A tricky Commodore PET repair: tracking down 6 1/2 bad chips
12 hours ago
- The Commodore PET was one of the 1977 Trinity home computers, along with the Apple II and TRS-80.
- The author attempted to repair a non-working PET for nostalgic reasons, but found it challenging due to notorious chip failures.
- Initial diagnostics showed random characters on screen, indicating the monitor, video RAM, character ROM, system clock, and power supply were working.
- Using an oscilloscope and logic analyzer, the author traced problems to bad ROM and RAM chips, which are now unobtainable.
- Two ROM chips failed and were replaced with adapter boards using 2716 EPROMs; however, one EPROM had a flaky byte due to under-powered programming.
- The logic analyzer revealed address errors caused by the flaky ROM byte, leading to wrong boot messages and a checkerboard character.
- After finding and replacing three bad RAM chips, the system booted but with reduced 6K memory; another RAM later failed.
- Reprogramming the faulty EPROM with a more powerful programmer finally fixed the boot message and checkerboard issues.
- In total, the repair required replacing 2 ROMs, 4 RAMs, and re-programming one flaky EPROM; the 6502 CPU was fine.
- Lessons learned: test all chips early, and while a logic analyzer is helpful, analog faults can mislead readings.