TRS80 Model 4P CRT contrast fail

Tony Duell ard.p850ug1 at
Sun Nov 26 00:15:11 CST 2017

On Sat, Nov 25, 2017 at 11:02 PM, Adrian Graham via cctalk
<cctalk at> wrote:
> Evening!
>> OK... Firstly, measure the voltage on pin 8 of the monitor PCB. Can you get
>> it to swing from 0 to around 12V?
> Yes, it tops out at 11.82V.

Can you get it down to 0V?

>> Scope the video output from the CPU board (on pin 2 of the monitor PCB). Is
>> it a good TTL level signal?
> Yes. Given that it’s TTL and a 3-wire connection (composite plus H&V sync) I

You mean 'video', not 'composite'. Composite is an analogue signal with the
video information and the syncs all combined.

> should be able to feed that into a PET monitor shouldn’t I? Wish I hadn’t given
> away my ‘big’ TRS80 Model 4 :/

I have no idea if the sync polarities are right, but you can try it with a Pet
monitor. A TRS80 Model 3 has the same monitor as the Model 4 of course.

(And I wish I could find a 4P over here....)

>> Measure the voltage on pin A of the CRT base PCB. Expect around 60V here
> 60.3V


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

What happens if C101 is removed? Also try running it with TR101
removed (or at least the emitter lead of that transistor disconnected).

>> Have you checked the TR103 on the CRT base PCB?
> It tests as 2 diodes which is correct for a bipolar transistor I believe.

Well, it's not open or shorted which is good. Doesn't mean it has
any gain, but for the moment let's look elsewhere


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