Longtime watcher, first time poster. I recently got an Apple II+ from ebay and am trying to get it working. Seller reported the power supply was dead, and that proved to be totally wrong. I've gotton to the point where it will boot up to all question marks (common fault of course), but I do have a power on and reset key beep - so it seems it is kinda working. I have not seen that scenario (beep with question marks) mentioned here - usually no beep when only question marks. Any help on what to focus in on would be appreciated.
Here is the saga so far...
1) Power lamp was dim on keyboard. Ribbon cable had been plugged in backwards, and a pin was broken. Corrected, and power lamp now works.
2) Improper 5V RAM was fitted in E3 (4164) - burning hot. Probably someone had a RAM expansion card at some point. Replaced with correct 4116 RAM (12V).
3) Power on and reset beep, but no video displayed. Isolated to dead D13 (74LS161). Replaced all 4 video counter ICs, video now working.
4) Only question marks are displayed, with some random and flashing characters flashing in a few spots (changes with power cycle - not reset). Computer seems to kinda start-up as beep is heard at both power up and reset button press. Floppy drive seeks at startup, and stops when reset is pressed.
5) Typing on keyboard does nothing - screen unchanged and only shows full display of '?' (no Apple II text), and does not echo keys.
All voltages test correct. Problem persists with floppy card in Slot 6 or not (no other cards in computer). With the exception of the 8304 IC (and 6802), I have removed and tested all logic IC's in a generic 74LS tester. The IC's all auto-detect the proper type and test ok (except D13-74LS161 which I replaced to correct the video issue). I made a 2716 adapter socket and dumped all 6 ROMs with a DataMan 48Pro programmer, comparing with posted files - all match perfectly. Same for the character generator ROM. I have removed extra RAM banks, leaving only 16KB in row C. Same symptoms exist when all 3 banks are populated. All RAM ICs have been tested in a RAM tester (Inquisitor), and I've tried various other RAM ICs as well (Motorola and MOS). Removing one of the 8 RAM ICs does result in a no beep scenario, so it seems the RAM is being accessed ok - at least partially.
I ordered a 'new' 8304. Changing it had no effect. No way to know if the replacement is good, but having no IC present results in a no beep scenario so it seems like it is ok. I have not purchased a 6802 as it seems to be working enough to get a beep, which means a fair amount of code is executing from what I understand. So I'm guessing the CPU is ok and the fault lies elsewhere.
The board is emaculate with no mods top or bottom. I'm suspecting some sort of marginal IC (one that tests ok in a tester but not when stressed in the computer), but I'm not sure what to target for replacement. I only have this Apple II+ and no other computer to start swapping IC's, so any expert advise on what area to scope out would be welcome.
Thanks for any help!