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SERVICEMAN'S LOG Not every write-off is written off It is one thing to accept that a set is a writeoff; genuinely too expensive or impractical to repair. But if one is prepared to take a punt privately and spend some time, the reward can sometimes be worthwhile. My first story is not about a writeoff but is about an NEC FS6330 TV set belonging to one of my regular customers, Terry Ford. He brought it in complaining of no colour which look­ed like a straightforward problem, except that it didn’t turn out that way. And he was right – at switch-on, there was no colour. Pressing “Picture” on the RC1073E remote control and pushing “control ^” (up) put the colour back on but the control had to be set almost fully up. And if the set was switched to standby and then switched back on again, the adjustment had to be redone. To overcome this, I brought up the standard picture-on-screen display settings. This showed that the colour had been turned right down. However, after resetting it and switching off the Standard PST switch, I found it didn’t hold its setting. After mucking about with the controls for some time, I came to the conclusion that the set wasn’t memorising anything and this included the tuning. Whatever the set had been programmed with when it came in, nothing I could do would permanently alter the settings. When I looked at the circuit for the PWC-3518A CPU module, there is an IC1002 marked “MEMORY” and I felt sure that this was the problem. I started by removing the module and checking and soldering any suspect joints I could find. There were none worth writing home about, so I did the same with the motherboard (PWC 3517). Unfortunately, this made no difference to the memory prob­lem, so 74  Silicon Chip I checked the voltages on the IC and the microprocessor to find them all OK. I also replaced (C1021) 100µF to the oscil­lator but still wasn’t getting anywhere. Next, I ordered a new memory IC (CXK1006L) and fitted it as soon as it arrived. This time I had a new range of problems because the IC wasn’t programmed for anything. However, I was still unable to store any instructions permanently. I checked all the small components around IC1002 and they were all OK, so was it the main microprocessor? At this stage, I confess I took the easy course and phoned technical support at NEC. The technical officer asked me what was the first display I saw when switching on. This caught me a little by surprise as I hadn’t really noticed but the next time I did switch it on from cold, it went straight to the external AV mode with “V2” displayed on the screen. The technical officer told me that this almost certainly meant the microprocessor was faulty and should be replaced. He also advised me to fit three 5.6V zener protection diodes on the three data rails (PA1, PA2 and PA3) which go to pins 7, 6, and 5 respectively of IC1001. This I did, after installing the expensive CXP80420-130S with its 64 high-density pins. Fortunately I found a position already drilled, punched and marked FD1005 to fit these zener diodes - somewhat similar to the 6.8V zener diodes in FD1003. And that fixed the problem. The set now tuned and stored all its settings correctly each time. I was intrigued as to whether it was just the CPU or the memory IC as well, so I unsol­dered the latter, fitted a socket and plugged in the old memory chip. The original problem was still here, so obviously both the ICs had been destroyed. I left the set on test, fully confident that it was all fixed - but it wasn’t. After a while, it began giving a “bluey/ yellowy” sort of colour which I recognised as indicating U(B-Y) only; ie, loss of V(R-Y). I thought initially that this may have been due to an incorrect system setting (eg, NTSC) but it wasn’t. I went back in and finally found the V(R-Y) loss was due to faulty joints on IC701, the chroma/ jungle IC, which is another high-density 32-pin device. The owner, in the meantime, had been popping in and asking about his set on a regular basis. He had become somewhat dismayed at the length of time and different courses the saga was taking, especially as I had so confidently assured him that it was just the memory IC that was the trouble. However, he stuck it out and hopefully won’t have any more problems. A large-screen Telefunken My next story concerns a large screen stereo TV set. This was a 68cm Telefunken SDX290H employing a Thomson ICC7 (or more precisely ICC7000+) chassis. This set had been struck by lightn­ing and had no sound or picture. I ordered a circuit for it and received a photocopy of the basic ICC7 circuit, which covered Nordmende, Saba, Telefunken and Thomson models. From this, it didn’t take long to work out that the problem lay in a complex AV module MAV7000 on the rear of the set. This was blocking all the signals from the tuner/IF system, or any other external signals, from reaching the jungle IC IV01 and the rest of the set. Unfortunately, the circuit contained no reference to this AV module. This set can select no less than four different AV connections: two rear RCA/ Phono, one front RCA/Phono, three SVHS DIN and 1 Scart (both input and output). There