25 Nov
2017
25 Nov
'17
10:15 p.m.
On Sat, Nov 25, 2017 at 11:02 PM, Adrian Graham via cctalk
<cctalk at classiccmp.org> wrote:
You mean 'video', not 'composite'. Composite is an analogue signal with
the
video information and the syncs all combined.
Fine.
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
-tony
Evening!
Can you get it down to 0V?
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 :/
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.