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.