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SERVICEMAN'S LOG The neighbour who made things worse Why do some people allow friends or neighbours to “have a go” at their VCR or TV when it fails? If their motive is to save money, the ploy usually backfires. More often than not, an amateur serviceman only makes things worse and the job ends up costing more. My first customer of the morning was Mr Davis who brought in a Sharp VCA34X mid-drive video. He cheerfully admitted he knew nothing about it and informed me that he “was only deliver­ing it for his wife”. When I turned it on, I noticed that it was patterning severely in playback mode and when the test switch was on. In addition, I was unable to receive any channel in the E-E mode. The patterning was very similar to the symptoms displayed when an electrolytic capacitor has failed in the power supply and the effect actually seemed to change when I hit some of the small electros in this part of the circuit with the freezer. It seemed initially that one or possibly two electros were susceptible and as I felt sure that I was on the right track, I decided to change them without further ado. In fact, I changed all the electros because if one was faulty, it was possible that the others wouldn’t be far behind it. As it turned out, I was just guessing because changing them made no difference at all. The CRO and multimeter showed that all voltages were clean and correct and so my suspicions now turned to the RF modulator. To check this, I decided to feed the audio/video outputs from the VCR directly into a monitor. When I played a tape the picture was now clear but there was still no TV channel reception (just snow). It was then that I noticed that when the VCR was switched off, a TV channel appeared quite clearly. This meant that the TV was tuned into a TV station and that the VCR was transmitting right on top of it. As the output of this VCR is on UHF and can be adjusted easily, I retuned it to approximately Ch.37 and the pattern­ing disappeared completely. It is difficult to comprehend the misleading effects I had in the power supply. Perhaps they were due to the proximity of my hands and arms to the antenna but it’s hard to tell sometimes. The real complaint At this stage, I thought that it would be a good idea to talk to Mrs Davis (the VCR’s owner), to find out what was really meant to be wrong with the unit. As it turned out, her only complaint was that it was unable to record TV stations. I asked about the pattern­ing on playback but she said she really hadn’t noticed it as their reception was pretty poor any way. Well, at least I now knew what the complaint was. The only thing that couldn’t be explained was why the RF output was so far off the factory set Ch.37. Perhaps someone had had a fiddle? So I now had to address the real 38  Silicon Chip knowledge being a dan­gerous thing. By the way, out of curiosity, I check­ed the faulty transis­tor on my multi­meter – it still read perfectly, with no leakage indicated. I then held it up to the light and looked at the legs with a magnifying glass. At last a clue – I could just detect some signs of corrosion where the legs entered the transistor case. My conclusion was that this may have been enough to at least make it intermittent. Play it again Sam problem – no TV tuning. To reach the tuner, the top board has to be removed and the bottom board extracted from its well in the plastic case. After gingerly laying out the boards on the bench, I switched it on and measured the voltages to the tuner. All were correct except for the tuning voltage which was permanently stuck on 33V, even when I invoked the tuning mode and sent it searching from band I to band IV. The only changes that occurred were on the band switching rails. Because I didn’t have the correct service manual, I worked from a VCH83/85X manual. This model is a hifi mid-drive unit which is similar to the VCA34X and I found that I could follow the relevant part of the circuit quite easily, the main dif­ference here being circuit reference numbers. Tracing back the circuit from the tuner, I arrived at the top board and eventually came to Q1401 (2SC1740­ SQR) which also had an unchanging 33V on its collector. However, its base voltage was changing as the tuning was adjusted. It was at this stage that I noticed fresh soldering all around the area. Someone had definitely been there before me. I measured the transistor in circuit and it was fine. I then put the CRO on the base and could see the mark-space ratio of the pulses change in response to the tuning but there was still no change in the collector. The transistor must be lying – there just had to be something wrong with it! I decided to replace it with a BC547 as I didn’t have the original 2SC­1740SQR. It worked – all the stations could now be tuned properly and locked into memory. When Mrs Davis picked it up I asked her if someone had looked at it before. She wasn’t nearly as fierce as I had imagined her to be and she graciously admitted that the culprit was her next door neighbour who had also mistuned the RF output. I put it down to a case of a little I was