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SERVICEMAN'S LOG All the same – only different It’s not often that one set of notes deals almost exclu­sively with the same make of set. However, that’s the case this month because I had quite a few Sony TV sets to fix. Even so, each model is still a separate device, with its own quirks and problems. It’s really been quite a month, with TV set repairs pre­dominating. Computer repairs tend to be confined to older ma­ chines – machines which their owners can’t bear to throw away. It is hard to overcome the perception that most electronic applianc­es have a life-span of around 10 years and computers only about three years. This is not because they are not well made or because the manufacturers have incorporated “built-in obsolescence”. Instead, it is dominated by the demand for more features and, of course, faster operation – all ultimately involving the latest technolo­gy. I am happy to oblige by fixing older devices, provided the customer is prepared to accept the non-availability or high cost of spare parts. As already stated, this month I dealt with quite a few Sony TV sets. These are normally very reliable sets and the number of repairs that came in simply reflects their popularity in the marketplace. Unfortunately, apart from the brandname, the only thing they had in common was the diffi- Fig.1: the microcontroller circuitry in the Sony KV-2064EC colour TV set. Disconnecting R068 restored all the set’s functions but it was the microcontroller (IC001) itself that had to be replaced. 18  Silicon Chip culty I had in analysing and solving the problems. The KV-2064EC The first Sony was a 1983 stereo TV set, model KV-2064EC (XE 3 chassis), with no picture. Mr Hardy had already previously taken it to the local service agent but had become annoyed about the 3-week backlog, a non-refundable quote fee of $35 upfront, and an estimate of $290. I didn’t have the heart to tell him that some companies charge twice that price and require even longer queues. Although there was no picture, the sound was quite OK which seemed to suggest a straightforward problem. I don’t normally service TV sets more than 10 years old but I thought that this would probably be a piece of cake. And I even had the origi­nal service manual! Unfortunately, my optimism was to be short-lived. With the set on the bench, it didn’t take long to confirm that all the voltage rails were spot on, that the EHT, focus and screen voltages were correct, and that the CRT filaments were at 6.3V and glowing – in short, all the main “life signs” were there. The only voltages that were wrong were the tube cathodes. These were high which indicated that they were being cut off by the video processing circuits. I thought I would take a quick short cut here although I don’t really recommend it to others, on the grounds that there is a fair risk of damaging other components. By momentarily shorting each cathode to chassis in turn, there should be an intense single colour raster on the screen. This will indicate whether there is full vertical and horizontal scanning and if the tube is working. If, for example, the vertical timebase has failed, many sets will blank off the raster to prevent screen burn. In this instance, all was OK, with three bright fully scanned coloured screens. and Q316 (BLK). The base of Q316 is shown on the circuit diagram as 0V but was in fact at 5V, thus switching it hard on. Tracing back further, I found this same 5V on pin 9 of the micro­ controller IC (IC001) but this point is also shown on the circuit diagram as 0V – see Fig.1. Disconnecting pin 9 made no difference but removing R068 (22kΩ) which goes from pin 9 to the 5V rail restored everything, with a perfect picture on the screen. The set even continued to work OK when pin 9 was resoldered but with R068 left disconnect­ed. So what is the purpose of R068 and what does it do? Well, I really don’t know; there is no block diagram or description of the IC. Furthermore, the circuit shows IC001 to be a CX519-004P but I found a CX523-110P fitted. Worse still, neither IC is now available as a spare part, which isn’t unusual for a 16-year old set. Fortunately, I found a CX519004P in a scrapped Sony KV-2764. When fitted, this IC held its pin 9 low even when R068 was reconnected. I have no idea what caused the problem, which is very frustrating – but at least I had another satisfied customer. Mrs Mulligan’s KV-F29SZ2 It was now time to get really technical and bring the big guns into play. Using a signal generator and a CRO, I soon estab­lished that the video was getting as far as the colour decoder, IC301 (UPC1365), but was going no further. I checked all the waveforms on all the pins, especially pins 19 and 23 where I measured the shape of the horizontal pulses. Apart from no output on pins 26, 27 & 28, out of the RGB