TRS80 Model 4P CRT contrast fail

[Brent Hilpert]

On 2017-Nov-26, at 7:42 AM, Tony Duell via cctalk wrote:
> On Sun, Nov 26, 2017 at 3:02 PM, Adrian Graham via cctalk
> <cctalk at classiccmp.org> 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).



With a scope the objective would be to look at the levels when pixels are 'on' in the video signal.
For white/on pixels, you need conductivity through TR102.E-C and TR103.E-C,
to allow electrons to flow from GND, through those transistors, into the cathode of the CRT,
and be emitted from the cathode towards the screen.

For a white pixel then:
	- the video signal (and to a lesser level, the base of TR102) must be going positive, to turn TR102 on.
	- TR103 collector should be going 'lower' during the pixel period, the black level is approx. 60 V,
	  the white level should drop well below that.

You probably won't see much voltage variation at CRT PCB point H / TR102.C / TR103.E
as it's just current-switching there with no collector pull-up.

You could try connecting CRT PCB point H briefly to GND through a 150-300 ohm R, or short TR102.C-E (not 103).
That should bring up a white screen.
If it doesn't it would suggest the problem is around TR103.
If it does the problem is likely around TR102.

If your multimeter has an hfe mode, you could try pulling TR102 & 103 and see if they show gain.

The monitor board schematic on page pdf.173 of
	https://amaus.net/static/S100/tandy/systems/model%204/Tandy%20Model%204P%20Service%20Manual.pdf
presents DC voltage levels to be expected at the transistors and other points.
That's for the green screen monitor, looks to be the same as the white-screen (pdf.172) except for some minor component value diffs.