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.