matrix block (which was understandable), all was correct. The video could be seen as far as pin 6 on the video ampli­fier part of IC301. Next, I measured the DC voltages on this IC, concentrating on the luminance section, especially the bright­ ness, contrast, blanking (BLK) and beam current limiter (ABL) circuitry. I found that the voltage on pin 4 was only half the 8.5V shown on the circuit and neither the remote controller nor the set controls could correct it. I now followed the circuit back from pin 4 to transistors Q308 (ABL) Items Covered This Month • • • • • Sony KV-2064EC stereo TV set Sony KV-F29SZ2 TV set Sony KV-2585AS TV set Sharp SX-51F7 TV set Sony KV-S29SN1 TV set The next Sony lived by the sea in a lovely unit overlooking the beach. The only trouble was that the onshore breezes blew the salt atmosphere right into the back of Mrs Mulligan’s 1995 KV-F29SZ2 (G3F chassis) TV set. When I called, she complained that all she could receive was channel 2 and play back the video cassette recorder. The on-screen display showed that it was switching bands and searching, with lots of snow on the screen. I checked the antenna and the video leads and they were all OK. The VCR could receive all channels perfectly and from all of this, I guessed that it was either the tuner or the IF module. However, I would have to take it back to the workshop. When it finally ended up on the bench, I removed the IF module block and resoldered a number of suspected dry joints around the coils and filters. Unfortunately, this produced no miracle cures so I laid the set on its front to provide access to the “A” board and started making measurements. And this provided the first clue – the DECEMBER 1999  19 band-switching voltages to the tuner weren’t changing, even though the on-screen display said they were. (Sony uses an alphabetical code to identify various PC boards. An “A” usually indicates the main board, while smaller boards may be desig­ nated down to “V”.) I traced the relevant PC track back to the microprocessor. Everything was perfect on the underside of the “A” board but it was a different story on top. Although everything looked fine from a casual glance, a closer inspection revealed that the fine copper tracks disappeared beneath the green solder mask. By scraping away the mask in several places and testing with a continuity meter, I found breaks in two of the tracks and narrowed them down to within a few centimetres. Connecting links across these breaks fixed the problems and the set could now tune all the Band III and UHF stations. Finally, I wiped the boards and sprayed them with a fine mist of CRC 2-26 to try to prevent further corrosion. However, considering where the set is located, I fear it was a fruitless task. Almost certainly, the set will be a write-off in a few years. Picking up the pieces The third Sony had been all over the place before finally landing on my 20  Silicon Chip bench. I hate taking on jobs like this; inevitably, I have to fix the previous technicians’ faults before getting on to the real repair. This set, a KV-2585AS, has a GP1A chassis, designated SCC-F35A-A according to the label stuck on the CRT metalwork. However the service man­ual says the same model uses a chassis designated as SCC-D23L-A. It may appear that I am being pedantic but this set had spent the better part of a year in various workshops and many parts had been replaced – not necessarily with the right ones. The main question mark hung over IC601, in the switchmode power supply. An STRS6708 was fitted but my circuit said it should be an STRS5941. And just to confuse matters, another GP-1A set that came in had an STRS­5741. In this case, the set was dead and the main fuse was open, due to IC601 having gone short circuit between pins 1 and 2 (ie, between collector and emitter of the internal switching transistor). Unfortunately, these ICs aren’t cheap, costing around $30 each. The chassis itself was a mess; several parts were either unsoldered or missing and others were quite clearly the wrong types. Fortunately, it wasn’t difficult to identify the compon­ents that had been replaced, from the fresh soldered joints. As it turned out, there were over half a dozen incorrect parts fitted. I replaced these with the correct types and I also replaced the horizontal output transistor, Q802 (2SC4927), which had a short circuit. Finally, I was ready to fire the set up but as an addition­al precaution, I connected a 200W globe where the main fuse should be. The set came on with both sound and picture but before I could measure the main voltage rail there was a loud bang and it died again, the globe turning very bright. This exercise cost the horizontal output transistor (Q802), a 1.2Ω feed resistor (R340) and the switchmode IC (IC601). I replaced all these parts and I tried again, this time using a Variac