VAXmate PSU fixed, but no video

Sat Apr 18 12:05:36 CDT 2020

This is the circuit: https://rjarratt.files.wordpress.com/2020/04/transformerringingtester.png

Regards

Rob

> -----Original Message-----
> From: Rob Jarratt <robert.jarratt at ntlworld.com>
> Sent: 18 April 2020 17:25
> To: 'Toby Thain' <toby at telegraphics.com.au>; rob at jarratt.me.uk; 'General
> Discussion: On-Topic and Off-Topic Posts' <cctalk at classiccmp.org>; 'Mattis
> Lind' <mattislind at gmail.com>
> Subject: RE: VAXmate PSU fixed, but no video
> 
> It is not my design, Tony Duell sent it to me and I think he got it from a
> magazine many years ago. I will try to find it.
> 
> Regards
> 
> Rob
> 
> > -----Original Message-----
> > From: Toby Thain <toby at telegraphics.com.au>
> > Sent: 18 April 2020 16:51
> > 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>;
> > 'Mattis Lind' <mattislind at gmail.com>
> > Subject: Re: VAXmate PSU fixed, but no video
> >
> > On 2020-04-18 11:09 AM, Rob Jarratt via cctalk wrote:
> > > I checked Q6 (in circuit), it is not shorted. I have already checked
> > > all the
> > electrolytic capacitors for a bad ESR and replaced those where it was high.
> > There are no shorts on the electrolytic capacitors.
> > >
> > >
> > >
> > > I will have to desolder the transformer and do a ring test on it (I
> > > built a ring
> > test circuit some years ago). I don’t think you can do ring tests in
> > circuit can you?
> > >
> >
> > I think more than one of us would be curious about your ring test circuit
> design.
> >
> > I've been slowly learning to design a pwm flyback controller which is
> > more or less the same thing in principle?
> >
> > --Toby
> >
> > >
> > >
> > > Regards
> > >
> > >
> > >
> > > Rob
> > >
> > >
> > >
> > > From: Mattis Lind <mattislind at gmail.com>
> > > Sent: 18 April 2020 13:31
> > > 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>
> > > Subject: Re: VAXmate PSU fixed, but no video
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > > Den lör 18 apr. 2020 kl 13:26 skrev Rob Jarratt via cctalk
> > <cctalk at classiccmp.org <mailto:cctalk at classiccmp.org> >:
> > >
> > > Some of you may recall seeing me post about the VAXmate PSU failure.
> > > Thanks to members of this list I found the failed part in the PSU
> > > and the PSU is now working again. However, it looks like the PSU
> > > failed because of a failure on the monitor board. There is a burning
> > > smell coming from it, possibly the flyback transformer, but I am not
> > > 100% sure. I don't see physical damage, but of course that doesn't
> > > mean there isn't a problem. When I took the monitor board out again
> > > after this, I wasn't sure if the EHT lead was making good contact
> > > with the CRT anode. The monitor board is described in section 4.4 of this
> document:
> > > http://bitsavers.org/pdf/dec/vaxmate/EK-PC500-TD_VAXmate_Technical_D
> > > es
> > > cripti
> > > <http://bitsavers.org/pdf/dec/vaxmate/EK-PC500-TD_VAXmate_Technical_
> > > De
> > > scription_1987.pdf>
> > > on_1987.pdf
> > >
> > >
> > >
> > >
> > >
> > > I need some advice on diagnosing the problem, I have a few questions:
> > >
> > >
> > >
> > > 1.      If the EHT lead was not properly connected to the CRT anode, could
> > > that cause problems?
> > >
> > >
> > >
> > > No, that is unlikely in my opinion
> > >
> > >
> > >
> > > 2.      Is there anything I can safely do with a bench power supply to
> > > isolate the problem?
> > >
> > > 3.      Any other suggestions for diagnosing the problem?
> > >
> > >
> > >
> > > Check Q6 transistor. Looking at the picture 4-14, I think it is
> > > wrong. But
> > nevertheless check the capacitors C47 and C53.
> > >
> > > It happens that the EHT transformer get short-circuit turns. But it
> > > has only
> > occurred to me once. More often a capacitor is bad, a rectifier or the
> > switch transistor.
> > >
> > >
> > >
> > > If the EHT transformer is bad it performs bad when doing a ringing test.
> > >
> > >
> > >
> > > Try to find out what is smelling. Check all power semiconductors.
> > >
> > >
> > >
> > > 4.      There is an outline spec of the flyback transformer in the section
> > > 4.4.3.2 of the VAXmate technical description, what chance of finding
> > > a "modern" replacement?
> > >
> > >
> > >
> > > Usually very low. There are companies selling replacements but there
> > > are so
> > many variants so the chances to find one is very low. And there are
> > less and less of these on the market. But it is worth a try to check
> > what they have. It might be the case that they used the same monitor
> > design as some other DEC product. VT420? Then it could possible a
> > better chance finding one. donberg.ie <http://donberg.ie>  seem to have
> VT420 LOPT.
> > >
> > >
> > >
> > > /Mattis
> > >
> > >
> > >
> > >
> > >
> > >
> > > I have posted about the PSU repair here:
> > >
> > >
> > >
> > > https://robs-old-computers.com/2020/04/18/vaxmate-h7270-psu-fixed-bu
> > > t-
> > > no-vid
> > > <https://robs-old-computers.com/2020/04/18/vaxmate-h7270-psu-fixed-b
> > > ut
> > > -no-video/>
> > > eo/
> > >
> > >
> > >
> > > Thanks
> > >
> > >
> > >
> > > Rob
> > >
> > >
> > >
> > >