grbrady
Experienced Member
Hi There,
I'm working on a PET 2001-16B (2001N motherboard with "business" keyboard, "CBM 2001 Series" branding on the front label.) After a long time of replacing bad ICs, sockets and remediating corroded solder, etc., it seemed like I had it all working and I was ready to put it back in the case. However, when I put it in the case, instead of showing ~15k bytes free it shows 45 bytes free. When I first started troubleshooting this the problem was intermittent and seemed like tilting the board one way or another would cause it to start up seeing the full complement of memory, which makes me suspect a bad solder joint or other poor connection somewhere. However, I can't seem to find it. All the address lines from the CPU, through the buffers, muxes and resistors seem to have good continuity and I've reflowed all those joints. I've swapped around and replaced the buffers and muxes as well and don't see any change.
I have also probed all of the address and data lines with an oscilloscope and don't see anything terribly unusual. Signals seem to be present where I would expect to see them, although the high logic level is only typically about 4 V, but I think this should be fine for TTL logic.
I've also run PETTEST v.4 on the machine. It got past the countdown and into the memory test, but finds an error shortly thereafter. However, the address and bit at which it finds the error seems kind-of random. Example output from a series of attempts:
mem fail 0 0 02f7 02 42!
mem fail 1 0 0275 80 7f!
mem fail 0 0 206e 20 20!
mem fail 1 0 0275 04 ff!
mem fail 1 0 042e 01 08!
mem fail 0 0 02f7 80 00!
mem fail 1 0 042e 02 09!
mem fail 1 0 042e 02 09!
Looking at this now, I see a few addresses up there multiple times which might be significant.
I have not replaced the memory yet, although I have piggy-backed 4116s on all of the memory chips without any change. Is my best bet to start socketing and swap out the memory? Is there anything more I should try short of that?
Can anyone suggest any strategies for figuring this out?
Thanks,
Greg
I'm working on a PET 2001-16B (2001N motherboard with "business" keyboard, "CBM 2001 Series" branding on the front label.) After a long time of replacing bad ICs, sockets and remediating corroded solder, etc., it seemed like I had it all working and I was ready to put it back in the case. However, when I put it in the case, instead of showing ~15k bytes free it shows 45 bytes free. When I first started troubleshooting this the problem was intermittent and seemed like tilting the board one way or another would cause it to start up seeing the full complement of memory, which makes me suspect a bad solder joint or other poor connection somewhere. However, I can't seem to find it. All the address lines from the CPU, through the buffers, muxes and resistors seem to have good continuity and I've reflowed all those joints. I've swapped around and replaced the buffers and muxes as well and don't see any change.
I have also probed all of the address and data lines with an oscilloscope and don't see anything terribly unusual. Signals seem to be present where I would expect to see them, although the high logic level is only typically about 4 V, but I think this should be fine for TTL logic.
I've also run PETTEST v.4 on the machine. It got past the countdown and into the memory test, but finds an error shortly thereafter. However, the address and bit at which it finds the error seems kind-of random. Example output from a series of attempts:
mem fail 0 0 02f7 02 42!
mem fail 1 0 0275 80 7f!
mem fail 0 0 206e 20 20!
mem fail 1 0 0275 04 ff!
mem fail 1 0 042e 01 08!
mem fail 0 0 02f7 80 00!
mem fail 1 0 042e 02 09!
mem fail 1 0 042e 02 09!
Looking at this now, I see a few addresses up there multiple times which might be significant.
I have not replaced the memory yet, although I have piggy-backed 4116s on all of the memory chips without any change. Is my best bet to start socketing and swap out the memory? Is there anything more I should try short of that?
Can anyone suggest any strategies for figuring this out?
Thanks,
Greg