Reviving VT220?

[Tony Duell]

On Sat, Apr 29, 2017 at 2:34 PM, Aaron Jackson <aaron at aaronsplace.co.uk> 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?

Err, the PSU has 3 voltage outputs, +5V, +12V, -12V. I am not sure where you
are measuring 31V between, but that doesn't sound 'fine' to me.

On the other hand, if the +12V rail was 31V the CRT heater would be burnt
out. If the +5V was 31V then the logic ICs would be totally fried so it wouldn't
respond to the keyboard.

So I suspect a measurement error....


>
> 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. 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?

According to the schematic of the monitor section (p16 of the 17 page
.pdf file I have) there is a bleeder (discharging) resistor between the EHT
output and ground inside the flyback. This would discharge the CRT in
a few seconds I think. So you probably wouldn't be able to get a spark.

I assume you don't have an EHT meter.

> Are there any other bits I should be wary of and test properly?
>
> Thanks again for your help. Your voltage listing and advice in general
> has been very useful.

Do you have a TV-rate video monitor with a composite video input? If
so, connect it to the BNC socket on the logic board. That's a video
output. If you get no video there, then you need to troubleshoot the
video logic first.

-tony