TRS80 Model 4P CRT contrast fail

[Adrian Graham]

> On 26 Nov 2017, at 06:15, Tony Duell <ard.p850ug1 at gmail.com> wrote:
> 
> On Sat, Nov 25, 2017 at 11:02 PM, Adrian Graham via cctalk
> <cctalk at classiccmp.org> 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?

Yep, it runs from 0-11.82V.

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

Indeed. My brain was saying ‘video’ but my fingers typed ‘composite’ :)

>> 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.

I’ll make up an extension cable I think, got more PETs than I know what to do with. I used to have multiple Model 3s AND a Model 2 but they had to go when I moved to a smaller house back in 2010.

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

Perhaps embarrassingly I can’t remember where mine came from but given I’ve had it since the early noughties that’s not surprising. It may even have been from a car boot sale.

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

Nothing happens, in that the picture is the same as it is with both those components fitted.

Cheers

A