9 Apr
2020
9 Apr
'20
11:51 a.m.
-----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:
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-h…
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.
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?
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