26 Apr
2009
26 Apr
'09
10:05 a.m.
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
IIRC the Apple ][ text display was upper case only. The fact that your
clone does lower case means there must be hardware differences between it
and a real Apple. Which means, alas, the schematics for the latter aren't
going to be a lot of use.
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 would have expected ti to clear the screen to spaces at startup. That
_may_ be 0010 0000 if the machine uses true ASCII codes in the video
memeory. So maybe just bit 6 is playing up.
As others have suggested, reseat all the socketed ICs. If in doubt,
replace the sockets!. Then, assunming this thing uses 1 bit wide DRAMs
(4116s, or similar), change the arounds. See ig you can make the stuck
bit move somehwere else. If so, you know it's a RAM chip problem.
-tony