Dead Vestel 17IPS20 power supply repair (OK fuse, no shorts)
Good day folks. I thought I'd share with you how I repaired a Vestel 17IPS20 power supply in a Toshiba TV and hopefully this will help others who might be facing this issue as well. The TV appeared totally dead, that is, not even the LED at the front came on. With this being Vestel, I was expecting either a fried primary or a shorted rectifier diode, like it usually happens, but upon taking the board off and probing around, I got neither of these: no shorts and the fuse was also OK, so power was definitely going into this board. Taking a closer look, you can see the area around the FET in the top left of the board (well, RIGHT if you look at the picture that I took) looks a bit charred, although surprisingly the FET and all the components around it were OK, so I didn't bother to replace them for the time being. Flipped the board over and continued my searches on the bottom of the board: the first thing I checked was to see if power was going to the switching ICs. There's 3 of them: 1 for PFC, 1 for the main low voltages (12 and 5v) and one for the LED driver which is what the FET located in that darkened area of the board is for. Measuring the VCC pins of these showed some intermittent voltage, though too low for sustained operation (I think I got like 3-6v) so there seemed to be a short somewhere. The VCC for the 3 ICs is supplied by the same AUX winding of the transformer (the one in the middle) and I was lucky that the IC running the LED FET was connected via a jumper wire to this winding. This allowed me to pull out one end of the jumper to see which of the 2 ICs was pulling down the AUX winding. Sure enough, with the jumper out and no power going to the LED driver (U300), the AUX winding went up to 9v and I also got the correct 12v on the output of the supply, since the main switching IC was still connected and operates without a PS_ON signal - it's always on. This told me I needed to focus on the driver for the LEDs. Putting my meter probes across GND and VCC of U300 immediately revealed a short. Given the heat that area was subjected to, I wasn't sure whether the fault lay in the IC itself or the capacitor C397 placed between VCC and GND, so of course I took the easier approach first and pulled out the cap rather than the whole IC. This solved the issue: the short was no more and the cap was indeed shorted. Looking at the schematic, C397 is a 1uF cap. Not having a SMD one on hand, I took a regular electrolytic and botched it between a ground trace and the VCC pin of U300 just to see if it would indeed fire all the way up - it did. The TV is back in business 
So the next time you're faced with a Vestel PSU that doesn't seem to have any immediate faults, check the switching ICs - it might be something really simple to solve.
So the next time you're faced with a Vestel PSU that doesn't seem to have any immediate faults, check the switching ICs - it might be something really simple to solve.
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