Rob Jarratt robert.jarratt at
Thu Apr 9 13:51:38 CDT 2020

> -----Original Message-----
> From: cctalk <cctalk-bounces at> On Behalf Of Matt Burke via
> cctalk
> Sent: 09 April 2020 09:40
> To: cctalk at
> Subject: Re: VAXmate PSU
> 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?

Here is a trace taken using a 5us timebase.

As a reminder the probes are connected as follows:

Ch1. 555 timer.
Ch2. D19 Anode
Ch3. D19 Gate.
Ch4. Q1 Source.

It doesn't look like there are any glitches on the D19 gate. I don't think it is clear why D19 is triggering, because all the previous oscillations are about the same.

> 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).

I did check Q2 before, but perhaps I should check again. I have checked for shorts on the actual outputs, but there don't seem to be any. I have checked one of those rectifiers, I think one of my next tasks is to desolder all of them and check them.

> Matt

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