25 Jan
2014
25 Jan
'14
11:55 a.m.
Got yet another PET today - a CBM 8032 with the black bezel. In my PET
collection I'm trying to get, instead of each model, a representative of
each body style - and there seems to be no shortage of that. Of the five I
now have, each is different in some way.
Anyway, I'm confident the 8032's mainboard is alive - it chirps on startup.
But that's all it does. Nothing appears on the screen. I took the screen
hood off - the tube lights up, so there's power and action. If I disconnect
the data cable from the motherboard, I get a single dot in the center of the
screen.
THat last sentence is ambiguous. Do you mean that if you pull the cable
off once the machine is running youy get a dot on the screen (and does it
remain or fade out?) or do you mwean that you get a dot i nthe middle of
the screen if you power the machine up with the video cable unplugged for
mthe motherboard? If the latter, or if the dot doesn't fade out, I am
totally puzzled...
Anyway, here's hwat I would do...
Firstly, determine if the problem is in the monitor or motherboard.
There are, IIRC, 3 TTL level signals from the motehrboard ot monitor.
Horizontal drive, Verticla Sync, and Video. Check them witha logic proble
(much better than a 'scope here, IMHO, at least to start with, since what
you are really looking for is signals that are stuck in one state). If
the video signa lis never being assettedm, the screen will be blank (it
should be!) but no amount of wortk o nthe monitor wil lget it going.
Assuming the motherboard is outputting video, the quickest thign to do is
to measure the CRT pin votlages. IIRC, tue PETs use a 7 pin (modified
B7G) based CRT>
The pinout is (at least for every such CRT I've seen) :
1 Control Grid
2 Cathode
3 HEater
4 HEater
5 Control Grid
6 First anode
7 Second Andoe (focus)
One of pins 3,4 is likely to be ground. Expect about 11.5V-12V on the
other one. Expect a few hundred V o nthe 2 anodes, about 40V-70V on the
cathode, and ratehr less (may even be -ve) on the control grid.
Ifd all the higher voltages are misisng, suspect problems in the
horizotnal section (these voltages come form the flyback transformer),
but as you can get a dot on the screen, this is unlikely. If one is
missing, uspect the associateed rectifier/capacitor.
Anotehr thing that will remove the display is to great a -ve bis o
nthecotnrol grid. That is, the grid is too low in voltage _or the cathode
too high_. It might be aproblem with the video output transistor or
something.
-tony