Capcom Last Duel arcade pcb repair
Game played but with lines through sprites and corrupt sprite colours. Diagnosis was quick – it was clear a mask ROM was missing on the video board, and strangely it’s sister ROM was inserted with a pin outside the socket.
The board has pins for both a 23c2000 16 bit mask ROM or two 8 bit 27c301 ROMs, probably so the factory had flexibility to fit whatever type was cheaper during the production run. But.. the 27c301 is a 32 pin IC, and the mask ROMs fitted are only 28 pin.
This borrowed picture shows how it is possible – the top 4 pins are VPP (programming voltage), VCC (+5V), OE (output enable), PGM (enable programming). The 28 pin mask rom doesn’t need the programming pins, and it omits OE in favour of just using CE (chip enable), so it always outputs when enabled. The +5V line then just runs to the NC (not connected) spare pin on the 27c301. So this way the board can use both the 28 pin or 32 pin ROM with no other modification. In this case I used a fresh 27c301 and everything was fixed.