Jan Beta recently tackled the repair of a 250469 Revision B Commodore 64 short board, an interesting late-production model from 1991. This board was donated in unknown condition, but all chips appeared original based on their date codes. Unlike earlier short boards, this revision integrates the color RAM into the Super PLA, a design shift that could influence troubleshooting.
Initial Inspection and Power Testing
A quick visual check suggested no modifications or previous repair attempts. Hoping it wouldn’t be a rare case of a completely working board, Jan powered it up. Predictably, it displayed a blank screen—a common failure mode in C64s. The next step was checking voltages.
Using the user port as a test point, the 9V AC rail was fine, but the 5V DC rail was slightly low at 4.57V. That was enough reason to investigate further. While probing components, he noticed one RAM chip heating up significantly—an immediate red flag. Excessive heat usually means an internal short, which can drag the voltage rail down and disrupt the entire system.
Replacing Faulty RAM Chips
The first suspect RAM chip was desoldered and removed. A quick voltage test showed some improvement, but still not quite right. Jan replaced it with a fresh 41464 RAM chip, powered the system on again, and… nothing. The second RAM chip was also burning hot—another casualty of what seemed to be an overvoltage event. After removing and replacing it, the voltage readings looked much healthier at 5.17V, but the system still refused to start.
Investigating Further: CPU or KERNAL ROM?
At this point, the blank screen suggested a deeper issue. Jan tested with a dead test cartridge, which flashed four times—often an indication of bad RAM, but in this case, the newly installed chips should have been fine. Running another test, the diagnostics cartridge started but then crashed. Probing the CPU’s address lines with an oscilloscope revealed erratic signals, leading Jan to suspect the processor itself.
Out came the soldering iron again. The original 8500 CPU was removed and replaced with a known working 6510. Unfortunately, this didn’t solve the problem, meaning the issue lay elsewhere.
Finding the Real Culprit
Checking the KERNAL ROM’s data lines, Jan found unstable signals, confirming that the ROM was interfering with normal operation. Since the KERNAL ROM is critical for booting, replacing it was the logical next step. A previously prepared EEPROM with a stock C64 ROM image was installed, and at long last—the C64 booted to a blue screen!
Final Testing and SIT Chip Failure
With the system running again, full diagnostics were run using a test harness. Everything checked out—except the SID chip, which failed the sound and control port tests. Swapping in a Nano SwinSID provided functional, if not entirely authentic, audio.
Conclusion
This repair involved multiple failures—two bad RAM chips, a faulty KERNAL ROM, and a dead SID. Given the symptoms, an overvoltage event likely damaged these components. Fortunately, everything except the SID was repairable with available parts. After testing a few games via a Kung Fu Flash cartridge, the C64 was confirmed fully functional.