are also external DIN and wired loudspeaker connections. We ordered the missing circuit and were lucky to receive one but, much to my frustration, this was also incomplete! More precisely, it did not include a daughter board MOM7000 which is connected to the AV module via nine leads. This daughter board contains an IC (ICIM01 HCF4053BE) and various peripheral compon­ents. After exhausting every avenue right back to Thomson in Europe, the missing circuit proved to be unobtainable - and all this had taken three months or more to determine. The module was also unobtainable as a spare part and even if it had been, it would have been horrendously expensive. In any case, I was unsure as to whether there were any other problems with the set. By now, the owner of the set had lost patience and so the insurance company decided to write it off. However, it was an attractive unit, built in 1993 and boasting Teletext and a sub­woofer. By turning up the picture tube screen control, I could see there was a raster but that was all. And so, rather than let it be broken up for spare parts, I bought it and took it home to fix in my spare time (huh!) - much to my wife’s disgust (I alrea­dy have enough junk). When I finally tackled it, the first thing I did was to remove the module and try to make sense of it. I started by drawing out a simplified layout diagram of the ICs and especially concentrated on the daughter module, MOM7000. The surface mounted components on the double sided printed circuit board didn’t help much but I finally constructed a layout diagram that I could marry in with the circuits I had. In retro­spect, the final result didn’t look much but it helped enormously in tracing the signal routes through the board. Even so, the route the signal takes is rather tortuous. The tuner/IF signal comes in as CVBS1 on connector BEO1 pin 13 and comes out as CVBS on BEO1 pin 10 via IV03 TEA2014A, AUGUST 1999  75 IV02 HA118058 and IV01 TA8639P. However, without block diagrams of some of these ICs, it is impossible to understand the processing and switching that goes on internally. However, I was extremely lucky when I made a voltage check across the module, because there was no +13Vcc available any­ w here. This turned out to be due to a 3.9Ω resistor (RE22), which was open circuit. Replacing this restored the sound and picture from the tuner/IF system but not the AV inputs and outputs. After a lot of time spent measuring and replacing many components, I finally traced the problem to the daughter board. A BC548 transistor (TM10) was open circuit and the IC IM01 (HCF4053BE) had failed. Replacing these still didn’t fix the problem until I found that a part of the printed circuit, the audio com- 76  Silicon Chip mon return to connector BE01 pin 3, had been vaporised. Repairing this fixed the monitor output but most of the inputs and outputs were still not func­tioning fully. As luck would have it, replacing two TEA­2014A ICs fixed all the remaining problems. It appeared that these two ICs had jammed in one mode. The “write-off now has pride of place in my lounge room. And my wife has (fortunately) reconsidered her opinion that the set was “junk”. Making a 22.5V battery In the December 1998 notes, I described how I discovered a faulty silicon diode with reverse leakage. And although replacing the faulty diode solved the problem in the set concerned, I had been puzzled as to why the leakage did not show on test. A variety of multimeters and compo- nent analysers had been tried and all but one failed to detect it. The only one that did pick it was an old DSE Peak (Hokia) AS100D 100kΩ/V unit. Significantly, this uses a 22.5V battery for the resistance ranges and this was most certainly the reason that it revealed the fault – the diode leakage was voltage sensitive. And this highlighted another problem – where to get replace­ment batteries for there old meters. Unfortunately, 22.5V batter­ies – relics of the valve era – are no longer made. So what can be used to replace them and keep a valuable piece of test equip­ ment operational? It was while I was cogitating thus that an amateur friend turned up with a similar problem and in the process, suggested a solution. In his case, an even older multimeter was involved - the English-made AVO8. This was regarded as the “cream” of multi­ meters in its day and is still highly valued by its owner. This particular meter uses a 15V battery, once readily available in several forms but now quite rare. The last time it needed replacing, he used a Varta V74PX photographic type but that was very hard to find and very expensive. This time my friend took a different approach. The most readily available battery now is the 9V portable radio type, such as a 216, S3282, or similar. Two of these would provide 18V and the problem was to reduce this to 15V. Series resistance was obviously not a solution. Instead, but my friend’s idea was to add a 3V zener diode in series with the batteries. In theory, this should develop a constant 3V across it, regardless of current drain. However, he did have some reservations about zener behaviour at the very low current drain involved (µA rather than mA). As it turned