I spent a number of hours last night and tonight trying to figure out why my modified Apple II+ was not beeping, and instead showing a screen full of question marks (or sometimes a white screen) on start-up, even after I added a row of 4264 DRAM. I swapped all of the ICs back and forth with my functioning Apple II+ and still had the same problem. At that point, I realized that I should have done some basic testing before I modified the logic board. I removed my modifications, and still had the same problem.
I found out that I could get very similar symptoms on my working Apple II+ if I removed the F2 74LS139 decoder, which selects which RAM bank to use. So I started checking continuity on my non-working board. I discovered that here wasn't continuity between the low bank selection line and the row that I had my RAM in. I thought I had found a major fault in the board. Then it hit me. The apple II+ rows are labeled A through K, and start with A on the bottom ROW. I had put my RAM in row E (the high address bank), instead of row C (the low address bank). I moved the RAM chips and after swapping out a bad chip, BEEP, and APPLE ][ at the top of the screen.
I felt so dumb. Hours wasted. So now I'm re-doing my modifications. Still time for some progress tonight. At least it's working. That logic board has't been fully functional in probably 30 years.
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