Reviving VT220?

[Aaron Jackson]

Hi Jon,

I can hear a light crackle but not a whine.

Are you referring to Q202? The centre pin is 11v, the other two are at
0v.

Thanks,

Aaron


Jon Elson writes:

> 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
> circuit.
>
> Jon