VAXmate PSU

[Rob Jarratt]

> -----Original Message-----
> From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Rob Jarratt via
> cctalk
> Sent: 09 April 2020 19:52
> To: 'Matt Burke' <matt at 9track.net>; 'Mattis Lind' <mattislind at gmail.com>;
> 'General Discussion: On-Topic and Off-Topic Posts' <cctalk at classiccmp.org>
> Subject: RE: VAXmate PSU
> 
> 
> 
> > -----Original Message-----
> > From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Matt Burke
> > via cctalk
> > Sent: 09 April 2020 09:40
> > To: cctalk at classiccmp.org
> > 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.
> 
> https://rjarratt.files.wordpress.com/2020/04/primary-side-shutdown-detail-
> high-resolution.png
> 
> 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.


I have put probes on Q2 with the probes connected as follows:

Ch1. D19 anode (for triggering)
Ch2. D19 gate
Ch3. Q2 anode
Ch4. Q2 gate

The result is here:

https://rjarratt.files.wordpress.com/2020/04/primary-side-shutdown-secondary-scr-detail.png

It doesn't look like Q2 is being triggered.

> 
> >
> > Matt