TRS80 Model 4P CRT contrast fail

Brent Hilpert bhilpert at
Fri Nov 24 02:41:18 CST 2017

On 2017-Nov-23, at 5:07 PM, Adrian Graham via cctalk wrote:
>> On 23 Nov 2017, at 21:18, Brent Hilpert via cctalk <cctalk at> wrote:
>> ...
>> You might check the contrast control.
>> I can't find the actual schematic for the contrast control connections, but I 'm guessing the ends are connected across 12V (video board pins 5 & 1/4/10) and the wiper to video board pin 8.
>> The contrast control can be expected to take pin 8 somewhere + to get pixels fully on.
> You’re right with pin 8, tracing the circuit I can see why someone recommended I check Q101 and Q102 which look OK in my tester.
> The contrast control itself tests OK resistance wise, it goes from pretty much 0ohms to 610M.

610M ?
I hope that M is a typo.

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