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SERVICEMAN'S LOG A bounce with a twist No, that’s not a new section in an Olympic Games diving competition. Perhaps I should have called it the bounce that wasn’t. Anyway, we all know what a bounce means in servicing parlance, and that’s the theme. I’ve been talking a lot about bounces in recent notes - and the trauma and acrimony they can cause. Well this story had all the makings of a bounce situation, except for one factor; the time between failures. I wonder what the record is? Anyway, to start at the beginning. The set involved was a General GC187, 43cm colour set of around 1984 vintage and was one of several belonging to a 40  Silicon Chip local motel. It is a rather elementary type of set by modern standards, without any remote control or other up-market gimmicks encountered in later models. The first time I serviced this particular set was back in 1993 and the complaint at that time was that the image was very dark. Because all sorts of funny things - technical things, that is - go on in motels I find it best to view such problems in situ. Damaged antenna outlets, faulty antenna distribution systems, even faulty power points, are all possibilities in such installations. In fact, it didn’t take long to eliminate all these and confirm that yes, the set was faulty. But in spite of having serviced most of these sets in the motel over the years - and some from other customers - I had never seen this symptom before. So it was into the van and back to the workshop. As a general rule, faults of this kind suggest a low or missing voltage around the picture tube and immediate circuitry. My first check was to the picture tube screen (pin 8), which was something over 500V with the screen control as set. I decided that this was a perfectly reasonable figure. The EHT voltage is shown as 22kV and this was, if anything, a fraction high. At a more basic level I checked the supply rails. There are four altogether, +175V off pin 9 of the horizontal output transformer, T602, +127V from the switchmode supply (test point TP601) and two low voltage rails, one at +13.8V and another at +12V. All checked out as specified. That routine completed, it was time to look for something more specific. And the first thing I checked was the collector voltages on the red, green and blue driver transistors, Q201, Q202 and Q203. These are shown as ranging from +105V to +108V but in fact were much higher, over +150V. Which meant, of course, that the picture tube cathodes were similarly too high; around 50V more positive than they should have been. Which is only another way of saying that the respective picture tube grids were 50V more negative than they should have been. No wonder the picture was dark. OK, we were on the track. But why? The fact that this error was occurring on all three drive transistors suggested a common cause and the most likely Fig.1: General GC187. IC301 is at the top, with the sub-brightness and brightness controls below it, and D360 and resistors R624, R640 below again and to the left. One of the drive transistors (red) is at lower right. one would be something associated with the brightness circuit. The three drive transistors are driven from pins 26, 27 and 28 of IC301, described as the video amp/ PAL processor. And the brightness circuit connects to pin 4 of this IC and consists of a sub-brightness control VR304 (10kΩ) and the main brightness control, VR709 (5kΩ). And this leads back to a network, via a diode, D360, consisting of resistors R624 (560kΩ) and R640 (120kΩ) in parallel, connected to the 127V rail. More importantly, the voltage at pin 4, shown as 8.4V, was down significantly. (I can’t remember by exactly how much after all this time but it was significant.) I tried adjusting the sub-brightness and brightness controls but this had only a marginal effect; enough to indicate that they seemed to be working, within the constraints of the fault. I checked diode D360, which was OK, then resistors R624 and R640. And this looked like the answer because R640 had gone high. I replaced it and R624 at the same time, just to be on the safe side. Unfortunately, it wasn’t the real answer; it helped but it didn’t cure the fault. In fact, it made me aware of another fault; the colour was dropping in and out intermittently. I made a few more voltage checks but could find nothing wrong. It was time to check the dynamic aspects of the system; blanking pulses and such like. My first check was on a horizontal pulse to pin 23 of IC301, via a 1kΩ resistor, R327. This was shown as a typical triangular pulse at 3.2Vp-p. And it was; exactly. Next, I checked pin 19. This is shown as a similar pulse at 2.6Vp-p. Only it wasn’t; this waveform was missing completely. Well, I was hot on the trail now, even though it was a rather longish one; all the way back to pin 16 of IC401, the horizontal and vertical oscillator and drive stages. On the way it goes through a 15mH choke, L402 and a couple of resistors. On an impulse, I wiggled the choke. And bingo! Suddenly everything came good; full brightness, normal colour and a first class picture. I didn’t waste time finding out what was wrong with the choke; I reefed it out and fitted a new one. Everything came good again, I let it run for the next couple of days, with no sign of trouble and returned it to the customer. And that was the end of the story. Well, for 1993. But a few weeks ago the motel proprietor was on the phone asking me to have a look at a set. And what was September 1996  41 wrong with it? “Aw, the picture’s gone dark.” And yes, it was the same set, with the same fault, three years later. However, I must confess to stretching things a bit when I imply that this was a “bounce” in the normal servicing sense