Reviving VT220?

Tony Duell ard.p850ug1 at
Sat Apr 29 10:50:36 CDT 2017

On Sat, Apr 29, 2017 at 4:44 PM, Aaron Jackson <aaron at> wrote:
> Unfortunately I do not have an oscilloscope anymore. I've been trying to
> find a cheap one on eBay for a few months but the prices seem very high
> these days. I might be able to borrow one. For now I just have a true
> RMS multimeter.
> I've tried both extremes of the contrast and brightness but the screen
> is completely off. This is the amber version of the 220 if that makes
> any difference. I can't hear any high pitched whining from the flyback,
> which I can hear from my 420. All the large caps seem to be giving a
> very close reading considering I haven't taken them off the board.

If you have a few hundred volts on pins 6 and 7 of the CRT then the
horizontal output stage must be working. I believe you've checked
this. I assume you used the terminal's ground (e.g. the outside of that
BNC connector) as the reference for all voltage measurements.

The colour of the screen phosphor should make no difference at all.
AFAIK the only difference between the various colours is the CRT.

What voltages do you measure on the 'ends' of the brightness
preset? With respect to terminal ground (as above) of course?

> This terminal was working fine until about two months ago. It was in a
> air conditioned server room and probably hadn't been powered off for 5
> years or so until we had a long power cut, after which it didn't
> work. Does this provide any insight?

Normally the power supply fails to start when that happens, but we
know the power supply is fine since the logic side is working.


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