TRS80 Model 4P CRT contrast fail

Adrian Graham binarydinosaurs at
Fri Nov 24 13:06:12 CST 2017

> On 24 Nov 2017, at 17:23, Tony Duell via cctalk <cctalk at> wrote:
> 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.
> Indeed...
> 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).

Yep, see my correction this morning. Just checked it again and it’s 0-641ohms.

> 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?

Yep, that’s how I can *just* see the boot logo. 

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

I’ll have to partially reassemble out of the case to test, but I can get to the other end of pin 8 with the control panel removed.

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

Good point.

> 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.

I removed it and tested it last night, looks OK.

> Have you checked the TR103 on the CRT base PCB?

Not yet, sleep was more important at that point :) 

I’ll report back tomorrow, cheers!

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