Hey all, an Apple newbie here.
I took up checking my uncle's Apple IIc, that's spent the last 30-35 years in a cupboard.
The computer's actually in a pretty good shape, it's got the monitor, the stand and an external disk drive. All apparently in working order even after all these years. It's a PAL machine with the original 255 ROM.
There are several 5.25" disks in multiple formats. A few original Apple disks and the rest are copies.
The problem is the computer won't boot anything but DOS3.3 or custom bootloader disks (as far as I understand). PRODOS disks get stuck in the screen where it says "PRODOS <version numbers>" with a message saying "** Unable to load basic.system **" (or whatever .system file the disk is trying to boot). Pascal disks get stuck in a black screen a couple of seconds into the boot. Mission Impossible also loads for a few seconds and gets to the title screen, but then just stops loading and just sits there. I've managed to get a few simple games to work, for example Dig Dug.
I've cleaned the drives, tried booting from the external drive, swapped the external drive with the internal one, adjusted the drive speed (for some reason I can boot the Locksmith 5.0 disk and use the drive speed calibration program), popped out all socketed chips, cleaned their pins with IPA and reseated them, measured the power supply (+18V with no load and +16V when the computer's running) and gone through all the disks. All the PRODOS disks are unable to boot in both drives, so I don't think it's the disks or the drives that are faulty.
I made an ADTPro serial cable and was able to bootstrap the ADTPro software, but when I tried to receive a disk image, the transfer gets stuck on either the first block or the fifth.
I started the machine with no disk, did the ctrl-reset thing, and tried the serial cable by running IN#2 and PR#2 commands (with the ctrl-A ##B to set the bit rate). I was able to write perfectly fine from the PCs terminal program and the letters appeared on the IIc's screen. When I tried it the other way around and typed on the IIc, the PC only got garbled text with accented letters, e.g. "ÉÁÓÍÁ" and so on. No matter what bps, parity or stop bits I used, I never got correct letters when typing from the IIc to the PC.
There's one more constant symptom, that's appearing in several situations: four pixels in a vertical formation, like two : -characters on top of each other. They're always in the same place, in the lower left quadrant of the screen, sometimes even when a program is running.
Since I've done everything I can think of outside of replacing chips or other components, I'm in need of advice.
Could these symptoms be caused by failed RAM chips? They're all MT 4264-15's, which are prone to failing, if I've understood correctly. Or could there be a fault somewhere else?
So far nothing I've done hasn't made the IIc work any better or any worse, so at least the problem is constant.
Thanks in advance!