Konami TMNT arcade pcb repair
Game played, but with an overlay of text over everything. I suspected the tilemap custom chip wasn’t reading from tilemap RAM properly but nothing seemed obviously wrong until I found some bad traces on the bottom of the board. Someone had previously tried to repair them, and it had probably worked at the time but now the thin lines had corroded further. A patch wire on the top of the board was the safest fix.
Board booted to consistent screen of garbage text. I had a hunch this was actually the ROM/RAM check screen – but with the graphics addressing broken in some way so incorrect text was displayed. Logic probe check on tilemap related chips (found via schematics) found a dead input to the LS139 that controlled VRAM chip select.
Pin 3 (A16) comes direct from the main CPU and with this floating it caused garbage to be loaded to VRAM. I couldn’t find any broken traces, but there must have been one somewhere. A wire patch was added underneath the board direct to A16 on the 68000 and everything worked.