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Apple III "Diagnostic RAM" but only on power cycle

ravuya

Active member
Just got my heavily contaminated Apple III logic board cleaned up, plugged into my (mostly) recapped power supply and fired it up. The floppy drive and keyboard are not connected, because the keyboard needs a little more TLC and I didn't want to get involved with floppy drive madness at this point in the quest.

On the first startup I get the message "RETRY" and a blinking cursor, which I believe means it's waiting for me to insert a disk and strike a key. That makes sense, and I'm happy that it's working as well as it was (it was very filthy.)

I did a power cycle by flipping the switch off, waiting a second, and then flipping it back on. This time I get a "DIAGNOSTIC RAM" screen, where exactly half of the RAM on the zero page(?) is fingered as bad.

apple-iii-diagnostic-ram-failed.jpg

I remember reading that if your keyboard isn't attached (or the little neon bulb is blown out) that you will get stuck in the diagnostic, but I'm not sure if the diagnostic will manifest like this – I haven't been able to find that info again on google searches at the moment. And presumably it's getting "past" this diagnostic on the cold start, because I get to the "RETRY" screen.

I'll start working the schematic to figure out if it's something simple that interacts with exactly half the RAM.
 
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