I have an old, un-enhanced Apple IIe that that would give me errors while trying to load disks, like: UNABLE TO LOAD PRODOS or NO BUFFERS AVAILABLE . Now that I know a lot more about the Apple IIe (and electronics in general) than when I put it in mothballs, I tried my hand at fixing the problem again.
I recently leaned that by holding down the closed (filled) apple, while turning on the un-enhanced Apple IIe, it will do a self diagnosis. Here were the results:
RAM: F13 F12 F11 F10 F9 F8 F7 F6
This indicates that every built-in RAM chip was bad. I didn't quite believe this could be true, so I tried replacing some of the ICs around the RAM... to no effect. So, I splurged on an ebay auction for a set of "MICRON MT4264-10 64K 100NS DRAM". I replaced all of the RAM chips, and my un-enhanced Apple IIe sprang to life for the first time in 25 years! As it turned out, only 2 of the RAM chips were bad, but they both failed in a way that would cause the self-diagnosis to report that all of the RAM was bad. Anyway, based on my machine having 2 chips that failed in the same way, I think it must be a relatively common problem.
Now all this thing needs is a "V" key and a replacement key switch. I think the V key is around here somewhere...