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SERVICEMAN'S LOG I'd never seen that one before To some extent, I suppose, that heading would apply to most stories that warrant telling - if it isn't unusual, it's hardly worth relating. But this one, I feel, is more unusual than usual- if you know what I mean. It concerns a Panasonic video recorder, model NVG-30A, purchased by one of my regular customers, just two years before this episode. And this short life was just one of several confusing factors and red herrings that had to be bypassed before the real culprit was unmasked. The complaint was a very noisy picture; so noisy as to be virtually unwatchable. It was also intermittent, which didn't help, but this point was not fully appreciated initially. The story really began when the machine was only about one year old. At that time, it appeared to be nothing more than a routine fault. When the owner brought it in, it certainly was behaving very poorly. In fact, to illustrate his point, he had brought in a tape that he had recorded. This tape was very noisy on playback and was also noisy when played back on another machine. Fouled heads? My natural reaction was to suspect fouled heads and a visual check confirmed that they were pretty grotty. However, I was a little surprised that they were bad enough to create the symptoms we had observed. Still, it's hard to be as precise as one would like when making such judgements. Anyway, the first job was to give it a routine clean and lubricate, which I did, at the same time checking for anything else which might need attention. In fact , it appeared to be in excellent condition in all other respects, so I simply put everything back together and gave it a run on the bench. It turned in a first class perform40 SILICON CI-IIP ance, on both record and playback, thus seeming to confirm my original diagnosis. And, on that basis, the machine was returned to its owner. But I did take the opportunity to discuss with him the quality of the tapes he was using, because it did appear that there was more fouling than there should have been after only 12 months ' use. It transpired that some of his tapes, while of well known brands, were quite old, this not being his first machine. So I suggested that he cull out the oldest ones, particularly any showing noise in the first minute or so. TETIA TV TIP General Electric TC20T1 (Hitachi PAL3-A chassis). Symptom: Excessive width. The height is not affected so heads on screen look like footballs. The width control has a small degree of affect but not enough to correct the fault. Cure: One or other of C758 (0.33µF 400V) or C759 (0.22µF 400V) open circuit or dry jointed. These caps effectively earth the bottom end of the horizontal deflection yoke and without them the yoke circuit impedance becomes excessive. TETIA TV Tip is supplied by the Tasmanian branch of the Electronic Technician 's Institute of Australia. Contact Jim Lawler, 16 Adina St, Geilston Bay, 7015. It was about six months before I heard from him again and then it was much the same story. But this time, along with the machine, he also brought in a batch of seven tapes for me to check. A visual check of the heads confirmed that there was some build-up of oxide, though not nearly as much as on the first occasion. But it was more than I would have expected in such a short time. This prompted some questions, from which I learned that the machine copped a lot of use. It was used mainly to record the late night movies and this happened almost every night. When I checked his tapes on another machine, I found three out of the seven which I rejected. Not only were they noisy at the beginning but they exhibited odd streaks and bursts of noise throughout. They had obviously had a pretty hard life on the previous machine, which seemed to explain the head fouling. So I cleaned everything up again and returned the machine, along with the tapes and some appropriate advice concerning them. And as before, I heard nothing more from him for about another six months, which brings us up to the time of writing. Then the owner was on the phone with the lament, "This recorder is really playing up now. I made a recording last night and it's really crook. I'll bring the recorder in and the tape". I said OK, but I wasn't as confident as I tried to sound. Twice was bad enough but three times meant real trouble. When he arrived, I set things up and played the tape he had brought. And there was no doubt about it; it was crook, with all the symptoms of fouled heads. But when I replaced his tape with one of my own, it played perfectly, which didn't tally with the fouled heads theory. The final blow to this idea came when the owner wound his own tape forward some distance to where it was a perfect recording. In fact, this random on-and-off behaviour was repeated several times throughout the tape. Murphy's conspiracy All this put a different complexion on things. Not only had it ruled out dirty heads as a major problem, it had also - for the first time - clearly indicated that it was an intermittent fault, probably in the record mode. Until now, Murphy had skilfully organised the symptoms so as to disguise this fact - at least from me. So I suggested he leave it with me, and warned him that it might take some time. He accepted this philosophically; after all, as he put it, it wasn't much good as it was. I