Yet Another Retro Channel, aka YARC, is back with a new Commodore 64 repair mojo session, and this time the project involves rescuing a long-forgotten machine from years of attic storage. Covered in grime and corrosion, the computer looks rough on the outside, but the big question remains: can it be brought back to life?
The repair begins with a careful inspection of the case and motherboard. The board inside is a 250425 revision, notable for its reversed positions of the PLA and SID compared to earlier models. Dust, rust, and corroded shields signal that this system has seen better days. Before applying power, checks confirm the fuse and regulators are intact. With that, the first attempt to start the computer results in nothing more than a black screen.
To proceed, the board gets a full cleaning. After washing, drying, and retesting, low voltage on the 5V rail points to a faulty power switch. Replacing it brings improvement, but display output remains inconsistent. Suspicion quickly falls on the PLA chip, a common point of failure in the Commodore 64. Testing with a new oscilloscope finally reveals the culprit: outputs that are either stuck or floating incorrectly. Swapping in a replacement PLA restores video output.
Diagnostics are the next step. A full RAM test passes, followed by the standard Commodore diagnostic harness. The SID chip even surprises by producing working sound during testing. Running a game confirms the system is alive again, though voltage irregularities continue to raise concerns.
Thermal checks uncover a failing logic chip drawing power as it overheats. Replacing the 74LS258 resolves the issue, stabilizing voltage close to the required levels. With these fixes complete, the computer runs consistently, and both sound and graphics behave as expected.
The project highlights the challenges of working on aging hardware, where multiple failures often overlap. A dead PLA, a worn-out switch, a faulty logic chip, and even a blown fuse had to be addressed. Yet through methodical testing and persistence, the Commodore 64 was restored to working condition.
In the end, this repair was less about a dramatic discovery and more about YARC regaining momentum after time away from the bench. With the machine back in working order, the channel is ready to move on to the next retro challenge.
