VAXmate PSU

[Matt Burke]

On 08/04/2020 22:24, Rob Jarratt via cctalk wrote:
> I will look at all the suggestions, particularly of a failure on the secondary side. Something must have burned up, because there was a distinct burning smell after the initial failure, although I have never been able to see any physical damage to anything, despite looking many times.
>
>  
>
> But the thing that really puzzles me is that, after correcting the probes to include the D19 anode, there doesn’t seem to be anything that would cause D19 to trigger. Am I reading the trace wrong?
>

The only paths to ground for VCC are C12, E3 and D19. The fact that VCC
is stable for a period of time and then drops suddenly suggests it must
be D19. It looks like there could be a pulse on D19 gate but it is in
the order of 1-2 uS before VCC drops. There are more pulses prior to
that but I wonder if some of that is just induced noise?

It will be interesting to see what's happening on the secondary side,
particularly with Q2. I guess you have already checked the obvious
things like a short circuit on one of the outputs? Also worth checking
the rectifier diodes D11, D12, D22 and D23 (I think).

Matt