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SERVICEMAN'S LOG Projection TV – from many angles In the course of my service career, I have come across quite a few projection TV sets. Normally I shun these because of the logistics involved in servicing them. In-situ servicing can be difficult in many cases while the sets are just too big to easily transport back to the workshop. I was once asked to repair a projection TV set in a hotel and being more naive (and hungrier) than I am today, I attended the set to find that although it worked, it had no green. But the real problem was that it was switched on and the drinking clients were waiting to watch a world heavyweight title fight. The hotel manager had not mentioned this; he had simply told me that the set was in the lounge and left me to fix it. However, as I started to work on it, one belligerent and somewhat intoxicated customer decided that I was about to damage the set and felt that it was incumbent upon him to protect it. I count myself lucky that I was able to get out of there without personally being readjusted – and projected! Thereafter, I made it a strict policy: no house calls to pubs – ever. If they want their sets fixed, they can deliver them to me at the workshop and handle the delivery costs. I have also been forced to apply the same policy to proper­ty managers and other time wasters. The scenario normally goes along these lines. It starts with a request to pick up keys from a real estate office (usually in a busy street in a long No Stopping zone) and go immediately to flat 27 on the third floor in an old building (with no lift) and fix an ancient unnamed TV set with an unspecified intermittent fault – straight away. They then want you to return the keys and submit your account for payment within 90 days. Oh yeah! – if I’m lucky. And then only after them first questioning and whingeing about the cost. Many of my colleagues know how to deal with this – they charge like wounded bulls. Personally, I prefer to just politely refuse – it’s not worth the hassle or my time. If they want the set fixed, they can bring it to me and pay when the job is done, just like everyone else. However, I did make an exception recently when Mr Schultz, a well-spoken businessman, asked me to attend to a rear projec­ tion TV set he had just imported from Germany. It was a 117cm (46-inch) RP46 Thomson employing an ICC9 chassis, about four years old. Apparently it had been working perfectly in Germany but the picture was distorted and blurred when workmen had unpacked it and installed it in its new location in Australia. A local compa­ny had sent a technician along and he reported that one of the boards had been cracked and that the set was probably a write-off. I was asked to check it out and give a second opinion Sets Covered This Month • • • • Thomson RP46 projection TV set Seleco SVT 150 projection TV set Dual Digital Concept TV4170 TV set Sony KV-X2931S TV set on behalf of the insurance company. Fortunately, I already had experience with this series of sets and have had similar problems with cracked convergence boards which are held along one edge, needing only one quick jar to put an unacceptable strain on the mounting. And so I agreed to call and make a brief examination, to confirm or refute this diagnosis. Sure enough, the picture on the screen had colour but was distorted in all geometric settings. In addition, both the static and dynamic convergence settings were way off. To get to the convergence module, I had to remove the loud­speaker baffles and find the concealed screws that allow the front panel to come off. This done, access to this board is easy. However, although none of the controls worked, the board looked pristine with no sign of a crack. I replaced the front panel and removed the back. The irri­tating part of this is the need to use a Torx anti-tamper screwdriver for some screws and a 4BA spin-tight for others. That done, I was able to locate the convergence power supply on the righthand side. I measured voltage going in but none coming out and decided that this was where the problem might be. I removed this unit and told the client that I would take it back to the workshop. The power supply was a conventional switchmode FET type but the major problem was trying to match the component numbers from the circuits with those on the boards. For example, the chopper transformer is shown on the circuit as TR01 but on the board layout it is marked T7100. There is a note on the circuit and a small chart under the heading – and I quote – “Conv. of europ. Names to US names.”, where these items are shown. But it gets worse when identifying the connecting plugs and sockets. For example, BS02 becomes J7002 (mark­ ed J2 on the board), which connects FEBRUARY 2000  75 with J8204 on the convergence generator board, which then translates to J204 and then BE04. It was all very confusing. Anyway, there was no sign of the power supply even oscil­lating. I went straight to the start-up resistors RP09 (R9U) 100kΩ and