TRS80 Model 4P CRT contrast fail

[Adrian Graham]

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.

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

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

It tests as 2 diodes which is correct for a bipolar transistor I believe.

Cheers Tony,

A