In the latest video from More Fun Fixing It, what looked like a straightforward Commodore 64 repair quickly turned into something much trickier. A simple display fault set off a chain of testing, dead ends, and unexpected discoveries — the kind of repair that keeps retro computing interesting and unpredictable.
Viewers of the channel had already submitted their guesses: most backed the character ROM theory. Others suggested the PLA chip, U14, or U26 — all parts connected to the video output in some way.
Swapping the character ROM? No change. Testing the PLA? Still broken. U14 and U26? Both healthy.
This wasn’t going to be a quick fix.
The Real Problem: CIA 2
After exhausting the obvious candidates, deeper testing revealed the real fault — CIA 2 (U2). This chip, better known for handling I/O duties, was responsible for generating video address lines VA14 and VA15. Those signals were missing or corrupted, explaining the display issues perfectly.
Replacing CIA 2 brought the machine back to life.
Bonus Fixes
While diagnosing the main fault, other issues were found and sorted:
- U28 (4066 chip) affecting game ports
- Minor keyboard cleaning and repair
- General inspection and solder touch-up