RP18 (R18) 220kΩ and measured them. The former was OK but the latter was nearly open circuit. I replaced it and the whole thing fired up properly. I returned with it, refitted it to the set and after some adjustment of the controls, the set gave a very good picture. Mr Schultz was happy on one hand that his set was now OK – but unhappy on the other hand because he had to pay for the repair rather than the insurance company. However, it just wasn’t possible for him to claim on his insurance. After all, how could one faulty resistor be put down to accidental damage through shipping? Another projection set The next projection set I had to repair was for a club that had bought a secondhand Seleco, which wasn’t 76  Silicon Chip working. Unlike the Thomson, this was a front projection system, the picture being projected onto a wall screen. The set itself was a 1987 model SVT 150 and was worth around $12,000 new. This one had to go directly back to the workshop and I don’t know why I took it on, as I didn’t even have a circuit – I suppose I was just curious to see what one gets for $12,000. Removing the covers revealed immediately that it was very corroded. The only positive aspect was that I recognised the construction and layout of the boards as being similar to the Fujitsu General series of colour TV sets made in Italy at about the same period. Indeed, the main deflection board was marked BS950, the same as for a Fujitsu General FGS­281PTXT. Even the remote control was the same (30D3). This was a stroke of luck because it meant that I now had a circuit. I started by switching it on and there was a brief display, then an “electronic click”, followed by silence – something had just died. After a more detailed investigation, I found the set had been endowed with two power supplies and the one marked BS820 had failed. Fortunately, it was a very conventional supply and I re­placed the chopper transistor (BU508A, T601), four electrolytic capacitors and two resistors and fuse F451. Coils TR454 and TR453 (20µH) had been slightly melted but apart from that had held up to the strain. This time, when I switched on, it tried to start but closed down after a few seconds with just P30 displayed. Because this set was very corroded, I next looked at board BS776 which I deduced generated the EHT for the three tubes. After removing it, I went over it very carefully, testing all likely components I thought might fail. The only thing I could find here that was that R615, a 100kΩ resistor, had gone high. My broad plan of attack was to check each board, one at a time, looking for obvious signs of corrosion related failure or damage. However, after spending a great deal of time, I couldn’t find anything. The only thing I could work out was that something was killing the horizontal drive after a few seconds. I worked through all the boards until I reached the digital board (BS815). This is very similar to the BS816 in the Fujitsu General colour TV sets and indeed other digital TV receivers under names such as Schneider, Dual, Teac, Akai ITT and Nokia, to name a few. These all use the same chip sets but are not neces­sarily interchangeable as they are often encoded for the instruc­ tion set used within that model. The horizontal drive was generated from the DPU2553 deflec­ tion processing unit IC (CI5) which is in turn controlled by a central processing unit and its EEPROMs. Though this set was made in Italy, it was designed in Germany and there was another clue. The display constantly showed P31 or program 31, no matter which button of the remote control was pressed. This suggested that it was highly likely to be the CPU (or CCU as the Germans call it), or even more likely, the EEPROMs. The Fujitsu General showed these EEPROMs to be MDA2062s, whereas this set used a sub-board (BS842) soldered into the main board in location CI3. This sub-board carried a single 8-pin DIL EEPROM NVM3060 instead of the original 14-pin DILs. I thought I might be snookered here in not being able to get the spare parts but as luck would have it, the local agents were able to sell me the IC. When it arrived, the set powered up and stayed on and there was sound – but no picture – on all channels. It took another saga to locate and replace CI8 DTI2223, the “Digital Transient Improvement” IC, to finally bring up the picture. But that wasn’t the end of the matter. After it had been on for a while, the picture broke up into black and white horizontal bars and the Teletext no longer operated. With the aid of some freezer, this fault was traced to CI12, the Teletext RAM chip (TMS­ 416415N). Fortunately, I managed to find a replacement on an old computer motherboard. After that, I was home and hosed and the old projection set performed quite spectacularly. Dual TV set Reverting now to conventional TV sets, my next story con­cerns a 66cm Dual Digital Concept TV4170. This set uses a DTV2 chassis and is made by Schneider in Germany. It is also made under the brand names of Teac, NAD, Nokia, Salora and ITT. The entire chassis is no