On Sat, Nov 25, 2017 at 11:02 PM, Adrian Graham via cctalk <cctalk@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? > >> 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 Fine. > >> 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 -tony
