VAXmate PSU fixed, but no video

[Aaron Jackson]

>> 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.

I recently decided to take another look at a VT220 I've got which
appears to have a bad FBT. Q202 switching transistor has been replaced
and is outputting a 14.7KHz signal but the flyback also seems to draw
too much current and causes the terminal to hiccup.

It appears to be a problem with the primary winding, which has an
inductance of 5.6uH, although has now increased to 6.4uH after smoking
again (I left it on while hiccuping to check that the transistor was
still switching)... If you have an LCR meter I'd be curious to know what
inductance you measure on the primary winding.

I did eventually notice a small crack in the plastic on the primary
winding side. I'm not sure if it is superficial but I suspect this is
where the smoke escaped.

>> 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.

If you want to check the secondary winding, there is a diode which has a
high forward bias voltage. If you pass 20v or so through the secondary
and look at it with a volt meter you should see the voltage drop. It
won't conduct at all if the voltage is too low.

>> 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 :-(

There were two VT220 designs (I think), using either an onboard or
offboard flyback. The part number of the onboard flyback is 16-26299-01,
and there are some available on eBay if you are feeling rich.

Cheers,
Aaron

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