-----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: