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SERVICEMAN'S LOG The customer takes a holiday My main story this month concerns an intermittent problem in a VCR. While not the longest intermittent fault I have tackled, it was long by any standards. Also, the symptoms were quite misleading and it would fail for the customer but not for me. The device in question was a National NV-G7 A video recorder, owned by a lady who has been one of my regular customers for many years. Fortunately, she is very observant - a factor which proved invaluable in finally solving the problem. But there was no hint of the frustration to come when she first approached me. Her description of the fault was that, on odd occasions, there would be lines across the screen. And, in answer to my questioning, she described what I took to be a couple of noise bars, such as can occur with faulty tracking. Then she went on to explain that she planned to visit relatives in Queensland and would be away for about five weeks This meant that I could have the machine during that time and work on it at my leisure. It was a very convenient arrangement and she duly delivered the machine on the eve of her holiday. The NV-G7 A first appeared about four years ago and it transpired that this particular machine was still within its 3-year warranty period; it was now May and the warranty expired in July - a point of some importance, as things turned out. I set the machine up at the end of the bench, connected it to a monitor and proceeded to put it through its paces. I played pre-recorded tapes, I recorded programs off-air and replayed them, for hours on end - sometimes almost continuously all day. And did it play up? Not on your Nellie, as the saying goes. In fact, the whole five weeks went 48 SILICON CHIP by and I hadn't seen even a hint of the trouble the customer had described. Had she been a less reliable person I would have been tempted to doubt its very existence. In any case, all I could do was explain what had happened - or not happened - and suggest she take the machine back and see how it performed. With any luck, the fault might become continuous and thus easier to deal with. That didn't happen but within a week she was on the phone to report its brief appearance once again. And this time she had an additional observation. On this occasion, at least, she was convinced that the fault had occurred during recording. She had recorded a program and the lines had appeared on playback. She had immediately extracted the TETIA TV TIP Kriesler 59-01 Symptom: Total lack of horizontal hold. Line oscillator running slow and although its speed can be varied with R719 in CU701 , it cannot be brought up to correct speed. Cure: R729 (4.7kQ 1W) gone low. Metal film resistors rarely lose resistance but this one had been coated with carbon from a nearby burn-up on the main board. TETIA TV Tip is supplied by the Tasmanian branch of The Electronic Technicians' Institute of Australia. Contact Jim Lawler, 16 Adina St, Geilston Bay 7015. cassette and substituted a pre-recorded one, which played perfectly. Swapping the cassettes again produced the fault, which meant that it was recorded on the tape. In fact, she had put the tape aside for me. But did the fault occur only during record? Or had it also shown up during playback, on previous occasions? Naturally, we had no way of knowing and the tapes that had been involved had long since been re-used. Nevertheless, it was an important clue. Second attempt Of course, I was quite happy to have another go at it but it transpired that the customer was planning another interstate trip, again for about five weeks, and this would start in a few weeks time. In the meantime, she wanted to keep the machine. It did not always play up and even when it did, the results were watchable, albeit with some irritation. So a few weeks later, the machine was back in the workshop . We were into August now and, technically, it was out of warranty. But I had been careful to record the date when the complaint had first been made. I didn't want there to be any argument about what I felt was a genuine claim. Again, I set the machine up for testing, only this time I had the faulty tape to assist me. I viewed it at length and came to the conclusion that it was a typical tracking problem, probably involving a servo fault of some kind . But that was as far as I could go; nothing I could do would make the fault occur on my bench. So it was a repeat of the first episode. The customer returned, I explained the situation (which she accepted), and I returned the machine to her. Thankfully, she was very patient. And thankfully also, she was planning another holiday. This was to be even longer than before, starting in November and running until late January (it actually ran into Febru- THIS TESTER CAN PAY FOR ITSELF IN LESS THAN ONE DAY ~ □ ~CHECK™ ,., . i1 i 4 f~~lv'""" 11111! SIMM/SIP MEMORY MODULE and DRAM TESTER ~ * * . • * r, * ' ~ * * * * * * * ary). So I could have another go then. But, significantly, she had been home only a few days when she rang to say the the fault had re-appeared. And, again, it had happened in the