BryanMcPhail.com

Professional software development, amateur BMW tinkering, old arcade stuff

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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.

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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.

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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.

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I removed a RAM chip from another broken NeoGeo and swapped it in.  All tests pass!

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Added the 4 slot board back on top (the lower board can run on it’s own), all 4 slots and audio tested good.

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