TRS80 Model 4P CRT contrast fail

Tony Duell ard.p850ug1 at
Fri Nov 24 11:23:15 CST 2017

On Fri, Nov 24, 2017 at 8:41 AM, 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.


I am working from the Model 4P service manual which I downloaded from I
think. I can't find a schematic for the control panel, but the parts
list says the 2
pots are 500 Ohms (contrast) and 500k brightness. One of diagrams at
the start of
the manual suggests the former has 3 wires (wired as a potentiometer),
the latter
2 (wired as a variable resistor).

>From the monitor PCB schematic, I think you're right. The contrast pot is wired
with the ends to +12V out and ground, the wiper to DC Control (pin 8) The
brightess pot is wired between pins 3 and 4 (ground) and forms part of
the potential
divider for the CRT first anode (g2 across the Pond :-)). Note that
the brightness
control might have quite a high voltage on it.

I am assuming the fault is correctly described, a lack of contrast and not
brightness. Can you get a bright raster if you turn the brightness control up?

OK... Firstly, measure the voltage on pin 8 of the monitor PCB. Can you get
it to swing from 0 to around 12V?

Scope the video output from the CPU board (on pin 2 of the monitor PCB). Is
it a good TTL level signal?

Measure the voltage on pin A of the CRT base PCB. Expect around 60V here.

Check that C101 is not shorted. You could try the machine with it removed
to see what happens.

Have you checked the TR103 on the CRT base PCB?


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