Retro Hardware Diagnostic Flowchart
Step-by-step fault finding for the C64, Game Boy DMG-01 and ZX Spectrum 48K
Retro Hardware Diagnostic Flowchart
Choose a machine and a symptom, then work through each test with a multimeter or logic probe. Every step names the exact chip, pin number and expected reading. The pinout inspector on the right shows the chips referenced by the current step — click a pin for its signal and voltage. When you reach a diagnosis, copy the summary to share on a retro computing forum.
How to Use These Flowcharts
Retro fault finding goes wrong when it starts with a guess about which chip has died. These flowcharts are built the other way round: you pick the symptom you can actually see, then take measurements that eliminate whole groups of causes at a time. A blank screen on a Commodore 64 has perhaps a dozen plausible causes, but three voltage readings and a look at the reset line rule out most of them before any socket is touched.
Check the power supply first — and treat it as a suspect
On vintage machines the PSU is not just a prerequisite, it is a common cause. The original C64 brick is notorious: the 5 V regulator inside fails by drifting upwards, and the over-voltage destroys RAM, the PLA and other logic. If a machine has just died for no obvious reason, measure the supply on a dummy load before plugging it back into a board you have just repaired, or you will lose the replacement chips too. The same caution applies to the Spectrum’s on-board rails.
What each machine tends to fail on
The Commodore 64 concentrates its faults in the PLA (906114-01), the RAM bank, the CIAs and the character/kernal ROMs, with a black or garbled screen the usual first sign. The Game Boy DMG-01 is more mechanical than electronic: missing columns or lines on the LCD are almost always the solder connections on the display ribbon rather than the LCD itself, while power and audio complaints usually trace back to tired electrolytics. The ZX Spectrum 48K has a quirk worth knowing before you start — its lower 16 K of 4116 DRAM needs −5 V and +12 V as well as +5 V, both generated on the board itself. If that little supply circuit fails, the machine appears to have catastrophic memory failure when the memory is fine.
Record what you measure
Write down actual readings rather than “looked OK”. A rail at 4.7 V is not a pass, and a reset line that never releases tells you more than any chip swap. The export option lets you paste a complete measurement trail into a forum thread, which is far more likely to get a useful reply than a description of the symptom alone. And when swapping chips to test a theory, change one thing at a time — two simultaneous substitutions have lost more afternoons than any single faulty IC.
If the machine is due a service anyway, the recap and repair reference has verified capacitor lists for these boards, and the resistor and capacitor calculator will confirm the markings on whatever you pull off.