VAXmate PSU fixed, but no video
cctalk at beyondthepale.ie
Sat Apr 18 06:57:25 CDT 2020
> 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:
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
> 18.104.22.168 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
> I have posted about the PSU repair here:
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