Here's the story: The CRT is good. The logic board is good. The analogue
cables are good. The problem is, when I switch on the 128k, I get a bong,
but the screen is completely dark. No static, no pin prick, nothing. Here's
Here are a few things to try (if you haven't already):
1) Very carefully inspect the tall, 3.9 (or 4.7 in some runs) uF
*nonpolar* electrolytic capacitor near the top of the analog board.
It'll be held down with a blort of hot glue. The top surface of the
capacitor (the shiny can part) should be absolutely flat. If there's any
bulging, it's toast. If this capacitor fails, it fails open, and
prevents generation of HV. The replacement part is a little hard to find
at times, and not all that cheap. And it's the wrong thing, anyway. To
replace, simply take 4 or 5 1uF ceramic capacitors (rated at 50V is a
good idea), and put them in parallel. There's plenty of room for them,
and they're *much* more robust than the ridiculous electrolytic that was
designed in there in the first place.
2) Double check the 4-pin yoke connector. These often corrode, both at
the mating surfaces, and where the connector solders to the analog
board. Resolder, with fresh solder (don't cheat), the connector to the
analog board even if you can't see any obvious problem. Keep the
connectors mated when you do this, to prevent heat from distorting the
shells.
3) The suggestion to check out the HV transformer is a good one. I've
included pin-to-pin resistance measurements in the schematic set at
Gamba's site (home.earthlink.net/~gamba2). Click on the link to
schematics, then download the pdf of the Plus Analog Notes (or whatever
it's called; it's been long enough that I don't remember what I named
it). The resistance numbers are a rough guide. Don't suspect a problem
unless your numbers are *very* different from the ones given there
(i.e., 20% differences are not important; factors of several are).
4) Checking the horiz. output transistor (HOT) is a good idea, too. You
can't make meaningful measurements in circuit, however, because a
transformer winding is connected across B-E. You'll need to de-solder it
to test it. You'll test three pairs of connections: B-E, B-C, and C-E.
You should see a diode in the first two, and an open circuit in the
third test (when C is positive wrt to E).
Good luck, and let us know how you make out.
--
Prof. Thomas H. Lee
Center for Integrated Systems, CIS-205
420 Via Palou Mall
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu
650-725-3709 voice, -3383 fax
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