13 Dec
2015
13 Dec
'15
10:22 a.m.
Yes, I've certainly seen that on the schematics, and was considering them
as a likely failure too - after opening cases though, my drives all seem to
have a single larger SRAM (even the Rev A board, so that's presumably "Rev
A of the type that doesn't use 2114's" :-)
Yes, later versions used a single 6116 RAM (which as you said is not likely
to be the problem).
I have had a ROM failure in a 1541. I am pretty sure mine now has an EPROM
in a home-made kludgeboard (to move the enables round, etc) in it.
I think at this point you have to do real tests, there is no stock fault. I do
have the Sams book on the 1541 somewhere, but IIIRC it suggests swapping
out the socketed devices (ROMs, RAM(s), VIA, CPU, etc. If that doesn't help
then you pull the CPU and force the logic levels of the address pins to check
the address decoder circuitry (but the address decoder almost never fails).
I am pretty sure the firmware is not changed for different PCBs. I would keep
ROMs in pairs just in case there is a different revision and it matters, but I think
you could put the ROMs from the defective drive into a good one and see what
the LED does.
-tony