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SERVICEMAN'S LOG Little things can be big time wasters Frustration & annoyance is the theme of this month’s notes. It’s about the many & varied things that can contrive to slow the job & which collectively can add up to a lot of wasted time. As most readers will appreciate, most faults that turn up on the service bench are fairly routine. Generally, they are faults that have been seen before in the same model set or faults which, by their very nature, can be recognised in any set. These are the ones that provide the bread and butter – and a smidgin of jam occasionally. The real stinkers – the ones that call for a lot of patience and electronic detective work – may make it into these notes but seldom earn much income. In between these two extremes are those which, in spite of being relatively straightforward in a technical sense, can be quite frustrating – often for all kinds of silly reasons. This is one such story and it has a very silly twist in the tail. It is about a Panasonic model 2970V TV set, a 73cm model with stereo 40  Silicon Chip sound. This set is fitted with the M15D chassis which has been very reliable. Incidently, the “D” in this type number indicates a “dead” chassis; ie, one which is isolated from the mains. There is also an M15L model, the “L” signifying a live chassis. This set is only about three years old but it had been used for only part of that time. The owner had just moved into a new home and the set has been in storage for about 15 months while he and his family were living in smaller temporary accommodation. Obviously, the storage period may have had something to do with the problem. The owner was typically vague about the nature of the fault, saying only that the set was still working but that there was something funny about the picture. This was confirmed when I set it up on the bench. Yes, the set was working and, yes, there was something funny about the picture; it was suffering from severe pincushion in the east-west mode; ie, it bowed inwards on each side. This was one of the first frustrations, because it is quite a rare fault these days. Worse still, it was one I had never encountered before in this model chassis. I wasn’t quite sure where to start. Another frustration involved the complexity of the service manual. While the material in it is very well presented, the circuit and other data are spread over many sheets, making it difficult if the circuit has to be traced from one sheet to another. On the other hand, it is much better than having the circuit reduced so much that essential detail is lost. Getting back to the fault, the pincushion circuitry is on a separate board – designated the K board – and this same module is used in several models. Initially, I was unable to find the circuit and wasted a lot of time searching for it. I eventually ran it to earth in the section for the M15L chassis (of course – where else would it be?). It is reproduced herewith and at least I don’t have to apologise for the quality. But this only moved me on to the next stage of frustration. The circuit did not carry any waveforms or even any voltages on the transistors. I did eventually track down the voltage data – on yet another sheet – but not before I had wasted more time trying to rationalise the voltages as I found them. I also searched through the manual for any explanation as to how the pincushion circuit functioned, but in vain – I was on my own. So, all in all, I wasted a good deal of time before I even started. The K board The K board is about 150mm long by 100mm wide and slides vertically between two rails mounted on the right-hand side of the cabinet (as seen from the back). It connects to the rest of the set via two plug and socket assemblies, K1 and K2, on leads long enough to allow the board to be removed and worked on while still in circuit. This, at least, was a plus. Pin 1 of K2 connects to the 113V main HT rail, while pin 2 carries vertical pulses. These were traced back to a network connected to pin 2 of IC401, the vertical output chip. Pin 3 has no connection and pin 4 goes to chassis. Plug K1 connects directly to the horizontal scan coils via pin 1 (H-) and pin 3 (H+). Pin 4 connects to chassis. The hori­zontal amplitude here is quite substantial. The lower part of the circuit shows three transistors: Q701, Q702 and Q703. Q703 is the first one in the chain and is fed with vertical pulses from pin 2 of plug K2. The output from Q703, at its collector, goes via the pincushion Fig.1: the K-board (pincushion) circuitry for the Pana­sonic TC-2970V. Vertical pulses come in on pin 2 of plug K2 & are fed to the base of Q703 (lower right). Q703 drives the pin­cushion control, the output of which then drives Q702 & Q701. Finally, Q701’s output is coupled to the horizontal scan coil circuitry to provide the necessary pincushion correction. Note the absence of waveforms and transistor voltages. control to the base of Q702 which, in turn, drives Q701. Q701 is a power transistor (TO66 package) and is mounted on a heatsink. Its output is coupled into the horizontal scan coil circuitry to provide the necessary pincushion correction. Waveform checks I tried checking various waveforms, hoping I might find an obvious dis­ crepency, but without success. The vertical pulses appeared to be making their way through the chain OK but, without any waveforms for reference, I had no way of knowing whether the amplitude and waveform shape were correct at every stage. The only hint was that the gain of Q703 was not what I would have expected from a superficial assessment of its configuration. But then, I couldn’t be sure. Also shown in this part of the circuit is width control R708 (5kΩ) and pincushion control R710 (20kΩ). I tried adjusting the width control and this behaved as expected; it varied the width and nothing else. But when I tried the pincushion control, August 1993  41 circuit. But no, it was spot on value. Next, I lifted C707 and measured it. It was down to around 700µF, which made it bad enough to need replacing, even if it wasn’t the main fault. And it wasn’t, because a new one made little difference. My next stop was C708 (47µF) and this was where I struck oil; it