Model A2S4000 Serial #D470BDG
The problem
An Apple //c displayed random, flickering characters on the screen. Basic programs would crash. Booting from a disk wouldn't complete. The computer would crash into the monitor.
The fix
The //c (which had been upgraded from the original ROM 255 to ROM 0) had the built-in test program in the firmware. The test is activated by pressing both apple keys, the control key, and RESET simultaneously. Here are some helpful discussions on Applefritter:
http://www.applefritter.com/content/apple-iie-failing-ram-test http://www.applefritter.com/content/memory-address-ram-chip
The comments in the //e firmware listing imply that the bits are listed in the "normal" order, i.e. most significant on the left, least significant on the right. The self-test would sometimes report back GLU error, other times "RAM ZP 0 0 0 0 0 0 0 1".Â
Thinking this was likely to be a memory problem over a GLU failure, I replaced chip 0 in the first bank with a Toshiba 150s 4164. The native //c DRAM chips are Micron 4264, while the enhanced //e I have uses 4164. There is some information that the 4264 is back compatible with 4164, but not the reverse. However, the //e's and //c's should be similar (identical?) in their memory handling and it's likely that the //c doesn't use the unique features of the 4264—whatever they may be (page addressing?)
After replacing the first chip, the self-test then reported "RAM 0 0 0 0 1 0 0 0", so I replaced chip 3 in the first bank. The next system test reported "*RAM 0 0 0 1 0 0 0 0", chip 4 in the second AUX bank. After replacing chip 4 in the second bank, all is well. Here's a shot of the two lower chips and the last two self-test runs.
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