Reviving VT220?

Jon Elson elson at
Sat Apr 29 10:50:04 CDT 2017

On 04/29/2017 08:34 AM, Aaron Jackson via cctalk wrote:
> The heater and control pins appear to be giving sensible voltages. The
> logic of the board is fine, I can type on the terminals keyboard and I
> get the correct characters on the other end of the serial cable. The PSU
> is putting out 31v which seems fine?
> I'm used to discharging the tube before fixing things inside CRTs
> (usually I only attempt to fix simple things like a broken toggle
> switch), but I have not managed to get a spark off this monitor. The
> tube doesn't seem to get charged up at all.
If everything is working, even a small monochrome monitor 
should give a bit of a crackle (might have to listen closely 
in a quiet place) and you should be able to feel the 
electrostatic field on the hairs on the back of your hand 
for a few seconds after powering on.
>   I've measured the
> capacitance of nearly all caps and they seem fine, diodes seem to be
> working fine. Does this mean it is most likely the flyback transformer?
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 


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