VAXmate PSU fixed, but no video

[Rob Jarratt]

> -----Original Message-----
> From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Peter Coghlan
via
> cctalk
> Sent: 18 April 2020 12:57
> To: General Discussion: On-Topic and Off-Topic Posts
<cctalk at classiccmp.org>
> Subject: Re: VAXmate PSU fixed, but no video
> 
> >
> > 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
> > on_1987.pdf
> >
> 
> Congratulations on getting your PSU failure sorted out.  I suppose you
realise
> this means there will be a line of people wanting you to look at our PSU
> problems the next time we meet up :-)

Given how long it took me to fix this one, I don't think anyone will want to
ask me!


> 
> >
> > 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?
> >
> 
> Possibly.  I have VT220 terminal which was making a smell of ozone when it
> was running which I should have done something about but never got around
> to.
> This could have been due to corona discharge around the CRT anode
> connection or around the flyback transformer but I never found out.
> Eventually, it stopped working, drawing excess current from the 12V power
> supply.  The flyback transformer appears to have been damaged.
> 
> >
> > 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?
> >
> 
> One approach to testing flyback transformers seems to be to use a circuit
that
> causes them to ring and observing whether the ringing is damped by shorted
> turns.  I've never got around to trying this myself.
> 
> >
> > 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?
> >
> 
> I wish you good luck with this.  I never had any luck locating one for my
> VT220 :-(
> 
> Regards,
> Peter Coghlan.
> 
> >
> > I have posted about the PSU repair here:
> >
> >
> >
> > https://robs-old-computers.com/2020/04/18/vaxmate-h7270-psu-fixed-but-
> > no-video/
> >
> >
> >
> > Thanks
> >
> >
> >
> > Rob
> >
> >