record mode. She had played another tape but there was no sign of it. All I could do was make a note of it. Third attempt Thus began episode number three, in early November. I put it through its paces as before and, for a while, no joy. Then one day, when I recorded a 3-hour tape , then played it, there were the noise bars. I played a pre-recorded tape and the picture was perfect. I crossed my fingers and made another recording. And there were the noise bars again; it was still in fault condition. Well, at least I had proved the point. The fault occurred in the record mode only. But that really only complicated the problem. Still convinced that it was a tracking fault, I could not, by any stretch of the imagination, work out what kind of a fault would create a tracking error only in the record mode. Also, why had it taken so long to show up on my bench and how significant was this? Completely at a loss, I decided to put the thing aside for a few days, think about it, and hope for inspiration. In any case, there was pressure of more urgent jobs , plus the need for some extra bench space, so I disconnected it and pushed it aside. The inspiration idea didn't work. When I was able to set the machine up again, several days later, I was no nearer an explanation. But Murphy was on holidays and fate was on my side. When I first ran it, there was no sign of the fault and after a couple of abortive attempts to make it misbehave, I gave it away but left it switched on. Pressure of other jobs predominated for the rest of the day and I left it that way overnight. The next morning I tried it again and, lo and behold, it was in the fault condition. Then the penny dropped with a bang! Suddenly, a number of apparently unrelated facts came together and began to make sense. The explanation was - in a word temperature. It was summer now, but all my previous tests had been during the winter, when the workshop temperature would seldom exceed 20 All chips are tested simultaneously. Tests 64K x 8/9, 256K x 8/9, 1M x 8/9, 4M x 8/9 and 16M x 8/9 bits. Stand alone and portable - no need for a computer interface. User friendly LCD interface shows clear instructions and results. Zero insertion-force sockets for fast and easy operation. AC adaptor included. Expansion slot for add on products. Current measurement terminals. Two programmable voltage sources. Automatic current limiters provide full protection for your modules. 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Similarly, leaving it turned on for 24 hours was enough to restore the fault . So I could now create it, more or less on demand. But apart from that, I wasn't really any closer to finding the fault. I simply could not envisage the kind of fault which would create a tracking error - or servo fault - in the record mode only; which was what seemed to be happening. Well, I know when I need help. I called one of my contacts at the National Panasonic service section and put the problem to him, giving him all the clues I had collected so far. He was as helpful as he could be, fishing out a manual and pouring over it with me at length. But in the end, he had to admit that he was battling; he could think of nothing that would fit the symptoms. In fact, his summary of the situation was really more helpful than he imagined. Regardless of what the noise bars looked like, he was adamant that they could not be due to a servo fault if they occurred only on record. As he put it, "It's got to be a signal path fault." And while he admitted it was a long shot, he suggested that I check the vertical sync pulses and associated circuitry. Apart from that, I was on my own. The only good point was that the weather was getting warmer and the fault was present most of the time. So, at the next slack period, I decided to try the freezer approach. After all, it was definitely a temperature fault and, with a bit of luck, I might just hit the spot. Half a can of freezer later, I gave up. I'd been over almost all of the main board, paying particular attention to the areas mentioned by the National technician, plus the servo circuitry which, in spite of his reservations , I still felt needed to be cleared. The best I could say for that exercise was that I had found a lot of things that weren't wrong. So it was back to the think tank. Whenever I had a spare moment I fished out the manual, read and reread anything which I thought might help, poured over the circuit and prayed for inspiration. I even took the manual home at night - which wasn 't looked upon very favourably by Mrs Serviceman. But time was slipping by. It was after Christmas and the owner would be back in a few weeks. Finally, I dropped everything else, opened the manual and went over the circuit, section by section. This was quite an operation in itself, because the circuit gets quite a spread on. On the basis of the symptoms and TV TEST EQUIPMENT Is Your Product Or Service Getting The Exposure It Deserves? 