VAXmate PSU

[Rob Jarratt]

> -----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.html) 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