> That is absolutely the classic circuit. I assume the AC terminals of the 
> bridge
> rectifier come from the mains input, possibly via fuse,switch,filter.
> 
> To understand it, draw the bridge as 4 separate diodes. Now assume the triac 
> is
> turned off (open-crcuit) and it's just a standard bridge rectifier circuit. 
> Then
> consider the triac turned on (effectively shorted). You have the classic half-
> wave voltage doubler using the 2 capacitors and 2 of the diodes. The other
> 2 diodes end
> up in parallel with the capacitors (remember the triac acts a short circuit 
> now)
> and are always reverse-biased so don't do anything.
> 
> For 230V mains input, I would expect the triac to be turned off. So it's a 
> full
> wave rectifier with 230V AC in. Giving around 325V out.
> 
> I suspect those capacitors!
> 

I have now managed, after a bit of a struggle, to replace to two big rectifier 
output smoothing capacitors. The DMM shows about 325V output now, so I think 
this side of the problem is fixed. However, the PSU still does not work.

The first time I tried the new caps the PSU fans twitched for a fraction of 
second, but I did not have any other load attached. Now they don't move at all.

Could the earlier defect with the rectifier output have caused any other damage?

Regards

Rob

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