grateful to the Jones family when they decided to bring their Samsung into the workshop because it is a large 68cm stereo TV. Their two sons carried it in from the station wagon and put it on the bench. I gave it immediate priority so that I could get it out of my cramped workshop, the only problem being that one of the two symptoms described was intermittent. The set was a CB7230WT using an S60MT chassis and apparently it didn’t always want to start unless they hit it! And now it had a white line across the screen. Serves them right for assaulting the poor monster! Removing the back revealed a flat horizontal chassis divid­ed into two boards. Access was poor due to the usual short con­necting leads to the front and because the large reflex cabinets for the speakers got in the way. The lefthand board had the power deflection functions, while the righthand board carried the small signal circuits. On switch on, it displayed the classic vertical deflection collapse symptom; ie, a white line across the screen, just as described. It didn’t take a mental giant to work out that the 9-pin flat-pack TDA3654 (IC301) attached to a heatsink was the vertical output IC (IC301), especially as it got very hot. Access to the underside of the board was difficult but manageable once it was unscrewed from its support frame. I fitted a new IC and this cured this fault. Unfortunately, having just fitted the last screw in the back, I found that it wouldn’t switch on – not even after I had assaulted it, as well! There was just the momentary sound of EHT static and then nothing, so out it all came again. With the power board delicate­ly balanced on its side and meter probes at the ready, July 1997  39 Serviceman’s Log – continued I could determine that all three motors could be made to rotate but only intermittently and not necessarily in the correct sequence. This erratic behaviour lead me to suspect that it was either a noisy mode select switch or a crook microprocessor. I took the line of least resistance and went for the form­er. I removed the front escutcheon, then the top board, and gingerly unscrewed the loading motor assembly, being careful not to let the loading arms spring out. I then unsoldered the old white mode select switch, installed the new blue type (part no. 79TD3895) and carefully aligned the pointer as before. When I got it all back together again, I switched it on with a cover over the ejector and pressed the off/eject button. Just like a trained dog, it immediately stopped sulking and proffered me my tape. I tested it thoroughly with all sorts of tapes before phoning Mr Bryant. My only fear was his statement about it “chewing tapes” –surely he meant swallowing them whole? My pal the Palsonic I switched it on again and it came on perfectly. Well, to cut a long story short, I jiggled and poked, bent and hit the boards until finally I established the fault was associated with relay RL801, which switches the 21V and 16V rails. So was it the relay that was at fault or the relay driver circuit? I traced the line from the relay back towards the driver circuit and eventually came to pin 2 of connector CNP801. It was then that I noticed that the orange 5V lead that goes to pin 1 was not quite properly in its plug receptacle. It didn’t take long to fix this and confirm that this was indeed the culprit for the intermittent start-up problem. I tested it frequently until the Jones’s gratefully picked their set up later that evening. The greedy VCR I had asked Mr Bryant to also bring his remote control and instruction 40  Silicon Chip booklet when he brought in his Daewoo-made NEC VN22 VCR and he was as good as his word. Although he complained that it was “just chewing tapes”, it simply swallowed my dummy test tape and alarmingly wouldn’t regurgitate it when the eject button was pressed. The only dis­play was a cyclical presentation of all modes. The instruction booklet describes this function indicator as the emergency mode and advises the user to push the reset button. However, pushing this and/or any other button was an exercise in futility. This sucker wasn’t going to give me back my tape whatever I did! I attacked it with my electric screwdriver and soon had the covers off and then tried to persuade it to give it back. It took me a little while to realise that, with no covers, the end sen­ sors were exposed to incandescent light and this was adding to my woes. Having overcome this problem, So far the day was going well. With any luck, I would be able to knock over two more jobs before the end of the day. The next in line was a Palsonic 3428 TV set which uses a Goldstar PC04X chassis. The guy who brought this in claimed that he was a technician and that the “electros from the picture tube had caused the fault to occur”. I remained unconvinced and I suspected that he had had a go himself and so I was very reluct­ant to proceed. However, the bank keeps telling me I need money and so I can’t turn everything away. When I switched it on, the set displayed a bright uncon­trollable raster which is quite a common symptom for this model. Unfortunately, it has many causes, the most usual being