to wind up the voltage, a 100W globe and a meter across test point TP91 (135V) to chassis. As a further precau­tion, the base and emitter leads of Q802 were shorted. The 100W globe lit without stress and the voltage levelled at 139V. As there is no control to adjust this, I felt that the 3% error was within the design limitation of the set. Feeling much more confident, I removed the base-emitter short from the horizontal output transistor and again wound up the Variac. Once again, the picture and sound were good and there appeared to be no problems, although the test point was still at 139V. It ran for over half an hour and then, just as my back was turned, there was another loud bang followed by silence. There was a smell of electrical burning and the same com­ ponents had died again. And although I hadn’t seen what happened, I suspected the horizontal output transformer, T851. Once again, I had a problem deciding which was the correct one for this set, as the one fitted (NX-M 1601 1-439423-32) didn’t match the circuit or the other GP-1A set on hand. Eventually, I chose an NX-1604 1-439-416-41 and fitted it, along with all the other parts. This time the set came on and remained stable. I left it on for days, keeping an eye on it, and it performed faultlessly. Tube failures Just recently, I have had two cases of picture tube fai­lure, both in modern sets. The first was a 1995 Sony KV-F29SZ2 G3F chassis. I was called to this set, with the customer com­ plaining that there was no blue and that the set was “burning”. When I arrived, the symptoms I saw were no green and a burnt 1.5kΩ resistor, R713. This didn’t surprise me as it fed the green cathode. However when I asked the customer about this, he replied “Ah, well – I’m colour blind!” I replaced the resistor and all was well. However, I knew there could be only one reason why it had burnt and that was because the tube (M68KZT­ 10X) had an intermittent heater-to-cathode short. I told him the bad news and suggested that he “whinge” to Sony about it because, realistically, he had blown $1700 odd in four years on this largescreen “deluxe” TV set. At the same time, I pointed out that the set was well and truly out of war­ranty and so that would probably be the end of the story. A month later I bumped into him in the street and asked about the outcome. As it turned out, Sony had instructed him to take it to one of their service agents and if it really was faulty, they would replace the tube – he would have to pay for the labour costs only. He is still in the process of having this done. All I can say is, good for him and good for Sony. The second set was even younger, a 51cm Sharp SX-51F7. This set was only about two years old and the owners were watching it when it went “boom” and died. I was expecting something spectacu­lar when I called and I was surprised when I couldn’t find any­thing visually wrong on the circuit boards. All that was happen­ ing was that the picture tube (ASIK­ PD12SX) was arcing internally within the gun assembly. Obviously, it was “down to air” which is just another way of saying that the tube had lost its vacuum. I checked the power supply rails and the EHT. The HT was spot on at 115V and the EHT did not exceed 25kV. If either of these rails had momentarily gone high, they might have caused a flashover inside the tube, creating a fracture in the glass. However, both rails were steady and as there was no visible damage to the tube, I could only presume that there was a problem with the frit seal or with a part of the tube which cannot be seen. I thought that this too would be the end of the story but Sharp said it would come to the party with a free tube if the owners paid for an Fig.2: a section of the V board circuitry in the Sony KV-S29SN1. IC02 is at the top, while plug/socket CN01 is at left. Pin 3 (video) on the socket goes to pin 8 of IC02, while pin 2 (ST TV) goes to Q03’s emitter via R13. Fuse PS01 is at middle left, in the 8V line to the collector of Q02. authorised service agent (not me) to fit the tube. So full marks for customer service to Sharp as well. The giant Sony As soon as I saw this next set in Mrs Marsden’s home, I knew it was destined for the workshop. It was a 5-year-old 73cm Sony set with a KVS29SN1 G1 chassis and with lots of features, such as picture-in-picture, teletext and stereo sound. The set had developed two separate faults, one being inter­mittent no picture and the other no horizontal or vertical sync in the main picture (although the picture-in-picture was locked perfectly). To make matters worse, at least from a service view­ point, the set is big and heavy and access to the main module chassis is rather difficult. So there I was, on a Monday morning in the workshop, con­templating which was worse – fixing this set or working out how the GST was going to ruin me. I settled on the