10 Apr
2020
10 Apr
'20
6:47 a.m.
-----Original Message-----
From: Rob Jarratt <robert.jarratt at ntlworld.com>
Sent: 09 April 2020 20:34
To: rob at jarratt.me.uk; 'Rob Jarratt' <robert.jarratt at ntlworld.com>;
'General
Discussion: On-Topic and Off-Topic Posts' <cctalk at classiccmp.org>;
'Matt
Burke' <matt at 9track.net>; 'Mattis Lind' <mattislind at
gmail.com>
Subject: RE: VAXmate PSU
> -----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-det
> ail-
> 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 have found the problem and a possible other problem too.
I have discovered that D24 is shorted (no visible damage), although its twin D23 is not. I
will replace both. I know it is shorted because I lifted one end.
I have also checked the rectifiers, D11 and D12. I am not sure if there is a possible
problem here too.
D12 is an MBR3045PT. It tests correctly as a common cathode diode network. However, the
forward voltage seems to be 0.19V. The datasheet
(https://pdf1.alldatasheet.com/datasheet-pdf/view/53622/FAIRCHILD/MBR3045PT.…) would
suggest it should be 0.76V at room temperature. I can see no physical damage to it
though.
D11 is marked PHS 2402. It too tests correctly as a common cathode diode network, with a
forward voltage of 0.43V. I can't find a datasheet for it though, but I believe the
equivalent is MUR1610CT
(https://pdf1.alldatasheet.com/datasheet-pdf/view/82128/ONSEMI/MUR1610CT.html) and I think
the forward voltage for that should be 0.9V. Again no physical damage that I can see.
Is it possible both rectifiers are marginal? Or at least D12? Should I replace one or
both?
Thanks
Rob
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