This is a follow-up to my other thread, where the CFFA1 corruption was solved by replacing the four 74S257 with 74LS257:https://www.applefritter.com/content/cffa1-—-data-corrupted-only-cards-sram-range-1000–8fff
The machine ran fine after that. Meanwhile, I got another cool card, the Vossi's IECmem card, which can supply both the RAM and the ROM from its own SRAM/EPROM: https://github.com/vossi1/Apple1-IECmem . I wrote a whole new DOS and menu for it and it worked well. Today, I removed the E–W wire on the patch area and made a W–1 bridge instead, to get contiguous RAM (bank W at $1000), as the IEC board provide a basic rom.
And the machine refused to boot after that....
Symptom since then: power-up shows the normal _@ pattern, CLEAR SCREEN gives a single blinking @, and RESET does nothing at all ,no keyboard response. Same with the keyboard unplugged and RESET done by shorting pin 1 to pin 9.
Restoring the original E–W jumper did not fix it. This is when I began to panic....
What I have checked:
- All four supplies correct, no ripple, +5V at the CPU stays good under load.
- CPU: /RES high and drops on reset, clock present, R/W, A13–A15 and D0–D7 all toggling. A second 6502 behaves the same.
- PIA (6520): supplies, /RES, E and chip selects all fine, but PB0–PB6 all sit around 1.85V, i.e. still configured as inputs — so Wozmon never gets as far as initialising them I think.
- Patch area: continuity correct, no shorts between 0/1, W/X or 1/2.
The interesting part. I plugged Vossi's IECmem card in the machine, which can supply both the RAM and the ROM from its own SRAM/EPROM: https://github.com/vossi1/Apple1-IECmem
with all 16 DRAMs and both PROMs removed, the X–0, W–1 and Y–F pads open, and the card supplying RAM (DIP 1) and the Wozmon (DIP 7), the machine does reset, prints "" and accepts keystrokes — but the characters come out wrong and erratic, and nothing can be run. With the keyboard unplugged, a reset produces random characters on screen.
Putting back either the DRAMs alone or the PROMs alone — even with those pads still open, so they are never selected — kills it again: no reset.
So: CPU, PIA, terminal and keyboard all work, every testable chip passes, supplies are clean, and yet the machine only comes to life (sort of) when the motherboard supplies neither RAM nor ROM — and even then the data is unreliable.
ha, and today, it's my birthday, what a nice present, I was so happy to have a working Apple 1 at last, well, it's life :-)