was extremely leaky, which could easily account for the weird voltages and the failure of the pincushion circuit. And it did, because a new one immediately cured the pincushion problem. Having located the fault, I checked the voltages again, more or less as a matter of routine. I didn’t refer to the manual list this time, having memorised the values well enough – I thought – to satisfy such a check. And so it seemed; I measured 1.85V on the emitter, 2.5V on the base, and 9.3V on the collec­tor, near enough to the figures I recalled from the manual. SERVICEMAN'S LOG – CTD The final twist it behaved in a less logical fashion; it also changed the width and nothing else! That suggested that the fault was in this section of the circuit. Transistor checks My next step, was to check all three transistors but, as far as I could determine, all were OK. I had not at this stage unearthed any voltage data for these transistors but I made a few voltage measurements anyway, hoping that they might provide a clue. And they did, in a way. The voltages on Q701 and Q702 at least seemed reasonable, by rule-of-thumb guess­ timation. But Q703 was another matter; unless it was being used in a very unusual way, I couldn’t make any sense of it. I measured 17V on the collector, 17V plus on the emitter and 17V on the base. But, while this didn’t make much sense, it did remind me of the appar­ent low gain of this stage. But what should the voltages be? I found them listed quite by chance when, as so often happens, I was 42  Silicon Chip searching the manual for something else. At a quick glance I registered that those for Q703 were not only nothing like the values I had measured but seemed to be much more reasonable. All of which simply confirmed my idea that whatever was wrong was in the immediate vicinity of Q703, the transistor itself having already been cleared. There aren’t many components directly associated with Q703. I started with R712, thinking it might be open COMMON TEST POINT VOLTAGES E B C Q701 0.025 0.62 13.8 Q702 11.9 11.3 0.62 Q703 1.85 9.3 2.5 Q802 0 0.01 16.5 Fig.2: this is the relevant portion of the transistor voltage table from the Panasonic TC-2970V manual. The collector & base vol­tages shown for Q703 are transposed. And so, after a routine check and adjustment, the set was duly returned to the customer, putting an end to my time-wasting frustrations. Or so I imagined. My final frustration came as a nasty twist when I later took a second look at the voltage table in the manual. It was then I suddenly realised that the voltages were not listed as I had recalled them. Oh, the values were correct but not the transistor connections. The manual listed them as 1.85V on the emitter, 9.3V on the base and 2.5V on the collector. I did a double take on that. Those figures did not make sense and, had I been more observant, I would have realised this when I first saw the table. Instead, I read them as I imagined they would be, rather than as they were. The point about these figures is that – apart from anything else – they imply a base-emitter voltage of around 7.5V – an impossible condition according to my understanding of solid state theory. When I went to (solid state) school, the maximum voltage which could normally be developed across such a junction would not exceed 0.7V, and would be more like 0.65V in practice. So what had gone wrong. My immediate reaction was to sus­pect that the figures in the manual were a mistake; that they had been wrongly set out Fig.3: this diagram shows the front-end circuitry for the High Energy Ignition System, as published in the May 1988 issue of SILICON CHIP. The constructor’s problems were at the very front of the circuit. with the base and collector values trans­posed. I spent a lot of time, on and off, thinking about the problem and the longer I thought about it, the more convinced I became that the manual was wrong. Note particularly that, if we transpose the base and collector values as given in the manu­al, we then have 0.65V across the base/ emitter junction, exactly according to the rules. Finally, at the first opportunity, I rang my colleague in the Panasonic service department and put the problem to him. It didn’t take him long to fetch the manual and look up the circuit and chart. His reply was brief, to the point: “Ah yes, a typo” (typographical error). Anyway, that was the end of story as far as the various problems and frustrations were concerned, But I do suggest that anyone who is likely to be dealing with the M15 chassis, or the manual, make a note of the mistake. Finally, I do have some other comments on the fault itself. While electrolytic capacitor failures are not un­usual, I was surprised that one as large as C708 should deteriorate to this extent in only a few years. The fact that the set was stored for so long may have been a factor, although it should not have been. And what about C707? This, I think, might have been a vic­tim. Rated at only 6.3VW, it had about 17V applied across it while ever the fault was present. The wonder is that it didn’t break down completely. In addition, there is another electrolytic capacitor in this part of the circuit – C716 (10µF 50VW). This was also checked and was found to be down to about 5µF. It was replaced along with C707 and C708. While on the subject of electrolytics, I find that if one reads lower than its rated capacitance, by even a small amount, it is time to replace it. New capacitors invariably measure higher than their marked value. If they drop below that figure, they are generally on the way out. Kit projects To change the scene, but still on the subject of frustrat­ing situations, I am reproducing a letter from a reader, Mr R. S. of North Melbourne, Victoria. It is not a servicing story in the usual sense, nor was it a particularly profound exercise, but it is an excellent example of the problems which can arise from the supposedly simple job of building a kit project. Assuming a well-designed project and a properly prepared kit, it is reasonable to expect that it will work at first switch-on (provided, of course, that the kit has been correctly assembled). But it doesn’t always happen that way. And when it doesn’t, kit builders react in a variety