set the machine up in a corner of the bench, pushed in the customer's tape, and set it to record while I went on with other jobs. At the end of three hours it rewound automatically and I set it to replay. It came up with a perfect picture and I let it run, glancing at the monitor from time to time. It ran like that for at least an hour then, suddenly, it was into noise. I pulled the tape out, put it in another machine, and it was still noisy. The fault had occurred during recording. Investigating further, I went back to where the noise started and checked the onset more carefully. This revealed that it was quite gradual; taking about five minutes from the first noticeable effect until it was in full bloom. But the change back to normal was immediate; as if it had been switched off. I didn't have any real clues at that stage and pressure of other work prevailed. I simply put it through repeated record and replay cycles, in the hope that some further clues might emerge. And one did. During one test, I suddenly realised that the noise was not on the tape, which played perfectly on another machine. This time it was occurring in the replay mode. Well, that was a real turn-up. It meant that the fault had to be in a section which was common to both record and replay modes and there are not many of these. In fact, the only one that came to mind was the head amplifier pack, a small PC boa~d assembly in a metal can, which comes immediately after the head drum. It handles both the record and replay signals - separately, admittedly - but I suspected that there could be sufficient common circuitry to allow it to fail in both modes. Fortunately, I had a unit in stock and it is a plug-in device, so it was obviously worth a try. And the machine did come good. It ran almost continuously for several days, with- · out so much as a flicker - until I was about to give it back to the customer, that is. Then it was back to its tricks. That was a real blow because there was little else to suspect. But, as it happened, the solution came by pure chance. Have you ever performed some seemingly innocuous act which has suddenly revealed all? Well, I have - and that's what happened this time. I was watching a noisy replay of a known good tape when, for no reason that I can explain, I switched the machine from the normal mode to the still frame mode. The result was a perfect still picture! I then switched back to norm'al and it was noisy as before. A couple of more checks confirmed that this was no fluke. What did it mean? This is a 3-head machine, the third head being used in. the so-called "trick" mode. So what it meant was that one of the other two heads was intermittently noisy; something I had never heard of before. Nor has anyone else, as far as I can determine. Fortunately, I had a spare head drum in stock which I promptly fitted . And that did fix it - and confirmed the diagnosis. So ended what had been a most frustrating exercise. After all that trauma, I'm happy to sit back and gloat over someone else's problems. So here's a tricky one from J. L., south of the border. The fisherman's TV This story began when one of the local fishermen brought in an Hitachi CWP-131 colour set. This is a mains/ battery type with twin rotary tuners and a 30cm screen. It was popular many years ago and there are still a lot SEPTEMBER1991 41 R702 . ,o ""'" C702 0.01 '" H,(L)AFC ;~a, CR701 IN34A TR701 2SC45B<at>I<at> 2SC l740<at>!<at> C70 1 0 .0 1 W2W) R707 12K ( 703 CR702 0 .0 1 1N34A R 720 68 0 R72 3 680 C765 1/ 5 0 0 D06 at!00 R76!, 4.7K + TR6 0 11 TR602 2SC458 <at>/<at> 2 SC l740<at>/<at> R 60 3 4 .7 1( R601 6 .8K + C605 2 20/16 R614 R6 t9 220 C6 19 4 + + TR603 2SA673<at>/<at> '" V,(F) PRE DRIVE 10 R6 !8 27K J; ci'. ~ R6 ! 5 <8 R6 0 ~ ~ Q I( C60 • 3 _V.'.! ' HOLO J .068 RGO-. CR602 M V-1 3 C6 0 9 10/50 7 100 I 3 471 16 R60 4 1/ 50 1 C +'' 2 3 R6 20 22 K R6 l 7 2 .7 K 22 K R62 1 2.2K R60 9 820 C6 02 :r l 112Wl "" ( 74 4 - :~& R706 9 5 M60IL!I. HM6233 V.!FIOUT R60 8 2 ,71( '" R6 2 2 3.6 Fig.1: the vertical oscillator & vertical output stages of the Hitachi CWP-131. The vertical oscillator stage is based on transistors TR601 & TR602 on the left, while the HM6232 thickfilm output stage (shown on the circuit as HM6233) is on the right. of them around. Not many, though, spend most of their lives at sea, and this may have had some bearing on the multiple problems presented on this occasion. The first time the set came in it was a simple "no-go "; the lOA fuse in the 12V line had dropped its bu ndle. The owner said. " .. .it worked last night, but wouldn't go this morning". It's rare to find fuses open circuit without being blown. This one had simply broken in the centre, probably the night before as the set cooled after being switched off. I fitted a new fuse and the set worked as well as ever. It went back to sea that night and I hyard nothing more about it for about three months. Then it was back, again with an open circuit lOA fuse. This was too much of a coincidence so I asked him to leave the set so that I could investigate why it was so hard on fuses . By this time it was the off season for fishing and he· wasn't going back to sea fo r several weeks. So he was quite happy for me to keep the set and, hop efully, find the cause of the troubl e. Then , as a passing comment, he mentioned that the picture had sometimes bri efly collapsed to a few