easier of the two problems and took the back off the set but even with a photocopy of the service manual, it was difficult to know where to start. In the end, I began by confirming that the “no sync” prob­lem persisted even if I injected a composite video waveform from a colour bar generator into the AV socket. I then took a look at the circuit diagram of the “A” board and quickly counted at least 10 devices marked SYNC this or SYNC that – there is even a module marked A2 SYNC. Unfortunately, the block diagram was too small and too badly copied to read. The only thing was to start somewhere – anywhere – and track down a sync signal. After that, I should be able to trace the signal until it disappeared and then figure out what the problem was. Now unless the reader has access to a manual or circuit, this description will not be easy to follow. It is set out, as much as anything, to give some idea of the frustrating chase involved in tracking down this most elusive fault. So bear with me. I placed the set face down but at a slight angle so that, by using a small mirror, I could see part of the screen. I then unscrewed the main chassis, pulled it out by about 150mm and wedged it on the bottom of the cabinet. The whole thing was incredibly precarious and the neck of the tube could be knocked off at any moment. Next, I fired up the CRO and started DECEMBER 1999  21 at the output of one of the twin IF modules. This went to transistor Q1101, designated sync-detect, and then to plug CN1108 and micro­ controller board “M”. (Note: Sony uses the prefix CN to denote plug and socket assemblies). There was plenty of composite video signal on CN1108. Simi­larly, there was video signal on transistor Q1130, also designat­ ed sync-detect, which came from board B, plug CN301. I then noticed IC1101, marked sync-selection, and started measuring the inputs and outputs of six transistors, all marked “SYNC”. In the course of all this measuring, I encountered several dry joints and resoldered them, each time praying that this would fix the 22  Silicon Chip problem. It didn’t. To add to my woes, the picture was now displaying the second fault - ie, no picture – on a regular basis, thus disrupt­ing my efforts to track down the missing sync signal. Trying to follow the video often meant going to a module where the circuit is marked “SYNC IN” and “SYNC OUT” – the only problem is from which board’s perspective? There are also several plug/socket combinations where the plug was marked one thing and the socket something else. As I continued my somewhat random quest, I was also pulling out modules, soldering them, reinstalling them and where possi­ble, measuring any voltages. Gradually I was getting a feel for the thing – the intermittent picture appeared to be caused by the J board/B board connection, a rather clever plug and socket combination (CN308/2301 CN309/2302) that also acts as a hinge. This was either noisy due to dirty contacts or had dry joints. Anyway, I cleaned the contacts and resoldered all the joints around this assembly and that fixed the intermittent picture problem. Getting back to the sync problem, I eventually found that there was video coming out of IC1101 into pin 3 of CN1113 but no video was coming out of SYNC pin 2. This pin in turn connects to pin 3 of CN1107 (marked SYNC IN), after which the signal goes to sync module A2 CN4401 and then back to IC3501 on the A board, via Q3512 SYNC 3, etc. CN1113 is connected to CN01 of the Teletext “V” board and the circuit, on page 107 of the service manual, designates pin 2 as ST TV. I took the board out and did a routine dry joint check but could find nothing untoward at first glance. However, the circuit shows a 0.6A fuse (PS01) connected between an 8V rail (pin 6 of CN01) and the collector of 5V regu­lator transistor, Q02 (Fig.2). This transistor delivers a regulated 5V at its emitter and this rail feeds pin 10 of IC02. The PS01 fuse, by the way, is called a “circuit protector chip” and looks like a two-legged transistor with N15 marked on it. Anyway, as luck would have it, this fuse measured open circuit, meaning that there was no 5V on pin 10 of IC02 or on the collector of Q03 (SYNC-OUT). Replacing it restored the composite video and gave a perfect picture. Subsequently, I was talking to a Sony service centre a few days later. After concluding other business, I mentioned my saga and its victorious outcome only to be promptly informed that it was a well-known problem. However, my informant did admit that he didn’t know why it happened. In effect, this was a classic example of the effort which can be involved in tracing a fault in an unfamiliar chassis. The fault was simple enough in itself but the symptoms provided few clues and in the absence of someone who says “that’s a well known problem”, there is nothing for it but to do it the SC hard way.