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This one is just about as important as having a soldering iron in your toolbox! $75.00 + $10.00 p&p REMOTE CONTROL TESTER , (INFARED OR ULTRASONIC) Contact Paul To Reserve This Space (018) 28 5532 $85.00 + $4.00 p&p TUNER REPAIRS from only $17.00 SILICON CHIP + p&p We also repair most tuners on an exchange basis. Cheque, Money Order, Bankcard or Mastercard Y. V TUNERS~ • 50 Designed to test any lnfared or Ultrasonic control units. With the extension lead you can also test infared units which cannot be placed in front of the testing_unit. Requires a 9V battery. Output is via the LED diode and p,ezo speaker. • 216 Canterbury Road, Revesby, NSW, 2212 Telephone (02) 774 1154 everything I had checked so far, I mentally wrote off each section as it appeared not to be relevant. In particular, I was looking for any circuitry exclusive to the record function. I won't bore you with the details, but I finished up looking at a section called the "Luminance & Chrominance Pack Schematic Diagram". More specifically, it was at a section of this, designated as IC301, VEFY014 (see Fig.1). Now this IC is unlike any regular IC. It is a complete printed circuit board carrying two conventional IC chips: a 32-pin chip (AN3321) and a 24-pin chip (AN3215S), plus a swag of resistors, capacitors, etc, all surface mounted. And IC301 is novel in another respect; it is not even a conventional PC board. It is built on what is called "3D" board; a thin, flexible, translucent film. I have seen equipment - not in our field - in which such boards were even rolled up to conserve space. This particular board measured about 75 x 40mm and was folded in two lengthwise, making a package measuring 75mm by something less than 20mm. It is located underneath the main board, which is horizontal. Under this is a sub-board, attached to the main board at right angles, so that it protrudes downwards. And IC301, in its folded configuration, fits into a slot in this board, so that it sits parallel to and below the main board. There is a total of 78 connections to IC301, 39 on each edge. They are soldered to 78 copper tracks on the sub-board. Circuit details So much for the physical layout. Getting back to the circuit, one of the features of this section is the use of distinctive arrows to mark the record and replay circuits; pink for the record circuits and grey for replay. (Unfortunately, this difference is lost in the circuit reproduced here, although the pink tends towards a lighter grey). This distinction proved invaluable in my search for any record-only cir- .. •• w 0 0 4 w ::.::: u 0 0 <( a:, '.l: >- u w a: <( ....J z z 0... :c: :c: f- f- <( 0... <( 0... ....J <( ....J <( z z <..!l <..!l (/) (/) z z <( <( L 4 ' ! ~ < ~ i C, < ~ 0 0 ~ < ~ ~ = 0 00 ~ 0 < ~ ~ 0 z < z ~ 0 ~ = 0 ~ ~ 0 z z< ~ Fig.1 (right): the luminance & chrominance circuit for the NV-G7 A. Unfortunately, the colour coding indicating the record and playback paths cannot be shown here, although in practice they helped considerably in tracking down the problem. 0 ~ N . ~ I ~ :I: CJ NOVEMBER 1990 51 ing some nice remarks about these notes , he goes on to describe an unusual fault in his TV set. Mr R. T. describes himself as a "fixer" of electronic equipment in a radio station, so it was natural for him to tackle the fault himself. This is how he tells it. SERVICEMAN'S LOG - CTD Philips K11 ~ 0 O- ~ ONL"( SOM~~ Wl-\0 1-\~CS BatJ 11-\l;R~ Wll-(.... APPR~C\~.,..E:, 11\E:: SA,tSFAc,,o~ t Fe:t-i'" A, HAVING NAI t..£D n\E. C.Ul-f'Rl1"' APTER AL.-L11-\0$ MONTI-\S OF F'RUS"t'RAT\ON cuit which could be the culprit. And in a sense I suppose, I worked backwards. In the extreme top left of the diagram is a connection (13) marked "Picture VR", which runs directly to the video recording heads. Backtracking from here , we find that it connects to pin 78 of IC301 and carries a pink arrow. From here it connects to pin 28 of chip AN3215S, then to a block marked "FM Mod", and then out of the chip via pin 23, with pink arrows all the way. Was this "FM Mod" the temperature sensitive record-only circuit I was seeking? Hardly daring to hope, I hit the AN3215S with a smidgin of freezer. And that was all it needed; a 30 second recording confirmed that the fault had vanished. I let the machine run and in about 20 minutes - it was a hot day - the fault was back. Another smidgin of freezer (even less than before) produced another clean recording. The culprit nailed Only someone who has been there will appreciate the satisfaction I felt 52 SILICON CHIP at having nailed the culprit after all those months of frustration. Words cannot express it. (Well, I did mutter a few words!) And the repair? It did not involve, as one might imagine, replacement of the AN3215S, but replacement of the whole 3D board, IC301. This turned out to be relatively inexpensive and, in terms of labour, cheaper than attempting to repair it. That fixed it, of course. And now that I knew what was involved it was easy to confirm this, which I did over many cycles. So, when the customer returned I was able to announce with ill-concealed satisfaction - that I had cracked it. Naturally, she was delighted; the more so when I was able to tell her that there was no charge. Yes, National did the right thing about the warranty, without a quibble. But I was glad I had been so