VAXmate PSU fixed, but no video

[Peter Coghlan]

> 
> 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-video/
> 
>  
> 
> Thanks
> 
>  
> 
> Rob
> 
>