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Look before you leap "Look before you leap". That well worn adage might well be displayed in large letters on my workshop wall from now on. Even if I don't get around to actually printing it - and I probably won't - it will be etched in my memory for a long time to come. This all started when one of my customers turned up at the shop with a Sanyo colour TV set and a National NV300 video recorder. He pushed them across the counter and, in answer to my automatic question as to the nature of the problems, shrugged his shoulders and said, "They just don't work" . And I knew from the way he said it that any attempt to pursue the matter would be fruitless. Just why some customers adopt this attitude is hard to say. Some obviously feel unable to find the words to describe whatever signs or symptoms they have observed. Others, I suspect, have a more aggressive motivation; they are paying you to fix the device and don't feel that they should be obliged to contribute anything to the operation. Little do they realise that this at- ' 'tfr , ,. · . . -·· --- ~ ~ ~. :·.- (.?L1~\t,~3;}i&.'.-.. \ .1.) r=:.. ;x:r) s~i,;,.•.. . .... . ~c.: : ... . I~ \. , . ,\ . u--- ~{ ~ J m~rqi SOME. C\JSt"ON\E..R:S OtsV\OUSL'-( r, . F~L UNA~LE: TO FINO 11\E: WOR'DCS i'"O 'DE:.SC.1<.\~~ W\-\ ~-C-£.Ve:.:R CSlGNS o R S'<tl\'?T"OMS 11-\E."-f l-\P-.VE OD'SERV£t). ..... 34 SILICON CHI P titude can sometimes cost them a lot of money and inconvenience. Anyway, that's my whinge for the month; well, one of them! To get on with the story, the Sanyo TV set was routine. It was quite dead. The main reason for this was a failed horizontal output transistor which had taken out a safety resistor, both due in turn to a faulty power supply regulating circuit which had allowed excessive HT voltage to be . generated. That was knocked over in short order. But the video recorder was a different story. I set it up on the bench, plugged it into a monitor receiver, pushed in a test tape and pressed the play button. Nothing happened so I suppose that the customer could claim that he was right after all. The reason for the lack of action wasn't hard to find; the tyre on the play idler wheel (VXP0433) had " had it". So it looked like another routine job. Only it wasn't. While I have things set up like this I find it is a good idea to make as many tests as possible. It is surprising how often some other fault or symptom appears which one might otherwise overlook, at least initially. The first thing I did was operate the VTR/TV selector to feed the recorder's tuner into the monitor . This should normally have brought up the selected channel. And that was the first indication that there was something really wrong. All that came up was the sound, plus what appeared to be a blank video carrier; ie, a mid-grey screen, free from noise. My first reaction was to suspect that someone had been "mucking about" with the tuner presets and had detuned this channel. But a check on all the other channels produced exactly the same result, which seemed to rule out the "mucking about" theory but did not nee- ANTENNA ~~ RF IN [:] RF OUT RF OUT SPUTTER/ RF BOOSTER IN RF OUT VIDEO VCR TUNER MODULATOR VIDEO IN IF, DEMODULATION, LUMINANCE AND CHROMINANCE PROCESSING, ETC. SOUND SOUND IN SOUND VIDEO OUT OUT VIDEO PROCESSING FOR RECORDING. RECORD/PLAYBACK SWITCHING ETC. VIDEO HEADS SOUND HEAD Fig.1: block diagram showing the signal paths involved in a V~R/TV set . combination. Note that a VCR is virtually a complete TV set mmus th~ p1ch~re tube and associated deflection circuitry. For those a little rusty on this sub1ect, it may help in following the story (see text). essarily rule out a tuner fault. Another possibility was a faulty splitter amplifier, or antenna booster as it is designated on the circuit. These often suffer as a result of electrical storms and we have certainly had several of these recently. To check this I switched off the recorder via its own on/off switch but with it still powered up from the power point. If the splitter amplifier was functioning correctly, this would allow the TV receiver to function in the normal way and bring in all available channels via its own tuner. It did this, thus clearing the splitter amplifier. Well, that was one small step ahead. This still left the possibility of a faulty tuner or, more likely, a faulty modulator (designated RF converter in this unit). These are also susceptible to damage from electrical storms. So how best to check these theories? Mechanical problem I decided that this was as good a