bigger than that of a 34cm portable set and when one looks inside, the first question one asks is “where is the rest of the set?” Apart from the control panel, which is about 10 x 10cm square, the motherboard is divided into two sections: (1) the power/deflection board and (2) the digital/small signal/audio board, that latter using the ITT digital IC chip set. As with a few other recent jobs, this set came to me via the tortured route of failed repairs from several other service centres who really didn’t want to know. However, that’s not surprising really, considering that there are not many of these sets in the country and the agency has closed. We would all prefer to work on easy faults that earn money rather than complex ones that don’t. The set was reported as dead but strictly speaking, it wasn’t. There was no sound or picture but the switch- mode power supply was working and delivering all the secondary voltage rails except for the 5V U3 rail which was low at 3.3V and delivering 1.5A. There was no horizontal or vertical drive because this rail fed the “digi­ board” where the oscillators are located. The problem was whether it was a load fault or a supply fault. When the interconnecting St.DT (play) plug was removed, the 5V rail recovered to its full value and this suggested that it was a load problem. And it seemed that the only way to locate the fault was to desolder each device in sequence until the short vanished. This is rather difficult as the 16 odd ICs are all hard wired/soldered close together on the double-sided PC board – not to mention the tuner, IF, audio, AV and control panels. In addi­tion, many of the 40-pin ICs have multiple connections to the U3 rail, many of which are not marked. Nevertheless, I could see no easy alternative and so I persevered as best I could. Unfortunately, after spending a lot of time following this procedure, I had FEBRUARY 2000  77 signal never arrived at pin 23 of IC701. This turned out to be a fault of my own making; pin 23 had been poorly soldered when I fitted the IC socket. Because the IC is on the component side, it is very difficult to solder the pin from that side. All I could do was melt more solder onto pin 23’s pad from the other side until it finally connected with the pin. When this was finally achieved the whole set was trans­ formed into a well-behaved, responsive receiver with an excel­ l -ent picture. Everything worked, including the Teletext. The only thing that didn’t was the picture-in-picture facility and that was because the optional module wasn’t fitted. A weird fault made little progress. I could find no direct shorts, due partly to the amount of circuitry connected to this rail and partly because it is almost impossible to desolder awkwardly placed components on the double-sided boards. But the work wasn’t entirely wasted. I was beginning to suspect the Deflection Processing Unit, IC701 (IC1) DPU2553 and decided to remove it completely and fit a socket for it to the board. This was a drastic decision because it isn’t easy to fit a 40-pin IC socket to a PC board by soldering it on the component side – especially with virtually no room. However, I eventually man­aged to get the job done and then ordered and fitted a new IC. This turned out to be a good move, with the 5V rail making a big recovery – although not quite enough. And there were par­tial signs of life, with EHT and a white line at the top of the screen. I next suspected the two EEPROMs – IC1302 and IC1303 (MDA2062) – but then decided that IC1301 (CCU7916) was more likely to be the culprit, as neither the remote control nor any of the controls was having any effect. And I have to confess that I could only make intelligent guesses as to which section or IC might be faulty. 78  Silicon Chip Fortunately, my guess proved to be correct. When IC1301 (CCU7916) was replaced, the sound was re­stored and all the controls were working. However, there were still problems with the picture. The screen was intermittently trying to produce a full scan but there were huge quantities of what looked like hum moving up and down the screen. In addition, the horizontal deflection was off speed, as evidenced by picture tearing. The set was also suffering a great deal of stress, with a lot of heat being generated on the deflec­tion module. I switched it off and had a think about the problem. The most valuable symptom was the horizontal deflection system being off frequency. Why should this be so on a digital set when the oscillator is crystal controlled and has AFC feed­back? Because the frequency was only slightly off, a logical assumption was that the problem had to be in the AFC feedback loop. This is generated from pin 4 of the horizontal output transformer TR302 (TR2) and goes through CR410 (CR10) to pin 6 of the interconnecting harness plug St.DT. I followed this with the CRO to the anode of diode DT02 (D2) but the My final story this month describes one of the weirdest faults one could imagine. It was found in a set by our local Sony service agent. The