Reviving VT220?

Aaron Jackson aaron at
Sat Apr 29 10:19:03 CDT 2017

Seems to be working fine as a terminal hooked up to the TV which is good
news. I am sure you are correct about me having made a measurement error

I'll take another look at the schematics.

Thanks again,


> On Sat, Apr 29, 2017 at 2:34 PM, Aaron Jackson <aaron at> 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

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