Reviving VT220?

[Aaron Jackson]

I have measured these again because I thought they seemed a bit
strange. I must have probed the wrong pad on the circuit board earlier
(my probes don't fit in the CRT header).

Here are all the voltages and they don't seem too promising:

pin 1, green, 4.3v, should be 50v to 80v
pin 2, yellow, 6.6v should be 70v to 100v
pin 3, brown, 12v - fine
pin 4, black, 0v - fine
pin 5, white, 0v, fine, goes to case ground
pin 6, red, 0v, should be 400v to 800v
pin 7, blue, 0v, should be 400v to 800v

The voltage from the brightness control ranges between 2.4v and 5.5v,
and matches pin 1 above.


Tony Duell writes:

> On Sat, Apr 29, 2017 at 4:44 PM, Aaron Jackson <aaron at aaronsplace.co.uk> 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.
>
> -tony