centimetres high, then returned to normal. Even less frequently, it collapsed to a 42 SILICON CHIP bright line but alw ays recovered quickly, usually as the ship rolled to port - or was it starboard; I never can remember! (Simple J. L. The words "port" and "left" both have the same number of letters. So port is the lefthand side when looking towards the sharp end. No charge for that. Serviceman.) So, it looked as though we had two problems, not obviously related. It was not going to be an easy task and, in the event, the minor complaint became the major problem. Fishing for faults To open the account, I fired up the set on AC. The sound came up immediately but, when the screen lit up, I could see that the vertical scan problem had now become permanent. All we had was a line across centre screen. A quick check on the several rails showed that all voltages were close to normal, so I replac ed the lOA fuse and tried again, using the 12V battery I keep under the bench for such occasions. This time, the set came good with a full picture. When I restored the AC supply, the picture continued for an hour or so , then collapsed to about 25mm high. When I changed back to the 12V supply, the.fuse werit open circuit. So it appeared that something was shorting in the vertical stage and this was taking out the fuse in the 12V supply. It seemed strange, though, that the AC supply did not appear to be so critical and its fuse could withstand whatever overload was being imposed by the defective component. The vertical stage in the Hitachi NP6C chassis is a rather strange setup. Transistors TR601 and TR602 are designated as the vertical oscillator in a configuration that I cannot recognise. (One day someone is going to say, "That's easy! It's a so-and-so oscillator". And I'll say, "Of course it is. I just wanted to see if you knew!") The oscillator is followed by a simple series height control (R612), then a PNP pre-drive amplifier, TR603. This feeds the main driver amplifier and an output pair on a thickfilm unit, HM6233. The curious thing about this whole vertical stage is that the so-called oscillator stage appears to require substantial feedback from the output of the thickfilm unit (pin 8, normally 60V p-p) before it can produce the required voltage at this unit's input (pin 2, normally 1.2V p-p). I found the thickfilm module to be extremely sensitive to vibration. I took a long, soft knitting needle I keep for this sort of investigation and very gently prodded around and over the mod- ule. I couldn't find any part of it more sensitive than any other part, but the gentlest touch anywhere on its surface would toggle the unit in and out of operation. The picture was bouncing between normal and a bright line, and was very hard to stabilise in either state. One thing I have learned about these Hitachi chassis over the years is that these thickfilm units are subject to faulty soldered joints. The problem appears to be due to the ceramic substrate being extremely rigid, while the transistors and other components attached to it are far less rigid. These components have a tendency to change their size as they heat and cool, but the ceramic substrate refuses to move. So, over time, the assembly develops cracks in the joints. It's always worth trying to resolder the modules. I have been successful in perhaps half of my attempts but the only guaranteed solution is to replace the unit. This time it was to be one of my unsuccessful attempts. I removed the module and tickled it with a No.8 (400°C) bit in my Weller soldering iron. It took quite a long time to cool after that but, when I replaced it in the chassis, there was no sign of any activity. I tried all kinds of prodding and poking but it appeared to have failed completely. Oh well; it was worth a try! I have a colleague who is the local Hitachi agent, so I rang him, hoping he had an HM6233 in stock. I was out of luck, but he offered to include one in an order he was sending off that night. So , in a couple of days, I had a new unit; an HM6232. It looked the same as the HM6233 and virtually identical to a secondhand HM6231 which I had dug up from a junked set in the store room. When I had a closer look at the unit in the set on the bench, I found it was an HM6232, not the 6233 as shown in the circuit. I wish manufacturers could make up their minds - they make the life of your average serviceman truly a hell on Earth. Australian Made TEST EQUIPMENT YOU CAN TRUST Built-in Meter to check EHT transformers including split diode type, yokes and drive transformers. $78.00 + $3.00 p&p Appeals to Heaven I fitted the new module and switched the set on. I stamped my foot and thumped the bench, I called down from Heaven all kinds of imprecations on the designers of electronic equipment; all to no avail. All I had was a thin white line across the centre of the screen. At this point, I made the same mistake that I have made in the past. I reached for the high-tech tools of my trade and tried to deduce the cause with frequency counters, oscilloscopes and other assorted state-of-the art devices. If I had used my multimeter, I would have solved the problem hours earlier. I have already mentioned the peculiar nature of the vertical oscillator in this chassis. The oscilloscope showed only lO0mV at the input to the height control (it's normally 2.4V p-p) and also that the oscillator was running slow. The sync pulses were arriving at the oscillator at the correct intervals, but were obviously incapable of pulling the oscillator up to speed. The waveform vanished on the other side of the height control until I adjusted this to its maximum position, which produced a tiny trace of signal (about a lmV) at the base of the pre-drive transistor, TR603. There was no sign of any signal at the collector of this stage. Since the voltages on the oscillator transistors, the pre-drive transistor, and the input to the thickfilm module were all close to correct, I decided Strong magnetic field, larger than usual coil with multicore centre. Double insulated for your safety, also fitted with momentary on/off switch. 