Data East Bad Dudes arcade pcb repair #2
This pcb booted to obviously corrupt colors on the title and hi-score screen, but then in-game all the sprite colors and most of the level colors were correct. At first I assumed this would just be corrupt graphics ROMs but they all read with the correct CRCs. I switched a Hippodrome game board onto the Bad Dudes main board as they are compatible and the Hippodrome title screen was now corrupt, so this narrowed the fault down to the main board rather than the game board.
Because the sprite colors were good it’s unlikely the palette RAM was at fault as all layers share that RAM. The custom graphics IC’s output directly into the palette circuit on the board, so hoping the custom IC’s weren’t bad I piggybacked spare chips into the palette section as shown in the schematics. (If a chip on board has dead outputs the ‘piggyback’ gets the same inputs and can drive the outputs again). Success! I removed each piggybacked chip one by one until only one was left – LS157 at 17H. Old chip was removed and replacement soldered in and board 100% again.