Reviving VT220?

[Tony Duell]

On Sat, Apr 29, 2017 at 7:36 PM, Aaron Jackson via cctalk
<cctalk at classiccmp.org> wrote:
>> Since it doesn't smoke, I'm thinking the horizontal sweep
>> transistor may have gone out.  It may have shorted, which
>> then may have burned up the primary winding in the flyback
>> (if left on long enough).  I'd check that transistor for a
>> short, and then you can trace a few traces to identify the
>> primary terminals on the flyback and see if they are an open
>> circuit.
>
> Hmm, not good news. zero ohm between pin one and three of the transistor
> you mentioned. To be fair the board around it looks a little brown, but

Is this with the transistor soldered to the board or removed? If the latter then
the transistor is shorted base-emitter. If the former then you are measuring
the resistance of L200 and the secondary of T201 (horizontal drive transformer)
in seroes which will be essentially zero.

> I assumed this was just because it would have got hot. Could replacing
> this cause damage to the flyback? I seem to have a low resistance on
> what I think is the primary coil.

Most flyback windings have a very low DC resistance. In any case the most
common failure is a short between a couple of turns of such a winding which
will ruin the AC performance of the transformer but have almost no effect
on the resistance. That's why you need to test such a transformer with a
ringing tester, not an ohmmeter.

-tony