240V A/C 2.2 amps. 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L 651 Forest Road, Bexley, NSW 2207. Phone (02) 587 3491. (Tax inc.) from $17.00 exchange+ p&p Cheque, Money Order, Bankcard or Mastercard ~v. TUNERS). ·· · ·· · ··· · ····• •.•-•:-:-:-;-;.;.:-:-:-:•:•:•:•:•:•:•:-:-: :-:-:-:-:-:-:-:-:-:-:-:-:-:-:;:::::;:;::::::;::::-- 216 Canterbury Road, Revesby, NSW, AUSTRALIA, 2212 '"2:::r (02) 774 1154 (02) 7741154 ~A i SEPTEMBER 1991 43 SERVICEMAN'S LOG - CTD ~ 'R~~\/E-t:> -rt-\~ MOPUL,..~ & ,1c::Kt..-e:D 1--r w,-rl-\ ~ No.~ Br'f" \N M'-f WEU..ER 1~01'-.\oo••· that the only possible fault must be an open circuit capacitor in a feedback loop. There were several parts of the circuit that looked like feedback loops, and they all included electrolytic capacitors. So, with a sad heart and a weary hand, I began to extract all the electros that might conceivably figure in the vertical function. · An hour later I had checked every capacitor and most of the resistors in the relevant part of the chassis. There was nothing wrong with any of them. I then removed the transistors and diodes and tested them exhaustively but couldn't find any that were in any way doubtful. And that didn't leave much to test. I had already checked the voltages on the transistors and at the input and output terminals of the module and had found nothing dramatically wrong. So if there were no faulty ea- · pacitors or transistors in the circuit, then the trouble had to be a faulty resistor. But which one? I checked 44 SILICON CHIP every resistor from R601 onwards but they were all OK. That left only the output module - which I had already replaced. I had another look at the voltages around the module. I had already checked the supply on pin 1 and found 107V. This was high by about 30V but was accounted for by the fact that the output stage was not drawing any current. The voltage on pin 5 sets the bias on the driver transistor and this was correct at 0.6V. And the voltage on pin 11 was also correct at 0.1 V. At first glance, these had seemed to be the important voltages on the module. Pin 5 should turn on the driver, which should then turn on the output pair. But I had overlooked the network around pins 2, 3, 4 & 10. I had checked the resistance values for this network but had paid no attention to the voltages. This very soon revealed an anomaly which enabled me to solve the problem. The voltages should range from 54 V on pin 2 to 34. 7V on pin 10. In fact, they were all within 0.1 V of 9V. Since this voltage is derived from the 54 V rail which feeds the horizontal oscillator - which was working - then the low voltage on pin 2 could only be the result of an open circuit feed resistor. This turned out to be R636, a l0Q 0.5W metal oxide resistor which is shown on the circuit but not on the PC board pattern. I hunted all over the board pattern in the manual but couldn't find a trace of R636. Fortunately, Hitachi boards are quite clearly labelled so I was eventually able to find it. But it took some doing. It looked as though R636 had been added as an afterthought because it had been mounted high above the other components, on tubular supports, and the silkscreened part number appeared to have been handwritten after the overlay mask had been prepared. A quick check revealed that R636 was indeed open circuit and so it was immediately replaced. All the voltages around the output module then came good and I had a full scan - more than a full scan, in fact, because the height control was still turned up full bore. Legless trimpot It was then that I became aware of yet another fault in the vertical stage. As I adjusted the height trimpot, the picture collapsed to a bright line. I removed the trim pot and the two outer legs fell off. They had been riveted to the ceramic base but the rivets had corroded away. This may have been the result of the set's many sea voyages yet, strangely, there were no other signs of sea air corrosion. So the Hitachi was finally fixed but it leaves unexplained the sequence of original faults. I suspect that the faulty height pot was responsible for the intermittent nature ofthe original vertical collapse, ·while the demise of R636 accounted for the final failure. I'm not sure how all this related to the blown fuses - it might have been just coincidence, especially since the AC fuses showed no signs of distress . Or perhaps the ship's voltage regulator is out of adjustment, putting more than the usual 13.5V on the line. That's an idea I must put to the skipper, next time he comes into port. Thank you J. L., and congratulations on a happy ending - even if you were "all at sea" there for a while. (Ouch! Sorry about that). SC