SNK NeoGeo 1 slot arcade pcb repair
This 1 slot booted up with ‘Z80 error’. A previous owner had obviously tried to fix this and had removed the old Z80 and installed a socket and new one. With the Z80 removed though it was clear a tiny trace had been lifted off the board when the work was done. I removed and replaced the socket and repaired the trace but still got ‘Z80 error’.
Making sure all address and data lines from the Z80 reached the ROM was the next step, and indeed another broken trace relating to the original socket was found. Still ‘Z80 error’ after this was fixed. All address & data lines to the RAM were then checked with no problems found. Without much to go on I desoldered both the ROM in case the program was corrupt, and the RAM in case it had failed. Both tested fine off-board.
Next up is to try and check if the Z80 can communicate with the 68000, as if it can’t the 68000 will always print “Z80 Error”. On all but the early 1 slots a custom chip handles this – NEO-C1. The top left corner of this chip takes the 8 data lines from the Z80 to pass info to the 68000. I checked continuity from these 8 pins back to the Z80 data pins and only 4 were connected… It seems 4 of the lines had corroded near the battery although they visually looked reasonable. 4 patch wires were soldered underneath the board and everything works! Most likely this was the original fault all along, and the Z80 itself was fine to begin with.