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SERVICEMAN'S LOG Around the world for sixpence No, that heading is not a travel agency’s advertisement; it is one from way back which, as I recall, implies a whole sequence of frustrating events which ultimately result in very little gain. And that was exactly how I felt by the time I had finished this particular job. To be more precise, I can honestly say that this job turned out to be one of the most frustrating I have encountered for a very long time. I very nearly decided not to relate it, however, because the fault eventually turned out to be something quite simple. The only reason I have told it is that it might help someone else. Or that someone else might be able to answer some of the questions it poses. The story concerns a Hitachi-Fujian 34cm colour TV set, model HFC1421B, chassis type F87PT. These sets are made in China and were originally marketed in Australia, several years ago, through a Chinese Trade Commission, with offices in the Sydney suburb of Botany. However, I can find no reference to them in the current directory. The owner’s complaint was that the picture was rolling but a quick check revealed that this was not the case; the Fig.1: this diagram shows the relevant portion of the circuit around IC501 in the Hitachi-Fujian HFC-1421B. The IC is at the top, with pin 40 near it’s bottom lefthand corner. R710 is near the bottom of the diagram, in line with pin 34 of the IC. 40  Silicon Chip set had not lost vertical hold, it had lost horizontal hold – something likely to be rather more complicated. And as I had no technical data for this set, my first job was to try to acquire at least a circuit. However, with the marketing situation apparently changed, I was unsure as to who to approach. So I contacted my retailer colleague who has sold a number of this brand – but not this model – in the past. It transpired that he had obtained his stock from a wholesale outlet in Canber­ra and he gladly supplied their name and phone number. They were very obliging but could not help directly. In­stead, they referred me to a Sydney firm, Cumberland Electronic Services, 120 Old Canterbury Rd, Lewisham 2049. Phone (02) 564 1700. If you are involved in servicing, I suggest that you make a note of that. I rang them and they were also very obliging. They put me onto their service department and the gentleman in charge was most helpful. Yes, they could supply either a copy of the circuit diagram for $5.00 plus $2.00 postage, or a complete manual for around $25.00. I decided that, for what looked like a fairly routine fault, I could probably get by with a circuit, although the price of the manual is quite reasonable compared with some prices quoted these days. So I posted off a cheque and, in a few days, had a photocopy of the circuit. And, as photocopies go, it was not bad. By that I mean that it was probably no worse than the original, which was obviously much reduced. At least most of the values were readable, or at least decipherable. But some of the closely packed lines were hard to trace. Anyway, it was a start. So why had the set lost horizontal hold? The heart of this part of the circuit is IC501, a TA7698AP. This is a large, complicated 42-pin IC which handles the video signal, sync separation, chroma signals and matrixing, etc. It also has pulse...”. In fact it looks like the top half of a slightly impure sine wave. But “from shaped”? Anyway, terminology aside, I decided that it was good enough. So what did that leave? If the horizontal oscillator was running free at a reasonable rate and reference pulses were being fed back from the horizontal output transformer, I reasoned that it looked like a sync pulse failure. A composite video signal from the front end of the set (pin 15 of IC201) is applied to pin 39 of IC501 (INV IN), goes through an inverting stage, and emerges on pin 40 (INV OUT); ie, the signal is inverted but otherwise unchanged. From here, the signal goes to pin 37 (SYNC SEP IN), which seems quite logical, except that there is a network of resistors, capacitors and a diode in between – see Fig.2. And it is around this network that I have encountered faulty components in the past. So, initially, I went over this network and checked every likely component, either by measurement or substitution. And I could find nothing wrong. But the mere presence of the network itself kept nagging me. What was its function anyway? Could there still be a subtle fault in it somewhere that I had missed? This was something I was unsure about without knowing what the network was supposed to achieve. Come in sucker the facility to be wired to handle PAL, NTSC or SECAM signals. It is a very commonly used IC and is found in a variety of brand names. These include AWA, NEC and Orion. And it so happens that I have an AWA manual in which there is a section purporting to describe some of the associated circuitry. Among other things, it has an enlarged and detailed diagram of the IC, with designa­tions for the various pins – something not available on the Hi­tachi circuit. I have included these designations in brackets in the following discussion. I concentrated on the circuitry around the sync separator section, partly because I have had problems here before. In particular, I remembered encountering both vertical and horizon­tal sync problems in this area. But there were a couple of other symptoms supporting this idea. Most important was the fact that, by carefully adjusting the horizontal hold control (R709) and riding it, it was possible to hold the picture on the screen. This suggested that the free-running frequency of the horizontal oscillator was not grossly out and that it should be capable of locking if the sync pulses were being processed correctly. Further to this thought, I checked the horizontal pulses from the horizontal output transformer, which are used for com­parison in the AFC circuit. These come from pin 7 of the trans­former and are fed into pin 38 (H PULSE IN/GATE PULSE OUT) of the IC. There was a substantial