An unavoidable part of old home computer systems and kin like the Commodore PET is that due to the age of their components they will develop issues that go far beyond what was covered in the official repair manual, not to mention require unconventional repairs. A case in point is the 2001 series Commodore PET that [Ken Shirriff] recently repaired.
The initial diagnosis was quite straightforward: it did turn on, but only displayed random symbols on the CRT, so obviously the ICs weren’t entirely happy, but at least the power supply and the basic display routines seemed to be more or less functional. Surely this meant that only a few bad ICs and maybe a few capacitors had to be replaced, and everything would be fully functional again.
Initially two bad MOS MPS6540 ROM chips had to be replaced with 2716 EPROMs using an adapter, but this did not fix the original symptom. After a logic analyzer session three bad RAM ICs were identified, which mostly fixed the display issue, aside from a quaint 2×2 checkerboard pattern and completely bizarre behavior upon running BASIC programs.
Using the logic analyzer capture the 6502 MPU was identified as writing to the wrong addresses. Ironically, this turned out to be due to a wrong byte in one of the replacement 2716 EPROMs as the used programmer wasn’t quite capable of hitting the right programming voltage. Using a better programmer fixed this, but on the next boot another RAM IC turned out to have failed, upping the total of failed silicon to four RAM & two ROM ICs, as pictured above, and teaching the important lesson to test replacement ROMs before you stick them into a system.
This is dedication
In repairing old computers, these are the most common issues I have found:
1.
Broken RAM chips. Especially MT ram.
2.
Broken ROM chips. Mostly you’ll see that the IC’s pins have turned black. And if you try to pull the IC out of the socket, often one or two pins stay behind in the socket. Completely went brittle. And not just the ‘big’ roms, but also the small ones 7488 etc. I don’t know why, maybe they have a specific technology. I rarely see it on eproms.
3.
Broken 74×244/74xx245/74xx238.
4.
Bodge wires that were soldered with resin core solder. The solder joints often look really good, but I still find that sometimes they are not having a good connection. I noticed it especially on TRS-80’s. But well, maybe the TRS-80’s are the vintage computers with the most bodge-wires in the world anyway. ;)
And if a computer looks brand new as if it was never used, then it probably had a manufacturing defect. For computers like this:
5.
Check the pins of all socketed IC’s. There is a good chance that one pin is curled up or bent.
6.
Check for broken PCB traces. The computer probably went out of the factory seemingly perfectly working, then got shipped, and differences in air pressure and temperature caused the track that was hanging to dear life by its finger nails to sever completely.
Fusible link PROMs can fail because when the fuse is programmed, the passivation layer can bd disrupted, leading to corrosion. The black lead you mentioned could be the result. I don’t see how a mask programmed chip can fail that way, or even be prone to failure. But, if it happens then it does.
I once found the problem in a terminal. It had failed completely. In frustration, I told my partner “it could be any chip. Even that one”. When I touched it, it raised a blister. So yeah…
It used several state machines controlled by fusible link PROMs. 1970s tech.
“but at least the power supply and the basic display routines seemed to be more or less functional.”
I guess… But with a system that old do you really trust the power supply? The last thing you want is to put a bunch of effort into diagnosing the other problems just to have the power supply give up the ghost letting the smoke out of all the out of production, hard to find ICs as it goes.
its a pretty simple linear power supply
GTA Vice City had those funny pyramid computers. TIL it’s commodore Pet.
I’ve found that it’s all too easy to cook traces right off the board on some of that old equipment, while attempting repairs. A particular incident involved a known bad RAM chip that had been soldered on at the factory. To minimize heating on the board I did the old trick of clipping all the pins with diagonal cutters to first remove the chip, then desolder the pin ends individually. All went well until it was the ground pin’s turn. Two things were different for this pin: the ground trace was heavier copper, and unbeknownst the pin was really wedged into the hole. Could be the thicker trace rode higher up the taper of the pin, I guess. Anyway the extra heat needed to melt the joint (because the trace sinking it so fast) combined with the extra force needed to extract the pin caused good cm of the trace to lift off, pin still stubbornly stuck in the copper, an unexpected an unwelcome surprise. Fortunately the trace was intact, the pin was cleared, and the new socket served to hold the trace down well enough.
If I had a do-over, (1) might use low temperature solder to bring down the necessary heat input, (2) apply twisting force to free the pin first, before extracting, and (3) hold down the trace with a popsicle stick or some such while doing the pull. But 14 times out of 16, it’s not going to be that kind of a struggle anyway.
PETs were notorious for random issues when they were moved. The PC board was large and the main chips were socketed. Whenever you had to move one it was standard procedure to open it up and press to re-seat each chip because board flex would make them walk out a bit.
Looks like the motherboard could use some more standoffs/support.
The Sharp MZ-80K was better built here.
It used similar chassis, but quality level was different.
The Japanese cared about quality, while Commodore was just being Commodore here.
My very first job was repairing Commodore PETs. RAM chips and the 6522 IO ports were common failures, and occasionally the character generator ROM. I don’t ever recall seeing a failed PSU.