Yesterday I finally got the NTE6889 transceiver ICs, that were going to replace the faulty 8T28 IC's in my Apple II+. However, when trying to boot from disks, my Apple still crashes. And NO, it is not a disk problem. I have tried moving the disk drive + floppy to another Apple, which can boot fine.
My Apple will crash only after reading the floppy disk for some time. Different floppy disks will make it crash differently, but it is always the same type of crash for a certain disk. E.g. if I boot my DOS 3.3 system disk I get this:
0021 - A=23 X=FF Y=40 S=10 P=94
Sometimes the first value (the adress of the BRK instruction?) is different, but usually it is one out of three or four values. Another type of crash is for my Lode runner disk. The screen is cleared, and a very brief error message (?) is shown:
and then the speaker makes a low-frequent tone for a second or so, then the Apple seems to do a soft reset (if anyone can confirm this, I'd be grateful). It will start booting from disk again, then crash, reset, and so on, ad infinitum. I tried a couple more disks but I don't remember exactly how the crashes looked, only that they were not identical.
My theory is one or more faulty RAM chips. It would explain why the Apple crashes only when booting from disk, and the slight inconsistency from crash to crash (with the same disk). I got the idea of writing a short machine-code program for testing the whole RAM memory, but I haven't tried it yet. Isn't there a self-test function in the monitor or something?
Any ideas are appreciated!