Hi
@daver2 , no luck swapping the RAM banks sadly. I think I'll have to use your PETTESTER ROM to figure this out!
While the RAM bank swap test can sometimes be helpful, it has its limitations, similar in a way to piggybacking IC's as a diagnostic method. The reason is that two RAMs one in each bank share the same data pin, so considering a RAM pair in bank 1 or 2, if either is faulty in a manner that its output is interfering with the other, then electrically swapping the banks doesn't help. For the computer to boot normally it requires that all RAMs in the lower (electrically selected) bank are normal and that the ones in the upper bank are not interfering with them.
This is similar as noted to why piggybacking RAM IC's sometimes cannot help either. One thing about DRAM IC's they are internally highly complex (compared to SRAM) and have all sorts of different failure modes, where they can either fail to respond and not interfere with the bus, or other modes where they can interfere with the bus sometimes and not others, made worse by the fact that Commodore didn't use pull-up resistors on their outputs, and other times simply have stuck outputs. And they can have refresh issues too.
Generally I find the computer's machine language monitor (TIM) the most useful tool for inspecting and diagnosing the RAM, but it requires that the lower 1k (or better 2k) of RAM is functioning flawlessly (often not the case for defective DRAM). So my diagnostic system electrically substitutes in known good SRAM on the expansion connector. But the Romulan Ramulators and PETtester ultimately system gets the same or better results.
With a functioning computer, examination of the RAM will always lead to the location of the defective chips, however, in a worse case scenario, it still comes down to two possibilities (two suspect chips) because the pairs of the in the upper and lower bank, share the same output pin. In that case one of the two can gave its output pin solder sucked and freed up in the hole, to make the final determination.
Many PET owners, because of the frustration of the DRAM problems, remove and socket all of them and buy a DRAM tester too and then fit known good chips, but its more of a sledgehammer approach to deal with a defects with soldered in DRAM, but to deal with it otherwise requires investment in some form of diagnostic hardware and or ROM programmers. For those already blessed with sockets, its just a matter of fitting good chips and the computer itself then can test any suspect chips by putting them as a trial in any position in the lower bank.