pulse here, similar to one shown in the AWA manual and described as “a from shaped horizontal flyback I suppose that, at this stage, I allowed myself to be sucked in to some extent. I felt I should try to understand the circuit, at least for future reference, rather than simply check each component until I found a crook one. A noble sentiment, perhaps, but not necessarily a very practical one. One reason I pursued this approach was that, as I mentioned before, the AWA manual purports to explain the purpose of this network. If I could digest that, I might have learned something. Alas for my hopes. The explanation turned out to be a mess of badly-translated gobbledegook, with spelling, grammatical, and composition mistakes that went far beyond the ones we regularly encounter. Taken together, they added up to incomprehensible garbage (I should have been warned by that “from shaped” pulse reference). June 1994  41 heels. As far as I could tell, the external network between pins 37 and 40 was intact, the horizontal reference pulse from the horizontal output transformer, was correct, and I had replaced the IC. What had I overlooked? Back to the circuit According to the screed, “The composite video signal from pin 40 is applied (to pin 37) through a sync separator time constant circuit. The slice levels of the horizontal sync and vertical sync can be set independently”. So now we know! Or we might if we were sure what is meant by slice level – I could hazard a guess – and, more importantly, why it is apparently so critical. And what is the purpose of the “time constant”, which is also mentioned? I went over the screed several times, cooked up several theories which had to be discarded, and discussed it with several colleagues. They all shrugg­ed their shoulders and one, to whom I complained about the standard of 42  Silicon Chip service manuals, muttered something along the lines of “that’s life”. Well, I know when I’m beaten. I had wasted a lot of time and achieved nothing more than a sense of frustration. I seemed to have exhausted all the possibilities I could think of. So what was left? The IC? Not very likely and only as a last resort. But this seemed like a last resort situation. And I did have a spare on hand. Oh well, at least it would prove the point, one way or the other. It is not such a big deal these days, even with 42 pins involved. Twenty minutes later I was ready switch on again, fingers crossed. It was an anticlimax; the fault was still there, exactly as before. Well, that really set me back on my I spent some time pouring over the circuit again. As I’ve already stated, some of the lines were very crowded, making some long runs very difficult to follow. And this was part of the reason I had missed out on a vital section. I went back to pin 7 of the horizontal output transformer and once again, but more carefully this time, traced the circuit up to pin 38 (H PULSE IN etc). And I suddenly realised what I had missed. Branching off this rail was resistor R701 (8.2kΩ). And this led me to R706, a 2kΩ preset tab pot, shunted by R707 (2.2kΩ) and connected to chassis via capacitor C709 (.01µF). This tab pot is what some makers call horizontal AFC, or horizontal position, among other names. The moving arm of R706 goes to pin 35 (AFC OUT) via C707 (.022µF) and its obvious func­tion is to allow the level of the reference pulse to this pin to be optimised during manufacture. Of course, the fault just had to be in this little branch of circuitry which had been overlooked. And it was – R701, the 8.2kΩ resistor, was virtually open circuit. When it was replaced, the picture locked up instantly and set was ready to go back to the owner. But why was it overlooked? There is no point in trying to make excuses; if you goof, you goof. The best one can do is try to learn from it. But I think it is fair to say that the crowded nature of the circuit didn’t help, although that is something we have to learn to live with. Another factor, and again this is no excuse, was that the offending resistor was not mounted anywhere near the cluster of other components around the IC, which I had tested as a matter of routine without too much regard for their place in the circuit. No, it was mounted over near the horizontal output transformer, where it connected to pin 7. Of course, it was all perfectly logical from the physical layout point of view. So there it is; a happy ending for the customer but a very unhappy INVERTER OUTPUT 40 R550 2.7k R551 820  R552 620k D591 C563 1 C564 .01 SYNC SEP INPUT 37 R565 56k C513 560pF Fig.2: this network between pins 37 & 40 of IC501 in the Hitachi-Fujian HFC-1421B has been redrawn from an AWA manual & given the component markings from the HFC-1421B circuit. experience for yours truly. That one won’t be putting any cream on the custard – it won’t even pay for the custard! Bread & butter Well, after all that, I think that something a little less traumatic is called for; something more in line with the routine day-to-day, bread-and-butter jobs. (Why do I keep talking about tucker?) Anyway, this is a story about a video recorder; a Hitachi model VTM818E. This is a current model and the particular unit was virtually new, being only about three months old. That means, of course, that it was still under warranty. What follows is not about any long and involved diagnosis; the fault was plainly visible. The real point is the unlikely coincidence of the forces involved. The lady who brought it in was quite clear about the prob­lem; it would load a cassette, apparently correctly, but would not play it. It would not respond to the play button; at least not properly. It would commence to play the tape but would run for only about 10 seconds before stopping. I didn’t doubt the lady’s description but I must confess to being a little surprised by it. It sounded like a mechanical fault of some kind but that would be very unusual for a machine with only three month’s use – and particularly with this model, which is a very reliable one. After the lady had left and I had finished