meticulous about recording the vital dates. It's a point worth remembering. To finish off, here is an interesting contribution from a reader, Mr R. T. of Wollstonecraft, NSW. After mak- The story in the July issue, about being misled by test equipment when repairing the degaussing circuit in a Thorn 9104, reminded me of a similar occurrence which I experienced. The TV set in question was the family's 1978 vintage Philips Kl lA. The initial fault was that it was unable to remain tuned to any station. It could be temporarily fixed by tapping gently - and sometimes not so gently! - on the cabinet in the vicinity of the preset tuning controls. The fault was obviously one of mechanical origin but despite several attempts at cleaning the controls with cleaning lubricant and checking for dry joints and hairline cracks in the PC board, the module eventually had to be replaced. We now had excellent pictures - for a fortnight - then the old trouble returned. This time we established that the problem was purely electronic. Monitoring the output of the voltage regulator, IC203 (TAA550), at G2 (see . Fig.2) showed that the voltage remained constant. But the voltage at the output of the channel-select switches (SK9) , at Dl2, was varying by 200-300m V. Faulty module? What was causing this? A faulty, brand-new tuning control module? At this point, the investigation went a bit haywire. In an attempt to determine whether it was the tuning control module or the actual tuner (U200) that was at fault , the lead to the tuner from D12 was broken and the tuner fed with voltage from a variable power supply. With this setup , the tuner worked perfectly and could not be made to mistune. The lead to D12 was reconnected , the lead to G2 opened, and the variable power supply substituted for the internal regulated supply from IC303. The fault immediately returned. As a further attempt to isolate the fault , an 1 ___l, __ - CHAf.NEL SELECTOR- - I F AMPLIFIER - FOR EXCLUSIVE KITS AT AFFORDABLE PRICES: OATLEY ELECTRONICS +12c L UHF ***VISIBLE LASER DIODES*** Brand new 3mW visible Laser diodes, supplied with a high quality collimator, ·. ' and heatsink assembly. g Simple to use, since it requires a cu rrent of about 85mA, with a vo ltage of only 2.3V across it. We include the specifications, instructions and a suggested driver and digital switch circuit. This circuit is also suitable for driving our Infra Red Laser Diodes ! U200 r SPECIAL introductory offer: We will supply the diode, the collimator, the PCB and the components (complete Kit) for the driver for ONLY: $270.00 PCB and components kit for the driver and digital switch can also be purchased separate!\~~-~: I SEE IN THE DAR,K! . with our INFRA RED NIGHT VIEWER at a SPECIAL price, while stocks last! We have good stock of the 6032A tubes. This is your opportunity to build a product which commercially cost 1OOO's of dollars at a fraction of the cost. We supply the electronic kit, a 6032A Image Intensifier tube, and the case components for only. ONLY $339.00 BRAND NEW TUBES Fig.2: relevant section of the Philips Kl lA circuit, showing the tuner and its associated switching circuitry. external pot was connected between Gl and G2, with the moving arm to D12. I was not really surprised to find that the fault was still there. What was going on? Up to this point, everything had been done on the control panel, which can be accessed from the front of the TV set. Delving into the innards of the set, to pin 2 of the tuner, it was found that the voltage on it was varying by 700-B00m V. Sense at last Now things were starting to make sense. It seemed most likely that something inside the tuner was intermittently loading the control voltage. And the reason why the tuner had performed perfectly when operating from the external power supply was that the latter has a very low output impedance, so that any variation in loading would have been masked. Had the power supply been connected via a resistor, say 47kQ, then this would have simulated the resistance of the tuning pots, and the fault would not have caused as much confusion as it did. Having Scottish blood in me , I did not want the further expense of a new tuner for a 12-year old TV set, if it could be avoided. Fortunately, a few squirts from a freezer can revealed that one of the tuning diodes had an intermittent short. A new diode restored operation and the set has now been working perfectly for several months. Had the set belonged to a paying customer, a new or reconditioned tuner would have been fitted but, for an in-house repair, it was felt that the end justified the means. Well, that's R. T. 's story, and a very interesting one too. It shows just how easy it is to be deceived if all asp ects of our test equipment - in this case the power supply - are not taken into account. Thank you, R. T. The 6032A tubes are BRAND NEW and perform well with a good infra red torch. Torch and Lenses are not supplied STILL ON SPECIAL: Till the end of 1990!: Single Channel UHF Transmitter/ Receiver at $49.90, the pair! PIR detector with commerical case at $39.90, interface at $7.90! 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