More broken Apples...
ard at p850ug1.demon.co.uk
Sun Apr 26 12:05:37 CDT 2009
> 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
> ' = 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.
More information about the cctech