time as any to tackle the mechanical problem in the recorder. Not only would this give me time to think but, with this much of the machine functioning , I would be able to make a few more tests. So the faulty pulley was replaced, along with its mate, the idler unit (VXP0432). Then the heads, guide posts, etc were cleaned and the deck given a general once over. There were no hitches and I soon had the deck running and able to play a known good tape. But the result from the tape was the same as from the tuner; sound OK but only a blank carrier in place of the picture. Well, I reckoned that settled it as far as the tuner was concerned; it appeared to be in the clear. And, by the same token, it made the modulator the prime suspect. I normally carry one of these in stock. As I mentioned earlier, they are prone to lightning damage and I usually replace two or three during the stormy season. But I had none now, so I decided I should order one anyway, even allowing for the remote possibility that this was not the fault (although I was sure il was). The circuit setup At this point it might be a good idea to take a closer look at the broad picture of any video recorder/TV receiver setup. Fig. 1 shows the block diagram of a typical arrangement. It doesn't apply to any particular combination but, in a sense, applies to them all. Individual devices perform the various functions, such as switching, mixing, etc, in different ways but the functions themselves are essentially the same. The antenna is no longer coupled directly to the TV receiver but to the video recorder instead. Here the signal is split, half going to the TV set and half to the recorder's tuner. And to overcome the splitting losses involved, the signal is also amplified. From the recorder's tuner the signal goes to what is almost another TV set in its own right. Included in the VCR circuitry is an IF system, demodulator, luminance and chrominance processing circuits and a sync separator - in fact everything necessary to produce complete video and sound signals, albeit at a modest level. All that is missing is the picture tube, its associated deflection circuitry and the audio system. Instead, the video and audio signals are passed on to the recording processing circuits and recorded on the tape. At the same time, these signals ar.e fed to the modulator, which is really a miniature TV transmitter delivering a complete video and sound modulated carrier to a blank channel on the TV set. A choice of two output channels is normally provided, typically channels O and 1, or 3 and 4. Thus the TV set can be fed from the recorder 's tuner instead of its own. Some users take advantage of this fact to receive UHF signals on TV sets that don't have UHF tuners. In the replay mode the circuits are switched or mixed in such a way that signals from the tape modulate the carrier, and appear on the TV screen via the aforementioned blank channel. There are also various additional features, such as video and audio input and output sockets etc, for use with external gear such as cameras, other recorders etc. Now I know that my fellow servicemen may be muttering that all this is common knowledge, so why go over it. Well it is to us but not necessarily to all our readers, keen and experienced though they may be in other phases of electronics. So I thought that this was a good opportunity to bring everyone up to date and, at the same time, make this story a little easier to follow. Where was I? So where was I before I interrupted myself? Ah yes; I suspected JUNE 1989 35 2SC2206 which acts as a buffer stage (see Fig.2). There was no video here either. This stage is fed from another bufthe modulator and planned to order But the worst blow was the fact fer stage, Q3017 (2SB641), via a a replacement. Based on the above that I still had to find the fault. And 3-terminal phase compensating cirdiscussion, readers may be able to I suspected that it was going to in- cuit, FL3003. In turn, Q3017 is fed volve a fair amount of circuit tracfollow my reasoning. Since I was from pin 19 of IC3003 (AN6327). ing, both on paper and in the unit. unable to get a video modulated Note that pin 19 is fed from a switThe circuit is, of necessity, a comsignal out of the modulator from ching circuit which selects either plex document. It is spread out over two separate sources (ie, the tuner playback or record mode. many pages, most of them being or the video heads on playback), it I decided to go straight to pin 19. three or four page foldouts, and just seemed most likely that the If there was no video here, I was in