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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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PTV745 from hell, and you might not believe this
This is another one for the books. I file this with the "adjust
vertical height to cure picture turning red" and in another Sony, "a bad speaker can cause no high voltage". It is indeed from hell. When it got assignned to me the symptom was no sound and it would not switch video inputs. It would do pretty much the same thing as one with the option set wrong in the service menu, which I have seen before. At first I said "piece of cake, just set the option byte". Hardy har har. Well, as I was getting into the service menu the set quit. I asked "did it shutdown too ?", the answer was no. So I did my thing with the scope, pickng up radiation, first from the standby transformer, and then the main SMPS tansformers. I got a very low amplitude which shut down very quickly. This led me to look for a short, and sure enough the HOT was shorted, and it blew the replacement immediately. This is the same one, I reworked the PC board to accept a PTV500 series deflection output transformer. It requires about six or seven jumpers to do this, and of course you have to grind foils out. The tough one is the huge ground plane at the one corner. But I got it done and the set is up and running, but the same symptom exists. I have been all over the option code, in the service manual, set them to defaults, all that. I had checked all the supplies, I had done everything any sane and competent tech would do, except there is one thing. But I want you, you competent techs in here to tell me truthfully if you would have checked what turned out to be the problem, which I will reveal momentarily. I had three signal boards do the same thing. One was from the boneyard and we actually bought one, figuring this is some sort of fluke and it would not be the first time. We put a new board in it, well a PTS, and it was the same ! Well I had checked all kinds of stuff on the original board, as well as it's boneyard replacement. Until a certain point I had gotten myself into a corner. The new board, I went through the options, it was set right except for one A2 instead of B3, and that is the remote locator, this was not the problem. Finally I decide to go to IC1, the video switcher. This model has the ghost cancellor connection, which obviously has tuner video. I switched to video 1 and plugged in a cable to video 1. Actually I just touched the contact, if I did video would disappear, but when just touching the inner connector (without tripping the switch), I got sync. Like that I scoped the IC, and found a few other things along the way, the video output is actually switching. This SNAFU has to do with the COMB filter and video switching. Well, I decided to scope all the pins of IC 1, I downloaded a datasheet and got busy. I just went through all of them. Well eventually I got to the SDA and SCL pins. One was being clamped to a 3V high. I got it by the balls now, but where ? Am I in a situation where the convergence module is causing no sound ? Damn, that would be for the books. Being a bit bold, I disconnected the convergence plug from the small signal board. This did not affect the convergence actually, apparently it did need any data at the time. It was still plugged into the power/ deflection. I scoped SDA and SCL right there, and it was still no good. Thinking and thinking about it I was at a loss at the moment. But then I noticed that SDA and SCL also go to the power/deflection board. Why ? I found out why, IC551 (TA8859 vertical correction IC) uses SDA and SCL. That was the culprit. So we have vertical height causing the picture to go red, an open subwoofer causing total shutdown, STK 392-120s instead of 392-570s causing high voltage arcing and now a vertical driver IC causing no sound and no video switching. And here's a couple of things to ponder, first of all the video switcher IC was actually working. It was switching properly even with the reduced amplitude. But something else wasn't. I believe I have answered my own question though, this phenomenon has something to do with the PIP. Because I did notice the lack of a PIP window even in the customer menu, in PIP color and PIP tint. So that is the culprit, when the options aren't right, the PIP circuit is the one that REALLY requires the correct data. While I was in the service menu I had no reason to go into the VSCORR or the Vwhatever, which is what addreses that TA8859, so I did not know that it was not working. Well I bet it wasn't addressable at the time. I just changed it and the set works. After the debacle with the sweep transformer, we are planning on a celebratrion for it's leaving the shop. I am not the first tech to work on it, but I think I might be the last. But it does produce a good picture. Everybody happy now. I told the boss, and I think this is true, if it would've clamped a little higher, like at four volts, or if the PIP system would tolerate it and work properly anyway, we might've never known about the problem. It could've run for ten years that way. Not a bad learning experience, but this chipset is on it's way out. It came too late. Of course the principle applies to newer sets, but this was just wierd. Wanna buy a good PTV745 signal board ? We got two. My next project is to use parts and or adapt one to a GR5 chassis. I'll let you know how that goes. JURB |
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