18 Apr
2020
18 Apr
'20
8:13 a.m.
I don't have an LCR meter but I think it might be a good idea to get one.
Regards
Rob
If you want to check the secondary winding, there is a diode which has a high
-----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.
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 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. 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.
There were two VT220 designs (I think), using either an onboard or
offboard
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 :-( 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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