VAXmate PSU fixed, but no video

[Rob Jarratt]

I don't have an LCR meter but I think it might be a good idea to get one.

Regards

Rob

> -----Original Message-----
> From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Aaron Jackson
via
> cctalk
> Sent: 18 April 2020 15:13
> To: Peter Coghlan <cctalk at beyondthepale.ie>; General Discussion: On-Topic
> and Off-Topic Posts <cctalk at classiccmp.org>
> Subject: Re: VAXmate PSU fixed, but no video
> 
> >> 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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