Konami G.I. Joe arcade pcb repair

Very clean board but booted to corrupt screen with continual watchdog resets.  Like most Konami games of this era the watchdog can be disabled by bridging the jumper near the edge connector.  With the resets taken care of the 68000 CPU was stuck and not running – however it wasn’t stuck in HALT mode, instead it was waiting for a DTACK signal.  When the 68000 writes to an address it waits for the receiver to signal it has received the data via DTACK.  (Not all games do this, some just tie DTACK low and assume whatever was written got there).

There are no schematics for this game so I had to spent some time tracing out the board.  DTACK is driven from the LS00 NAND gate at 4g.  The LS00 is driven from the LS74 at 2g and the LS30 at 7g.  A logic probe on these chips revealed a dead output on one side of the LS74.  With a replacement soldered in the game was perfect.