What's worse than 2 transistor SB? 2 transistor with transil

[Behemot]

I got this Chieftec half-sized ATX PSU (don't know the model right now, but it is ATX sized, just shorter) that is smells and does not turn on.

It is a single transistor forward with 2 transistor stand-by. One cap bad at +5 V SB, OK replaced both. Than there was some diode burned, seemed like a -12 V eh? Replaced with more fat shottky. Wrong. It whistles and there is 16 V at +5 V SB!! It was 6.8V transil which was there to bring the voltage down. No wonder it was completelly baked and blown after those years. And it makes reason there was 16V cap after the stand-by rectifying diode and not sooner than in the end there was 10V, just next to the transil.

I will probably replace it with 6V transil but in bigger package, DO201 and for 1500W spikes. And replace the caps with 16V/1000uF KZHs. Also thinking about maybe relocating the transil a bit as it is under coil for passive PFC without much cooling possibilities.

[c_hegge]

Re: What's worse than 2 transistor SB? 2 transistor with transil

^
The problem is more likely that one of the small 50V caps has failed. give it a complete recap (with the exception of the two primary caps) and it will probably be fine. The transil is only there so that the failed cap doesn't cause the "16VSB rail" and cook the motherboard. In your case, though, it looks like it melted when presented with an over-voltage condition rather than doing its job. Normally, they will just and blow up the transistor, but the motherboard should survive.

[Behemot]

Re: What's worse than 2 transistor SB? 2 transistor with transil

There are only tiny caps, 0,22 and 0,47 uF, they seemed fine measured in circuit. Than some 10uF caps, they were also more or less in spec, that's the thing. But I will replace them for sure and test out of circuit…

[c_hegge]

Re: What's worse than 2 transistor SB? 2 transistor with transil

It could be one of the 10uF caps.

[Behemot]

Re: What's worse than 2 transistor SB? 2 transistor with transil

Neither of them. There were yet another two small caps, one of them, a 47uF one, measured 0,02 uF. ESR was out of range

[goontron]

Re: What's worse than 2 transistor SB? 2 transistor with transil

pics?

[Behemot]

Re: What's worse than 2 transistor SB? 2 transistor with transil

It's a Chieftec SI-A300P3, found some pics and info here http://www.vseprosto.net/2012/09/chi...rnogo-rezhima/