SNK Neo Geo 4 slot arcade pcb repair
This Neo Geo 4 slot motherboard reported ‘VRAM error’ on boot. There is a useful diagnostic rom at http://smkdan.eludevisibility.org/neo/diag/ so I burned this to board to see if it gave more info. Well.. not really, still a VRAM error at location 0000.
Despite the popularity of these boards I couldn’t find a guide showing conclusively what each RAM chip is on the board, perhaps because they change about depending on the many different motherboard revisions.
Eventually I worked out it was one of the top two surface mounted chips. The VRAM attaches to the graphics custom chip, rather than the 68K cpu directly. The error is a stuck data bit which is quite a rare error for a RAM – often this means a bad trace so I examined all the traces from the RAM to the custom chip and didn’t find anything. The worst case is that the custom chip itself is bad. As I didn’t yet really know which of the two RAMs was the upper (top 8 bits) or lower (bottom 8 bits, of which one was stuck) I desoldered them and swapped them. The error moved from the lower RAM to the upper RAM which pretty much confirms a bad RAM chip rather than traces or custom chip.
I removed a RAM chip from another broken NeoGeo and swapped it in. All tests pass!
Added the 4 slot board back on top (the lower board can run on it’s own), all 4 slots and audio tested good.