I had to do for the customer. Damage of this kind is not, of course, covered by warranty. But it was covered by the customer’s household insurance and I filled in the necessary details on his claim. As for the set itself – well, I wouldn’t nominate its designer(s) for any Oscars. I cannot escape the impression that they started off with a lot of surplus components and that they used as many of them as possible! An exaggeration? Well, maybe, but other designs have pro­duced the same end result with less components and greater reliability. More to the point, from a practical servicing point of view, I offer this advice to anyone presented with one of these sets. Do not, in any circumstances, touch it – and I mean that word “touch” almost literally – without the benefit of a manual. If a manual cannot be obtained, knock it back. To do otherwise is to do both yourself and your customer a SC gross disservice. Especially For Model Railway Enthusiasts Available only from Silicon Chip Price: $7.95 (plus $3 for postage). Order by phoning (02) 9979 5644 & quoting your credit card number; or fax the details to (02) 9979 6503; or mail your order with cheque or credit card details to Silicon Chip Publications, PO Box 139, Collaroy, NSW 2097. 44  Silicon Chip