Hey, keirf! I know that name!
After copying the ADF to a floppy using his GreaseWeazle hardware (but my FluxEngine software!) and finding the memory tester, I see the following.
https://imgur.com/a/IKJCLBz
I'm going to guess that whichever chip is handling bits 12-15 isn't getting selected properly when address bit 0x080000 is asserted.
Unfortunately it's DRAM, which means the 20-bit address is split up into row and column and probably divided by two anyway to allow 16-bit accesses, so actually figuring out what line is causing problems is going to be non-trivial. I don't know yet whether the eight 4-bit RAM chips are divided into upper-half-megabyte/lower-half-megabyte, which might mean a bad chip, or if they represent even and odd 32-bit words, which means the chips would have to be fine for the low half megabyte to test correctly.
Looks like it's back to the continuity tester tomorrow...
Thanks!