out, these reserva­ tions were justified – the zener value was no longer accurate. Nevertheless, he tried it and eventually finished up with a 3.3V zener and a 1N4148 silicon diode in series. And this worked very well. When he tested the unit using several close tolerance resistors, covering a wide range of values, the results were as close as could be expected from a service type ohmmeter. His reservations regarding the zener behaviour involved the characteristic curve at the knee; a gentle curve com- pared with the much sharper curve of an ordinary silicon diode. In fact, a string of five diodes might be theoretically more accurate. What about that 22.5V battery situation? One possibility is to use three 9V batteries in series with a 4.5V zener/silicon diode combination. I haven’t tried it but I can’t see why it wouldn’t work. The missing tube To finish off this month, here is a story, in lighter vein, from a colleague who is prepared to swear on a stack of service manuals that it is true. I’ll let him tell the story in his own words: A few weeks ago a customer, an elderly gent, appeared at the door of the shop. He was juggling a portable colour TV set at a dangerous angle on his knee, against the door jam, while he struggled with the door knob. I hurried to the door, opened it gently, took the set and placed it on the counter. The set turned out to be a 34cm AWA model. “So, what’s the problem?”, I asked. “Well, there’s only a thin white vertical line down the middle of the screen”, he replied. “The sound is OK”. On the basis of this description, the problem appeared to be fairly straightforward. The presence of an image on the screen meant that the EHT system was obviously working, so the lack of horizontal deflection could only mean failure of the horizontal yoke, or a connection to it. “OK, leave it with me and I’ll have look at it”, I replied. He hesitated. “Er-ah; I was wondering if you could have look at it now? It’s the one in the kitchen and my wife likes to watch it while she’s working.” I looked at the pile of work on the bench and considered how far I was behind – as one usually is. Still, I could perhaps stretch a point. Hopefully it should not take too long; possibly just few minutes. (Yes, I know - these are the ones that can turn sour). “Oh well, I guess so. Take a seat and I’ll have a look at it.” I took the set into the workshop and took the cover off. And for once I was right; there it was – a bad connection to the yoke socket. A few moments with the soldering iron was all that was needed. I turned the set on and it sprang into life. I put the cover back on and took the set out to the counter. He paid me and I took the set out and put it on the back set of his car, placing it face down, as I normally do, to keep the centre of gravity low so that it would travel safely. I waved him off and went back to the bench. Half an hour later, he was on the phone. “Sorry to bother you but have you got the screen there in the shop?” I was con­fused momentarily, then realised that he probably meant one of those anti-glare screens which some people fit to their sets. I didn’t remember there being one on this set but I could have been wrong. “Er! – No; Hang on . . . I’ll check . . .” I looked around on the workbench and counter but there was no sign of it. He’d probably left it at home. “Sorry, But I don’t see it here. Perhaps you left it in the car.” He said he would have a look. About 20 minutes later, he appeared at the door. I suggest­ed he look over my bench and counter. But there was no sign of it anywhere in the shop or on the street outside. I was sure he had left it at home somewhere. Anyway; back to work. Some time later he was on the phone again. “I can’t find it anywhere. All I can find is the ribbing”. I’m afraid the last part of that remark went over my head, “Oh well, sorry about that. Anyway, how is the picture?” “I don’t know; there isn’t any screen”, he replied. Suddenly, something dropped. It wasn’t “the penny”, because it was still a mystery, but I was jolted into realising that there was something wrong. “You had better bring the set back to the shop. When you arrive, come to the door and I’ll get the set out of the car for you, so you don’t have to struggle across the road with it”. He arrived a few minutes later and I went out and collected the set, carrying it face down as I had put it in the car origi­nally. I took it into the shop and placed it on the counter. As I did so he said, “I have looked everywhere. In the car, the path, the house; I can’t see any glass anywhere”. As I tilted the set up from its facedown position, I said, “The only place where I can see glass is there!” – this while I pointed to the picture tube. His face was a picture. “Oh dear, I’m so embarrassed. What’ll I tell my wife?” There was a funny side to it of course but it was not a time for laughter; I felt so uncomfortable for him. And an ironic twist was that he used to work on radar surveillance in the Catalinas during the war. But that was a long time ago. I tried my best to make light of it. “Don’t tell her”, I said, “Just tell her I found it and it’s all fixed up.” SC And that’s how we left it. AUGUST 1999  77