Reviving VT220?

[Tony Duell]

On Sat, Apr 29, 2017 at 5:45 PM, Aaron Jackson <aaron at aaronsplace.co.uk> wrote:
> 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.

This looks very much like a problem with the horizontal deflection
circuit. Maybe the flyback transformer, maybe not.

Without a 'scope, troubleshooting is going to be difficult. But
start by testing the horizontal output transistor (Q202 on
the printset). For a start, just check it for shorts and opens. But
if it has failed, don't just replace it, as a problem with the flyback
transformer can destroy the new transistor.

-tony