Faulty Vestel 17PW06-2 power supply (dropping voltages, no picture)
Greetings to all of the wonderful folks here. Hope you can help me with a bit of feedback, like you're always so kind to do, on this Vestel 17PW06-2 power board I'm tinkering with. When I first got this TV, it was completely dead, as not even the standby light would come on. Upon opening it up, the only problem I could spot right away was a shorted diode (D321 to be exact, not sure if you can quite spot all of the labels in the pictures), so I replaced it and now the TV sort-of "wants" to turn on, but doesn't actually do so. What happens is, I plug it in, the power LED now comes on and cycles between orange and blue, indicating the set is powering on and then....nothing: the LED remains blue and that's it - no picture and no backlight. Taking some measurements on the power supply, it looks like I still have a short somewhere, because when the TV is in standby, I read 24v on the 3 caps in the top left of the board (C209-C211), which are part of the LED drive, which is normal, and also 5v and 12v on the respective caps (the horizontal ones, though I can't remember which ones exactly), but after the LED turns blue, the 12v bus drops to around 7v and stays there and the 24 volts also drop (until eventually probably reaching close to 0, since I'm only reading the discharging caps at that point). As far as I know, the LED output should jump to around 100v shortly after startup :|
I flipped the board around and on the back I had a look at the two ICs which I pictured sepparately for a better view: the one with the last 4 legs joined together is the one I'm after (IC401). It's a MDS5652 dual transistor and my multimeter reads a short between pin 3 and pins 7-8. The strange part is that pin 3 is the source S of transistor 2 and pins 7-8 are the drains of transistor 1, so is it really possible for the two transistors to become internally "cross-shorted" and cause this "no power up" issue, or am I aiming at the wrong component altogether ? Is there something else I should check ? I've obsiously checked all the discrete components like diodes and trasistors and they seem fine, which is backed up by the fact that in standby I do get correct voltages - it's only when the TV fires up that I get the big voltage drop. I also tried disconnecting the LED connector (PL201 - upper left) but no change. Could it be the main board that's busted ? I'll try powering up just the the power supply and see what happens. Meanwhile, has anybody else faced this issue ? I've read around, but couldn't find an exact answer. Cheers.
I flipped the board around and on the back I had a look at the two ICs which I pictured sepparately for a better view: the one with the last 4 legs joined together is the one I'm after (IC401). It's a MDS5652 dual transistor and my multimeter reads a short between pin 3 and pins 7-8. The strange part is that pin 3 is the source S of transistor 2 and pins 7-8 are the drains of transistor 1, so is it really possible for the two transistors to become internally "cross-shorted" and cause this "no power up" issue, or am I aiming at the wrong component altogether ? Is there something else I should check ? I've obsiously checked all the discrete components like diodes and trasistors and they seem fine, which is backed up by the fact that in standby I do get correct voltages - it's only when the TV fires up that I get the big voltage drop. I also tried disconnecting the LED connector (PL201 - upper left) but no change. Could it be the main board that's busted ? I'll try powering up just the the power supply and see what happens. Meanwhile, has anybody else faced this issue ? I've read around, but couldn't find an exact answer. Cheers.
Attached Files
The schematic appears to be the correct one (at least I see the correct model and version in the bottom right of the first page)...replaced D327 for good measure (I actually swapped it with D302 which I pulled out earlier)
I can just tell by looking at the traces: VCC MAIN 12 is what goes through D327 and into VCC of ICBR. VCC MAIN then goes to D326. I'm thinking about that Q306 (a BC327): I get roughly 17v on all of its 3 terminals when BL is not connected and when it is, I get fluctuating voltages on the base and emitter and a steady 7v something on its collector (the pin attached to the positive leg of cap C318 for quick reference).
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