modulator, being the common com- mating them mentally can require real trouble. But there was and, acquite an effort. ponent, was at fault. cording to the accompanying The logical place to start was the So the modulator was ordered waveform, it was right up to video input to the modulator where and arrived in a couple of days. Fitscratch. The same applied to the presumably, the video wasn't gain~ ting it was quite simple. There is a base of Q3017, which is virtually in. There is far too much circuitry 5-pin plug and socket carrying a 9V the same point. So where was the involved to reproduce here, apart supply rail, video, audio and video going down the drain? from the vital section, but I may chassis connections, an RCA plug Well, it was somewhere between quote some references for those of coupling it to the splitter amplifier, the base of Q3017 and the emitter my colleagues who have the and a couple of plastic clips to hold of Q3018. I made a voltage check of it in place. In a few minutes I was NV-300EN and NV340EM manuals. both transistors and, while Q3017 I trust other readers can follow the ready to go. was almost spot on, Q3018 was general idea from the text. No prizes; it didn't work. Every"funny" to say the least. I pulled it thing was exactly as before which out and checked it and it seemed was was a bit of blow and in more No video OK. And while it was out I checked ways than one. It was a blow to my The CRO confirmed that there for video at pin 3 of the phase comego for a start; I had been so sure was no video at this point (pin 3, pensating filter. No joy. that I knew what was wrong that I Video In, of modulator ENC17502 The same applied to the input of hadn't even bothered to make any page 3-15). This pin mates with pi~ this filter, pin 1, but there was norchecks to confirm the diagnosis. I'd 3, Video Out, of plug BJ7004 on the mal video on the other side of the leapt before I looked. left of the luminance and chromlkn resistor feeding this pin, which inance circuit (page 3-9). And nearis also the emitter of Q3017. Open ly two thirds of a metre away, on circuit lkO resistor? No, it was the right hand side of the circuit, is intact. the video source which is supposed Faulty filter unit? A resistance to feed it; the emitter of Q3018, a check revealed almost zero resistance between pins 1 and 3 and an open circuit from either of these to pin 2 (chassis). Since it is virtually a sealed package I could only guess as to its circuitry but I had found ;1(;, nothing to suggest that it was the Ty culprit. Nevertheless I decided to ~ disconnect it whereupon I I'/) l/ I had video at both ends of <" I ~ ~ the lkQ resistor. 1....- I -ffe. f. So what if I bypassed the C, f filter and connected the / emitter of Q3017 to the erbase of Q3018? Maybe the picture would be a bit z onkus but I should get something. i Only I didn't; there wasn't a sign of z anything on the monitor. z <: I went back to Q3018 and checked the voltages again. As I had ~ 11-100 (:, -i already observed, they were out ~\S WAS A GO i and I had initially suspected that Of'P0~UN\i"f TO __ . ~ this was due to a fault in the filter. 2. :BR.\NG E:V~'-fONE ~ j{M ... z.. Ut=> OA-f'~ooo J ✓ z SERVICEMAN'S LOG -CTD i/1// < ... so :c ,O / • ~ ~~<Z-r Fig.2: the relevant portion of the NV300 VCR circuit, showing the two buffer transistors (Q3017 and Q3018) and the phase compensator (FL3003). The video signal comes out on pin 19 of IC3003. Now that this seemed to have been cleared, I had to look further afield. The circuit actually specifies two sets of voltages - one for the record mode and one for the playback mode. In fact, the differences between the two modes are quite small, whereas the errors I had measured were much greater. More specifically, all the voltages were down but the one that stood out was the base. This was supposed to be at 1.49V for playback but in fact was less than half of this. A resistance measurement from base to chassis showed a possible reason; it was almost a dead short, around H2. But where was it? About the only circuitry left was from the base to diode D3004 and thence to C3075 (10µ,F, 16V) and the junction of a voltage divider from a 9V rail, R3095 (6.8kn) and R3096 (1.5k!1]. A run from this junction wound its way off to the left and eventually finished up at another diode, D3006 [not shown here). I lifted D3006 but the short remained. That left the 10µ,F electrolytic, C3075, as the only remaining possibility. Modern electros seldom go short circuit; the low value ones frequently dry out but to break down with only a few volts across