Better Tools, Faster Fixes
In this Retrobits episode, two faulty Commodore 64s are diagnosed using a powerful C64 diagnostic upgrade—the DesTestMAX cartridge by Matt Desind. Traditional dead test cartridges rely on screen flashes to hint at faults. DesTestMAX skips the guesswork entirely.
This tool delivers readable output and doesn’t even need working RAM. Instead, it displays messages directly from the cartridge and uses color-coded borders to highlight multiple faults at once.
Seven RAM Chips Gone
The first case is a longboard 64C in excellent cosmetic shape. Internally, however, it’s a disaster. Seven of its eight RAM chips have failed. Normally, diagnosing that many issues would require multiple test cycles and chip swaps.
With the C64 diagnostic upgrade, DesTestMAX instantly reveals each failed chip. Users can even move working RAM chips across sockets to verify motherboard logic. This saves time and effort during repairs.
A Reset Circuit Mystery
The second Commodore 64 boots to a black screen. Even DesTestMAX can’t display anything. Voltage checks pass, so the focus shifts to the reset circuit.
This machine features an early 326298 board with wiring that doesn’t match official schematics. Instead of routing the reset signal through an inverter, the board connects a 555 timer directly to the CPU. That’s not documented anywhere.
Tracing the Real Fault
After testing each component in the reset circuit, only one issue stands out: a failed capacitor. Replacing capacitor C24 restores normal reset timing. The machine now boots perfectly.
This fix proves how a simple component can prevent an entire system from working. It also shows why older schematics can’t always be trusted.
What Viewers Can Expect
This episode highlights how a C64 diagnostic upgrade speeds up repairs and uncovers hidden hardware problems. It also explains the reset circuit in detail—something that’s often overlooked in C64 troubleshooting.
Whether you’re a casual tinkerer or a seasoned restorer, this video delivers useful tips and real repair insight.