of the word. The truth is that I was the only one to appreciate the situation. The proprietor had completely forgotten that this was the same set with the same symptoms of three years previously. So there was no aggro of any kind; just a funny feeling on my part as to what might have been. Anyway, down to business. I visited the motel again and yes, at switch-on, the picture was very dark. Instinctively I reached for the brightness control and gave it a tweak. And up came the picture to normal brightness. What was more, the set seemed to be behaving perfectly normally. The brightness control setting was quite reasonable and the range of control was normal. So what did this mean? There was a temptation to assume that it was simply finger trouble on the part of the last user but while I hesitated to accept that, there didn’t seem to be much point in assuming a fault on 42  Silicon Chip such rather flimsy evidence. I suggested that I leave it and for them to keep an eye on it. A couple of weeks went by and they were on the phone again; same problem. I checked it out in situ and yes, it was faulty. So it was into the van and back to the shop again. The only snag was, as soon as I set it up on the bench, it worked perfectly. In short, my worst suspicions were confirmed, it was intermittent. I set it up in “intermittent corner”; the corner of the bench which I reserve for such troublesome devices and let it run all day and every day. This proved only partially successful. The fault did occur on several occasions but by the time I attacked it, it had cured itself. But I did notice a couple of important points. It had a greater tendency to fail when first switched on in the morning and most particularly, when the weather was damp. If not touched, it would come good after about an hour. It was a most frustrating situation; one where one could waste hours of time speculating on likely causes and testing these ideas. In fact I did try a number of ideas. This model is rather notorious for dry joints and I went over all the likely ones and resoldered any which looked at all doubtful. I replaced any electrolytics which looked at all daggy. I went over the work I had done previously. I found nothing positive and in fact, it achieved nothing; the symptoms remained exactly as before. In the meantime, the motel was on my hammer wanting to know when I could finish the job and what it was going to cost. To pacify them, I voiced the only idea had in mind; a fault in the horizontal output transformer (T602) being effected by the damp weather. I quoted them for a replacement – should that prove to be the fault – and of course, it wasn’t cheap. They said they’d think about it. Which at least gave me some breathing space. So I simply let the set run from day to day, hoping for a more definite indication of the fault. And eventually it happened. We had a long bout a very wet weather and I noticed that, instead of a brief burst of the fault at switch-on - which I knew would not stand investigation - it was taking longer and longer for the fault to clear. So, finally, after the fault had remained for several hours, I moved the set out of the corner and tackled it. Thankfully, the fault held and I was able to make a quick check of the four supply rails, screen voltage on the tube and the EHT. All were spot on. Which exposed the fallacy on my faulty transformer theory. Bypassing some of the steps in my previous exercise, I went straight to pin 4 of IC301, fed by the brightness control circuit. Sure enough, the voltage was way down from the stated 8.2V. And incidentally, the fault now seemed to have worsened; it was almost impossible to get any image on the tube. I backtracked to the sub-brightness control, then to the brightness control, still measuring the very low voltage. It was only when I went the other side of the brightness control and measured the voltage applied to it from the 127V rail and the beam limiter circuit of pin 7 of T602, that I found a normal voltage. So it was being applied to the brightness pot but was not appearing on the other side of it. Time to look at the pot itself. Easier said than done, because the control panel had to come out. But sure enough, this was where I Fig.2: Sharp DV-1600X. IC701 is at top left, FB701 below it off pin 4 and C711 to the left of it. Mains power is applied to pins 1 and 3 at extreme right. found it. I had to dismantle the pot to pinpoint it and found that the voltage applied to the lug was not present on the moving arm. And there was no continuity between these two points. The reason? The centre lug of the pot, which ultimately connects to the moving arm, is riveted in place. And under and around this rivet, visible only under the jeweller’s loupe, there was faint evidence of corrosion. The result had been an intermittent break in the lug, under the rivet and effected by temperature and humidity and progressively getting worse. I didn’t have an exact replacement handy and had to put one on order. In the meantime I patched in a 10kΩ pot temporarily. Everything came good immediately. And it stayed that way. The replacement pot eventually arrived, I fitted it, put everything back together and let the set run for a few more days. It never missed a beat and was finally returned to the motel. And that’s my story of a three-year bounce. As I said earlier, it wasn’t a real bounce but the point is it could have been. Had that pot decided to play up a few weeks after the 1993 episode, producing identical symptoms and had it involved a less understanding customer, I would have been hard put to it to convince them it wasn’t the same fault. In fact, I doubt whether they would ever have believed me. So it was a near miss. Well, we must be thankful for small mercies. For a complete change of scene, I have a letter from a Mr. B. L. of Gwynneville, NSW, relating some of his DIY experiences involving faults in his own TV set. This how he tells it. “Daaada!” cried my two year old son - one second he was watching Sesame Street and the next, no more Big Bird. This was the result of a failure in our Sharp DC-1600X TV set; a complete blackout. I have to own up to not being a real serviceman, at least not by trade. However, having graduated last year with a B.E. (Comp.) I felt that, as an engineer, I would have to at least have a look to see if I could do it myself. (Do I hear you groan, “No, not another meddler!”