them is rare indeed. But it was a dead short. I restored the various circuits I had disturbed, fitted a new 10µ,F capacitor and that solved the problem. In fact, it turned out to be a very nice recorder. But I was a bit browned off at myself for trying to be too smart. The answer seemed so obvious that I thought I could make it a quick one without bothering to make any checks or measurements. The point I failed to appreciate, of course, was that the modulator was not the only common factor in the video chain. Switching between playback and record was not directly ahead of the modulator but further back and involving additional common circuitry - the two buffer stages and the filter. So I came a gutser. Oh well, no real harm done, except to my ego. A crook General My next story concerns a General colour TV set, type GC207. This set is about 6 years old and I had serviced it about 12 months previously, under warranty. This was one of a few models marketed by the General Corporation which had a full 5-year warranty for labour and material. There was only a month or so left of the warranty when I serviced it on that occasion. When the customer contacted me on this latest occasion he came out with the bald and somewhat aggressive statement that "the set's the same as it was the last time you serviced it" . Obviously, he imagined that would be all I needed to know in order to recall everything about the set. I wonder if he has the faintest idea of how many sets would have passed across my bench during that time. As it was, all I could do was suggest that he bring the set in. This he agreed to do and, in the meantime, I looked up my records to bring myself up to date. The complaint from the customer then was simply that the set wasn't producing a good picture. More specifically, my records reminded me that the picture had developed a green cast plus a marginal lack of brightness. On that occasion, I had performed a grey scale adjustment and reset the sub-brightness control, a 10k!1 pot (VR305), which feeds pin 4 (pedestal clamp] of the PAL Processor IC (IC301). There is also a lMQ screen control pot (VR406) on the neck board which provides a measure of brightness control. Considering the age of the set, the need ,Y i ., ~~ -:~==~ ,,t?'c ---So W~~'R£ WA-S THe:. VlDE.0 GO\NG DOWN 1\-\E: DAAI ~? JUN E 1989 37 Fig.3: the neck board circuit of the General GC207 TV set. The major symptom appeared to be low voltage on the collectors of the R, G, B driver transistors but the fault was elsewhere. for these adjustments was not surprising. In due course the man and his wife turned up with the set. They were still in a somewhat belligerent mood, the theme being that the previous repair had not lasted very long. And although they didn't spell it out, the implication was obvious: my previous repair, under warranty, was designed to last just long enough to get the set out of warranty (just how I could have managed this remains a mystery). Of course, I couldn't refute this implication because that was all it was; I just had to wear it. But when they began complaining about the reliability of the set, in a broader sense, I was quick to point out that they had had pretty good value for money; just on 6 years of TV entertainment which had not cost them a cent until now. That quietened things down a bit. The symptoms At a more practical level I plugged the set in while they were there, as much as anything to confirm whether the symptoms appeared to be the same as before. In one sense they were, in that the picture again had a green cast and was down in brightness. But it looked a lot worse than that. While it is rather hard to define, I had a strong impression that I was looking at a sick picture tube. There was not only the lack of TETIA TV TIP Kriesler 59-03 Symptom: ( 1). Line tearing or wrig- gling verticals. (2). Small, erratic changes in overall size of picture. The tearing and wriggling only lasted for a few minutes at a time but the size changes went on continually. Cure: Replace EHT tripler. The size variations are caused by small 38 SILICON CHIP changes in output voltage from the tripler. The tearing and wriggling must come from some kind of internal arcing but this has no effect on the line drive or line output. TETIA TV Tip is supp/iec:J by the Tasmanian branch of The Electronic Technicians' Institute of Australia. Contact Jim Lawler, 16 Adina St, Geilston Bay, Tasmania. brightness but also a lack of contrast, a subtle but definite lack of definition, and what might be described, in non-technical terms, as "sparkle". I couldn't be