Midway Mortal Kombat arcade pcb repair
Game booted fine, but everything was tinged red.
Luckily schematics for this game are available so this was a quick fix. Schematics show the palette RAM connects to a pair of 74ALS574 chips, which go to a pair of 74ALS541 chips which then go to a resistor network to form the analog RGB output. A logic probe showed one of the inputs at U3 was dead, but the corresponding output on the palette RAM was pulsing away.
So what looked like a tiny scratch on the PCB surface was actually a deep gouge that had completely disconnected a trace line. This was patched and the PCB 100% working again.