?) Well, after opening the case I discovered the chasm between the basic principles of television transmission and the implementation of those principles. I knew roughly how an image was received, processed and displayed on a CRT but I had no idea where to start looking for faults amongst the myriad of components before me. I decided to invest in a service manual and despite this model’s manual being out of print (it’s about 13 years old), I quickly located one care of High Country Service Data in Cooma NSW. The DV-1600X has a number of service bulletins, mostly relating to the power supply. I checked these items and found a number of apparent faults. R635 (39Ω) in the return path of the horizontal deflection coil measured open circuit. This would explain the symptoms - no sound, no picture. R616 (1.5MΩ) in the protection circuit was also open circuit, which apparently causes shutdown after a period of operation. The power supply IC701’s heatsink appeared to be poorly connected to the copper pattern. I replaced the self tapping screw with a small machine screw, star washer and nut. The horizontal output transistor was similarly modified. I was surprised to find that it was mounted with self tapping screws, soldered to the board and served as a link between tracks. To quote the manual, “Power goes from one track through the metal case of the line output transistor to the next track.” Any bad connections here would spell doom. While I was dabbling with these modifications, I noticed a number of browned-out resistors in the power supply. These were summarily replaced. At this point I wondered how the set ever worked. I fired the set up and found all voltages within specs. and the set operated fine. I was very pleased that I was able to solve this problem without too much drama. My bubble burst about two weeks later. Again it was a complete failure of sound and picture. This time I started out by measuring line voltages and found nothing; not a skerrick of life. I focused on the power supply and September 1996  43 Serviceman’s Log – continued quickly came to the conclusion that IC701 was not operating. Initially, I wondered about the likelihood of a faulty IC but remembered how all too often in my university lab experiments, I mistook my own faulty design, wiring, or measurements for a faulty IC. Then I measured 20V or so at pin 4 of IC701, relative to the negative terminal of the filter capacitor, C711 (10µF, 100V). Well, pin 4 is supposed to connect to this same point, via a device named FB701. Its schematic symbol looks somewhat like a fuse. I had never come across the “FB” monicker before, so I pulled the device out of circuit to examine it more closely. Here was my suspect; I had measured an intermittent 20V across it in circuit, yet a short circuit when free of the printed board. I had a chat with a friendly tech in town, who informed me that it was a ferrite bead used for RF suppression. (Of course, FB.) He suggested replacing it with a link. That was done and all is well some four weeks later - touch wood! I really don’t know if this was the original problem. I wonder if this multiple failure issue crops up often in the serviceman’s world. Well, thanks B.L. for a very interesting story. I particularly liked your comment about the chasm between basic principles and their implementation. Very true B.L. - very true. Also note your comment about the body of the horizontal output transistor forming an electrical path and which, as you suggest, is vital. On the other hand, the mounting of the IC701 heatsink would seem to be less important. As far as I am aware, this is a purely mechanical arrangement which does not involve any electrical circuitry. Regarding the FB701, I am a little concerned at your colleague’s idea of simply eliminating it. It is not clear what form of RF suppression it was supposed to perform or how important this is. However, since the manufacturers chose to fit it - and manufacturers seldom waste money on something which is not necessary - there must have been a reason. It might be advisable to fit a replacement, even at this stage. On a more general note, I would comment that the two main filter capacitors, C711 and C715, are notorious for failure; they dry out due to the heat from IC701. High temperature types are recommended as replacements. And that power supply board is notorious for dry joints. B.L. also raises the matter of connecting mains powered test equipment, such as a CRO, to live, or semilive chassis receivers, such as this one. Unfortunately, this subject is a very complex one, too complex to discuss here. Broadly speaking, each situation has to be assessed individually, using the skill and experience of the technician involved. However, a popular approach is to use a one-to-one 240V isolating transformer, with the secondary floating. The “earthy” arrangement may then be configured in any way required. So, thanks again B.L. for an interesting story – one which has provided an opportunity to discuss a particular set and some of its problems and which may benefit other readers. SC 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