18 Apr
2020
18 Apr
'20
7:40 a.m.
-----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 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 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
>
>