another job, I set it up and pushed in a tape. It loaded normally, so I pressed the play button. And it behaved exactly as described; it ran for about 10 seconds, then shut down. I pushed the eject button and the cassette unloaded normally. OK, time to pull the cover off and look at the works. The layout is fairly conventional, with the tape deck on the lefthand side and the PC board to the right. I could see nothing obviously wrong, so I pushed the tape in again and watched it load. This appeared to be normal. The machine employs the now fairly common “half load” procedure; as soon as the cassette settles on the deck, the tape is loaded against the control head, an arrangement designed to measure and display the actual playing time of the tape while it is being shuttled. Then, when the play button is pressed, the loading arms pick up the tape and Subscribe now to the largest faults & remedies library in Australia ✱ ✱ 1994 manuals are now available. Our database is regularly updated with information supplied by technicians such as yourself. ✱ Exclusive backup service by qualified technicians. ✱ ✱ Over 10,000 faults and remedies on file with flow charts and diagrams. Covers Colour TVs and VCRs of all brands sold in Australia EFIL Phone or fax now for your FREE information package ELECTRONIC FAULT INFORMATION Reply Paid 4 P.O. Box 969 AIRLIE BEACH 4802 Ph 079 465690 Fax 079 467038 June 1994  43 SERVICEMAN’S LOG – CTD wrap it around the drum in the usual way. And that is exactly what happened. The tape began to move, the take-up and supply reels revolved, and nothing appeared to be amiss. At least, not for the first few seconds. Then, suddenly, the take-up reel stopped and the tape kept coming, forming a loop. I was reaching for the on/off switch, when the whole system shut down. Well, that was normal too. The loss of sensing pulses from the take-up reel had indicated to the microprocessor that there was something wrong and it had done its job – including rewinding the loop back onto the supply reel, to avoid further damage. So all I had to do was find out why the take-up reel was malfunc­tioning. And it was malfunctioning in a rather peculiar way. Further observation, using a clear plastic dummy cassette, allowed me to see the takeup reel directly and revealed that it always made exactly one revolution before it failed. I unloaded the dummy cassette and made a closer examination of the take-up reel. Turning the reel with my fingers confirmed that it was meeting some obstruction after one turn. And looking more closely it appeared that there was something stuck to the side of the reel. But in the confined space of this par­ticular unit, I was unable to see what it was. I decided that the simplest approach would be to remove the cassette carrier. While this job is a bit fiddly, it is no big deal provided the screws are removed in the right order. I won’t bore the reader with the procedure – it varies from machine to machine anyway – but it does involve un­clipping the front panel of the machine and moving it out of the way. That done, the carrier can be slid slightly towards the front of the machine, allowing a couple of locking keys to be freed from their keyways. The carrier can then be released and laid to one side – the lead lengths are quite adequate – thereby providing ready access to the reel mechanisms. The nest step was to remove the reel itself. It is held in place with a small circlip, the only precaution being to ensure that the circlip doesn’t fly off to the other side of the workshop. Well it didn’t and I was then able to lift the reel free. Brake mechanism And now all was clear. What we were looking at was the brake pad from the brake assembly of the take-up reel. The brake mechanism consists of a brake shoe which is fitted with a felt pad. The brake shoe moves horizontally and presses the pad against the side of the reel, to prevent tape overrun when the transport mechanism stops. But there was no brake pad on this brake shoe, where it should have been; instead it was stuck to the side of the reel. And that was what was hindering the rotation of the take-up reel and causing the machine to shut down. On Sale Now At Selected Newsagents Or buy direct from SILICON CHIP Price: $7.95 (plus $3 for postage if ordering from Silicon Chip). Order today by phoning (02) 979 5644 & quoting your credit card number; or fax the details to (02) 979 6503; or send cheque, money order or credit card details to PO Box 139, Collaroy, NSW 2097. 44  Silicon Chip Now missing brake pads are fairly common, though not in machines only a few months old. More importantly, they don’t cause this kind of problem. Instead, they usually simply fall to the bottom of the machine and, in most cases, the user is not even aware of what has happened; the bare brake shoe usually continues to do the job, even if a little less scientifically. It is only when the machine is opened for some other reason that this fault is found. The brake pads are attached to the brake shoe with some kind of contact adhesive on the back of the pad. Exactly why this failed, allowing the pad to come free, is not clear. But what is apparent is that, when it did come adrift, it flipped over so that the adhesive side was pressed against the side of the reel, and it stuck there. And it was right in the path of the brake shoe, so that it fouled it after the first revolution. The cure was simple enough. Peel off the pad, clean off any adhesive it may have left behind, then refit it to the brake shoe. But I added some extra adhesive this time; a dab of Selleys Kwik Grip® – reputedly strong enough to mend a broken heart! Anyway, it should do this job adequately. As I said at the beginning, it was no big deal to find and fix, but it surely must have been one chance in a thousand that the pad behaved as it did and finished up where it did. So I can hardly expect that this story will have anything more than novelty value; I don’t imagine that anyone else will ever encounter the same fault. Still, stranger things have hapSC pened.