TRS80 Model 4P CRT contrast fail

Adrian Graham binarydinosaurs at
Fri Nov 24 02:44:27 CST 2017

> On 24 Nov 2017, at 08:41, Brent Hilpert via cctalk <cctalk at> wrote:
> 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.


It is, 610k or 0.6M :)



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