I have a Neo Geo cabinet with a 4 slot motherboard, of which only 3 slots were working. Games inserted into slot 1 refused to boot. After the usual cleaning of pins, it still did not work. Thanks to great work from Arcade King over on Aussie Arcade this was an easy repair. Referring to the traces published at this thread I systematically checked for continuity for each cartridge slot pin to the various chips on the motherboard. I found that on pin 1B-17 the was no connectivity to D3 pin 18. Put in a trace wire then fired up, and now all slots working!
Copy of the schematic below: