SNK NeoGeo 2 slot arcade pcb repair #2
This Neo 2 slot was stuck constantly resetting, which means the 68000 CPU is not resetting the watchdog timer, so the board continually tries to reboot itself. A previous owner had clearly changed out some faulty surface mount RAM previously so I spent some time verifying all data and address lines from RAM/ROM to the CPU were correct as some looked like they could be shorted. Turned out fine however. Without much more to go on I tried out the diagnostic ROM which surprised me by booting and showing a VRAM error. In this case both of the 6116 fast vram type chips that often go bad. Both chips desoldered, sockets added and replacements fitted. Now the diagnostic ROM reported no faults, but the board kept on resetting with the original ROM installed.
I dumped the original ROM and found it to be corrupt – no wonder the CPU kept crashing – really I should have tested that first. Some kind of internal addressing fault as in the dump you can see some ASCII text where the reset and stack vectors should be. Unibios was burned to a replacement eprom and everything worked again!