VAXmate PSU fixed, but no video

[Rob Jarratt]

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_Des
> > 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-but-
> > no-vid
> > <https://robs-old-computers.com/2020/04/18/vaxmate-h7270-psu-fixed-but
> > -no-video/>
> > eo/
> >
> >
> >
> > Thanks
> >
> >
> >
> > Rob
> >
> >
> >
> >