TRS80 Model 4P CRT contrast fail

Tony Duell ard.p850ug1 at
Sun Nov 26 09:42:21 CST 2017

On Sun, Nov 26, 2017 at 3:02 PM, Adrian Graham via cctalk
<cctalk at> wrote:

>> What happens if C101 is removed? Also try running it with TR101
>> removed (or at least the emitter lead of that transistor disconnected).
> Nothing happens, in that the picture is the same as it is with both those components fitted.

That suggests the problem is not in the contrast control or the clamp
transistor TR101.
I would suspect TR102, TR103 and associated components.

I don't have the schematic with me at the moment, but as I remember it
you can look
at the connection between TR102 (common emitter stage) and TR103 (common  base
stage on top of it) on one of the connections to the CRT base PCB. And of course
the output of the video amplifier on the CRT cathode pin. It would be
worth looking
at the signals on those points with a 'scope, see what amplitude of
video signal (not
the 'standing' DC level).


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