sure, of course. Other things can cause similar symptoms, though not many. So I contented myself with warning the customer that this was a strong possibility but one which would have to be confirmed by further tests. This left me with an "out" if I was mistaken. Later, with the set on the bench, I still felt convinced that the image on the screen showed all the classic symptoms of a sick tube. Nevertheless, I knew I had to go through the motions of checking all other possibilities. And the first thing that came to mind was the heater voltage. The heater voltage is derived from a winding on the horizontal output transformer and, by reason of its frequency and waveform, is virtually impossible to measure correctly with an ordinary analog meter or with most digital meters. At one time, Thomas Electronics marketed a moving iron meter for this job but I use a Fluke Model 8060A digital meter which is also suitable. The heaters looked bright enough but that is hardly good enough; it needs only a small voltage loss to produce symptoms similar to those in this set, so it had to be measured. In fact, the voltage was spot on so that ruled that theory out. My next step was to measure the collector voltages of the red, green and blue driver transistors. These are mounted on the neck board and are fairly easy to get at. This produced the first surprise. They are supposed to operate at 110V but all three were down to about half this value. There were two reasons to be surprised at this. One was the fact that all three were the same and the other was that this should have substantially increased the brightness of the picture tube, by reducing the positive voltage on the picture tube cathodes and thus reducing the bias. Instead the brightness was down. These two factors indicated that the incorrect collector voltages were a symptom rather than a cause. So, instead of tracing the collector supply back to its source, as I was initially tempted to do, I made some voltage measurements around the picture tube. No G2 voltage And that was the jackpot. The screen (or G2) electrode, which normally sits at around 500V, had no voltage at all. This electrode is fed from a 900V source via a voltage divider consisting of a 330k0 resistor (R416), a 1MO pot (VR406) and a 680k0 resistor (R415) to chassis, the voltage being taken from the moving arm of VR406 (see Fig.3). The culprit wasn't hard to find; R416 [33k0) was open circuit. With that replaced things started to come good. The screen not only brightened but was excessively bright and even winding down the screen control (VR406) was not sufficient. I went back to the red, green and blue collectors and found, strangely enough, that they were now at their correct voltage. Then I remembered that I had initially turned up the sub-brightness pot (VR305) in an effort to assess the set's behaviour. Returning this to its previous setting brought the brightness back to normal. More than that, it brought the whole picture tube performance back to normal. I was now just as sure that the tube was OK as I had previously been that it was crook. Which just goes to show how easy it is to be deceived. But that's not quite the end of the story. I went through the grey scale procedure again and the end result was virtually perfect but with one strange side effect. If the screen voltage was varied via VR406, the grey scale shifted, moving to excessive green when the voltage was reduced and to a lack of green in the other direction. However, if a new voltage was selected, the grey scale could be be reset to normal. This prompted a number of questions. Was the picture tube on the way out anyway and in a manner which made it strangely sensitive to the screen voltage? And was this the reason for the original green cast or was that the first sign of the 330k0 resistor going high before passing out completely? Or was it a bit of both? And why did the lack of screen voltage pull down the collector voltages of the red, green and blue driver transistors? It seems that this could only be because of excessive cathode current but the exact mechanism escapes me. The closest I can come to an explanation is to recall how the old power pentode valves used to draw excessive screen current if they lost their plate voltage but it is not an exact parallel. Anyway, the upshot of all this was that I was able to return the set to the customer with the assurance that the picture tube was all right, at least for now. But I felt bound to warn him that the set might drift into the green again. 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