The electrical connectors that Japanese used back in those days will corrode over time. Nothing hideous, the brass that they are made out of just gets a oxidized layer on them. This oxidation creates unwanted resistance in the circuit.
I recommend using one of the following: Flitz, Happich Simichrome, Wenol, or as a last resort, Brasso.
Clean the male ends with the metal polish and a rag.
Clean the female ends with Q-Tips dipped in the metal polish. Sometimes if they are really bad, I snap the Q-Tip in half and chuck it in a drill.
Don't just give them a once over, clean the connectors until the rag or Q-Tip comes out with no more discoloration. Clean the polish out of the connectors with CRC Electrical Contact Cleaner or similar aerosol electronics rated cleaner. DO NOT use carburetor cleaner! Brake cleaner can be used, but is not the best choice.
Not related to the polishing process, but some times the female connectors loose their shape and need to be gently pinched back into shape to provide a secure connection.