of ways. Some simply regard the design as a bomb, curse the designer, and chuck the whole thing in the garbage bin. Some strip it down and rebuild it; a time wasting and usually futile procedure. One enthusiast, in the days of build-your-own TV sets, stripped down and rebuilt a complete 17-inch TV set, in an attempt to cure a relatively simple fault – the pic­ture was transposed left to right. More enlightened souls, like our reader, assume that the design is capable of working and that its failure must be due to a construction fault –which is usually the case. They then set about finding it in a methodical way. Just how simple some of these faults can be is shown in this example. Here’s how he tells it. The night before Christmas Some months ago, my son-in-law to be raised the question of fitting an electronic ignition system to his motor vehicle. Because I had built a number of CDI (Capacitor Discharge Igni­tion) and TAI (Transistor Assisted Ignition) units, I was consid­ ered a suitable consultant. Although very pleased with the performance of all units tested, I have never been able to detect either an increase in fuel economy or engine power. Perhaps this is because I always cleaned and adjusted the ignition system on a regular basis – about every 3000km. What I have noticed is that the electronic systems require virtually no manitenance or adjustment, unless I disassemble the distributor for some other purpose. Since CDI is currently out and TAI is in, the choice was simple. I did not experience any crossfire with CDI but the inverter squeal could be objectionable. My last TAI circuit is dated at 1982 but the “High Energy Ignition System” unit produced by SILICON CHIP in May, 1988 was available in kit form. And, as his training was in the field of electronics, a kit was purchased and he assembled the unit. A few days before his initiation to son-in-law, he invited me to install the TAI in the vehicle, as he knew that I would be more familiar with the automotive side of things (self-trained also). However, I put this off while he August 1993  43 SERVICEMAN'S LOG – CTD was being moulded into married life during the next two weeks and I waited for his return. A telephone call was subsequently received on December 23rd and arrangements were made to perform the change over on the night before Christmas. He had already mounted the box in the engine bay and all that should have been necessary was for me to find a suitable 12V supply and make the appropriate connections to the ignition system. This particular vehicle incorporates the ballast resistance in the loom but I was able to find a suitable power supply con­nection at the fuse panel and run a wire to the unit. Laying out the wiring to the coil in a secure fashion came next. Suitable checking took place and I felt that we were ready for the all-im­portant smoke test. Switching the ignition to the run position produced no problems but switching to the start position failed to produce any fire in the engine. This enabled me to demonstrate how easy it was to revert to the old faithful 44  Silicon Chip Kettering system, if it was required. We had begun the work in the open and in dry conditions, but by now a light rain had become a heavy downpour and daylight had vanished, so we called it a night. No smart comments about Melbourne’s weather thanks; we aren’t overjoyed either! The next day, my daughter rang and invited me to lunch with the family. No great arm twisting was necessary, as it would give me the opportunity for further fault finding. After a pleasant meal, off came the lid and the investigation began. I was expect­ing to find a fault with D5 but it still behaved as a diode and was oriented correctly. Transposed resistors The resistors were checked next and it was found that the 22kΩ resistor to the input (pin 5) of IC1 was 2.2kΩ. A search was made for a suitable resistor and we were able to find two 10kΩ units. My son-in-law has a very limited stock of components, as electronics is not his hobby. I was concerned that the additional current through the zener diode in IC1 might be too much for it. A re-test took place but the engine did not start. At this point, it was not noticed that the 22kΩ resistor which should have wired into the input of IC1 had been placed in the collector circuit of Q2. In other words, the 2.2kΩ and 22kΩ resistors had been transposed. The unit was removed from the vehicle and, because there was the possibility of serious damage to IC1, I decided to take the unit with me for further testing and repair. The 22kΩ resis­ tor problem was corrected the next day. I then proved that Q1 was intact and with the aid of a spare coil connected, produced a spark when the output of IC1 was taken to chassis using a jumper lead. Since waveforms were going to be traced, the CRO was fired up. This was an overkill, as will be seen. The action of the points opening and closing was mimicked by connecting a flying lead to the power supply chassis. This produced a step at the 47Ω resistor, as was expected. Unfortunately, when the CRO probe was transferred to the other side of diode D5, the pulse vanished. On turning the board over to the track side, it came to my attention that there were more tracks and holes than were necessary for this project (as a last resort read the text fully). The constructor had placed the anode lead of D5 into the next hole up the board. Once this correction had been made, a pulse could be traced to pin 7 of IC1. By now reconnecting the test coil and spark plug, I was able to view a nice healthy spark. When installed the next day, the system worked perfectly. The constructor did emphasise that he had performed “high reliability hand soldering” techniques as his employer instructs. And to his credit the soldering could not be faulted. But my warped sense of humour considers that suitable connectivity is required before conductivity can take place. Fair enough, R. S. and thanks for the story. It emphasises one very important point – the difference between field servic­ing, where a device originally worked but has now failed, and production line servicing, where the device has never worked. Production line servicing is a completely different SC ball­game.