set, a 1990 Sony KV-X2931S AEB1 chassis, belonged to an elderly couple and it had given good service for about eight years. Recently, however, it had started cutting out intermit­ t ently, giving no picture (black screen) or sound but leaving the channel number displayed on the screen. After a few abortive house calls to fix it, it ended up on their workshop bench. Subsequently, a number suspect solder joints were found which were duly attended to and the set was put aside for testing. Initially, they were quite confident that the fault had been scotched by this wide sweep but it wasn’t to be. This was the kind of fault that just keeps coming back and after a few days, they were right back to square one. Over the next couple of weeks, the set rocked back and forth from the soak-testing bench to the workbench and each time some suspect joints were discovered and resoldered. Finally, it was considered good enough to go home where it worked for about a month before the symp- toms returned. It then came back to the workshop where a couple of more joints were resoldered it worked OK for a week before going home again. This see-saw between home and workshop subsequently went on five times, with the set always being OK at the service centre and in trouble at home. Of course, you can imagine how the owners were beginning to get a little tetchy about this. In the meantime, all hell was breaking loose at the service centre as to how to fix such an intermittent fault. Finally, the boss, whose expertise is normally confined to audio equipment decided to have a go. To cut a long story short, the attention of all concerned had gradually been homing in on a particular board – the J1 board which carries the audio control, AV input, Y/C input, SCART video out and E/W correction circuits. This board is mounted vertically at the rear supported by a cream plastic rear support bracket (11) and the whole combination is hinged at the bottom so that it can fold down for service. The copper pattern side of the board faces inwards, against the plastic support – see Fig.1. It seemed that moving this assembly could bring on the fault – sometimes! It was eventually established that the set would always work with the back off, but intermittently fail with it on. A lot of time was spent examining the plugs and sockets and hinge connections J1-41 and J1-51, as pushing the assembly backwards and forwards could induce the fault. Finally, the boss confirmed that if the cream plastic bracket was removed from the J1 board, one could do anything with the board and the set still worked. But when everything was reassembled into the normal positions and the plastic back of the cabinet was replaced, the fault could be induced by pressing on this plastic back. The point here is that there is a plastic cover mounted inside the cabinet back, apparently to protect the component side of the board. So pressing the cabinet back can move the board/bracket combination. Simple solution Finally, he reached a dramatic conclusion – it wasn’t moving the board that caused the fault – it was just pushing the board against the cream plastic bracket that could do it! Fig.1: an exploded view of part of the Sony KV-X2931S AEB1 TV chassis. The J1 board (12) is supported by the plastic bracket (11). Note the fitting (13) and the cover inside the cabinet back. Apparently, this bracket had become conductive and with the back on, the cover pressed against the component side of J1 board. This in turn meant that the copper side of the board was pressed against the plastic bracket. So far, no one has managed to measure the conductivity of the plastic or determine precisely which circuit was affected. But the solution was quite simple – use insulating tape around the edges of the plastic bracket where it touched the J1 board. He had previously cleaned the bracket with acetone to remove any residual conducting chemicals but without success. This finally achieved a lasting repair and the set is still working months down the track. Of course, a new plastic bracket may have fixed the problem but why go to the expense when just a few centimetres of tape was all that was needed? Who would have thought that this board/bracket assembly could cause such a fault? Apparently, not all plastics are good insulators as we have been lead to believe! Finally, note that this fault could be very similar to one found in a run of 1992 Mitsubishi TV sets, models C3420/C3421. This produced a product recall when a white plastic PC board frame had to be replaced with SC a black one (BFC 3420-01). Silicon Chip Binders  Heavy board covers with 2-tone green vinyl covering  Each binder holds up to 14 issues  SILICON CHIP logo printed in gold-coloured lettering on spine & cover REAL VALUE AT $12.95 PLUS P & P Price: $A12.95 plus $A5 p&p each (Australia only; not available elsewhere). Buy five and get them postage free. Just fill in & mail the handy order form in this issue; or fax (02) 9979 6503; or ring (02) 9979 5644 & quote your credit card number. FEBRUARY 2000  79