Namco Galaga 88 arcade pcb repair
Game booted to distorted text. Although it wasn’t readable I suspected the error related to the custom key chip, a security chip used in these boards. Swapping the keycus with a known working one enabled the game to boot, confirming the failure. Luckily the program ROMs can be patched to ignore the keycus completely.
The distorted look applied to all tilemap graphics in the game as well as the text. A logic probe on the graphics ROMs showed one of the address lines was stuck – that was the reason for the blockiness – repeated pixels because certain addresses could not be accessed. The tilemap and text is driven by the 123 custom and all signs pointed to that being the cause of the stuck address line. I swapped it with one taken from a Final Lap board and… no change. That sucks. It seems I didn’t check the inputs well enough to that custom as there was a dead input – which in turn caused the stuck output. Although this custom generates most of the address lines for the tiles LN0, LN1, LN2 come from the CPU board. This was a physical fault – the pin for LN2 was snapped off inside the connector. With that patched everything was fine for the main tiles. The text layer had a little extra fault which was corrosion under the ROM socket. One of the data lines sometimes did not make good contact between the text ROM at U8 and the 123 custom.