IC501, a Telefunken TDA35622A. For this reason, I have saved the chroma module and also the CRT board from a wrecked set so that I can quickly test these components. The chroma module comes in two different interchangeable sizes. It is also worth remembering that the heat­ sink around the power supply will often retain its high voltage for some time after the set has been turned off. A shock from this certainly won’t im­ prove your temper as you struggle to remove this chroma card. After substitution, the screen control on the tripler often needs realigning to obtain the correct operating point for the tube and it doesn’t behave like any other I know. The darkest picture is somewhere in the centre of its range with each end of the control giving a bright raster! What’s more, the effect on the picture can vary, with some colours disappearing completely at certain settings. In this case, the substitute chroma module made no dif­ference, so next I swapped the CRT board. This fixed the problem once the screen control had been adjusted for the darkest pic­ture. OK, so what was wrong with the CRT board from the custo­mer’s set. The circuit (see Fig.1) consists of seven transistors (two for each gun plus a common one), plus three diodes and various resistors and capacitors. Because the overall tint of the white raster was correct, the greyscale was also correct which surely meant that the three amplifiers were working OK, giving even quantities of red, green and blue. Anyway, I checked the voltages all over the board but could find nothing wrong. Next, I changed the five components that are common to all three driver stages (D901, Q907, C1, R922 and R923) but they made no difference. I disregarded the screen and heater components (C907, C908, R924 and R925) as being too fanciful and there were no cracks in the board or dry joints. So if it wasn’t something common, then perhaps there was some other component failure that was upsetting the whole cir­cuit. What I did discover was that hitting the components with freezer produced a positive result –the raster immediately began to darken and a picture began to emerge. And this could be fur­ ther improved by finding the correct point with the screen con­trol. Eventually, after repeated freezing and heating, especially on or around D902, D903 and D904, the picture was completely restored to normal. At this stage, I put all the old parts back in (except for C1) and carefully checked the diodes and their associated paral­lel capacitors (C904, C902 & C906) after first removing them from circuit. I couldn’t find anything wrong with these parts, so I replaced them and put the set aside to soak Fig.1: the CRT board circuit for the Palsonic 3428 TV set. It uses seven transistors (two for each gun plus a common one), plus three diodes and various resistors and capacitors test. As it subse­quently turned out, it was still behaving normally some two weeks later. So what caused the problem? My only theory (and I admit that it’s rather a lame one) is that one or more of the diodes was internally intermittent and this was affecting the beam current. And this in turn was being shifted well beyond the operating point of the tube so that normal operation was no longer possible. Or is the explanation much simpler? In all the swapping around of components, have I merely fixed a poor solder joint on the PC board. It is an unsatisfactory result and I invite readers to speculate. A dead Philips The last set of the day was a Philips KR66875 2B-S chassis stereo TV which was dead. There was voltage going to the chopper transistor but no B+. I couldn’t find any shorts on this rail or to the line output transistor and felt pretty sure that the line output transformer was faulty. To test this, I shorted the base of the line output transistor to its emitter and connected a voltmeter to the collector. On switch on, the collector voltage rose to the full B+ line. Fortunately, I had a flyback trans- former in stock but removing the old one is fairly hairy in that it is hard to remove the solder between the pins and the rivets in the chassis. A combination of solder sucker, solder wick and bad language does the trick and I finally managed to edge the old one out. The next challenge is to remove the EHT and focus leads which clip into receptacles. These were eased out using side cutters and pushed into their new homes. The new transformer was then installed and, as a precaution, I also replaced the 68µF electro feeding the vertical output IC and soldered a few suspicious looking joints on the main board. I also checked out the memory backup battery. By the way, access to this set is rather difficult and I find it easier to work with the entire cabinet upside down on a towel rather than trying to find a service position for the chassis. Fortunately, the new flyback transformer did the trick so that was another one knocked over. Why can’t they make TV sets easier to service? Still, I mustn’t grumble too much ­­– apart from the Palsonic, the day had gone fairly well. And even then, I managed to get the set fixed, or so it appears. SC Only time will tell. July 1997  41