18 Apr
2020
18 Apr
'20
4:57 a.m.
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_Descripti
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 :-)
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-vi…
Thanks
Rob