As technology continues to evolve, many enthusiasts remain captivated by the charm and nostalgia of vintage home computer systems. Among these iconic devices is the Commodore PET, a beloved computer model that has become a staple in the history of personal computing. However, with the passage of time, these systems often exhibit a variety of issues stemming from their aging components, which may not always be addressed in the official repair manuals. This is notably true for the 2001 series of the Commodore PET, which computer repair expert Ken Shirriff recently undertook to restore.

The initial assessment of the Commodore PET was relatively straightforward. Upon powering it on, the display emitted a series of random symbols across the CRT screen. This indicated that, while the system was functional at a basic level, there were significant underlying issues, particularly with the integrated circuits (ICs). Fortunately, the power supply appeared to be operational and the basic display routines were intact. Given these signs, it seemed likely that only a handful of faulty ICs and possibly some capacitors needed replacement for the system to regain full functionality.

In the initial stages of the repair, it became evident that two defective MOS MPS6540 ROM chips required replacement. These were substituted with 2716 EPROM chips utilizing an adapter to ensure compatibility. However, replacing these chips did not resolve the original problem, as the display continued to exhibit erratic behavior.

To further diagnose the issue, Shirriff employed a logic analyzer, which revealed that three additional RAM ICs were faulty. Addressing these faulty RAM chips significantly improved the display issue, yet some strange artifacts remained, such as a peculiar 22 checkerboard pattern. Furthermore, the computer exhibited bizarre behaviors when running BASIC programs, indicating that the issues were far from resolved.

Through continued analysis, it was discovered that the 6502 microprocessor (MPU) was erroneously writing data to incorrect memory addresses. Ironically, this problem stemmed from a byte error in one of the newly replaced 2716 EPROMs. The programmer used during the initial replacement was not capable of achieving the necessary programming voltage, which led to this oversight. Once a more reliable programmer was utilized, the issue with the EPROM was rectified.

However, just when progress seemed to be made, yet another RAM IC failed during the next boot cycle, bringing the total count of defective components to four RAM ICs and two ROM ICs. This experience underscored a crucial lesson for anyone involved in tech repairs: always thoroughly test replacement components before integrating them into a system.

This repair journey not only showcases the challenges faced when restoring vintage technology but also highlights the ingenuity and persistence required to breathe new life into these classic machines. As the community of retro computing enthusiasts continues to grow, stories like Shirriff's serve as a reminder of the joys and trials of preserving computing history.