26 Apr
2009
26 Apr
'09
4:38 a.m.
Well, I dragged those Apple IIs out of storage, and spent this evening
poking at them, with some success. Thought I'd post my results here, and
hope that someone has some suggestions for me.
There are three machines in total; one an Apple II Europlus, the other
two generic II+ clones.
The first thing I did was run them up - all dead, garbage on the screen.
Same as last time I looked at them. Step 2 - pull all the cards and try
again. After that, one of the clones ran up fine; turns out that the
'language card' (16k memory expansion) was killing it. Good stuff - one
running Apple II, even if it's not an original.
Unfortunately, the other two weren't quite so cooperative. I decided to
look at the other clone first, as its behaviour was pretty interesting.
The screen was entirely full of apostrophes to begin with, but randomly,
blocks of them would change to lower case 'p's, and back, flickering
very quickly. It responded to a reset by changing the pattern of ps,
though they tended to appear in the same place. I found the ASCII values
interesting:
' = 0x60 = 0110 0000
p = 0x70 = 0111 0000
So, on reset, perhaps it's clearing half the bits per byte, and the
other four have a problem. Reseting the machine tended to lead to random
behaviour for a bit, such as random display changes, and speaker clicks.
At one point, the display switched to high-res mode, and I could see
that a large amount of memory had the same sort of pattern through it,
and was flickering the same way text was; I guess the entire memory
space is like that. Perhaps bits 5 and 6 are permanently stuck.
I checked the CPU's behaviour; on reset, the address and data lines
would run for at least a short time, so I'm guessing the CPU is likely
to be OK. Often they would continue to run, at which time I'd see the
flickering. Other times, activity would stop, and the flickering would
also stop, usually leaving a screen full of 's.
So, my next step is to track down a schematic for the machine, and see
what I can figure out. I'm guessing I should look first at anything that
deals with the high four bits of memory. Thoughts, suggestions and
intuitive guesses welcome!
Cheers,
Mike