Thanks for the help in connecting two boards. As for removing oxidation from the terminals, I used "Electrosolve Contact Cleaner" by MG Chemicals from Amazon (15.4 fl oz for$14). The stuff worked great. I sprayed the terminals outside and with a respirator, after reading the warning label. I bought some covers for the unused RCA jacks to reduce the rate of oxidation and dust deposit.
Too bad Channel Master is NLA: after trying several other products of its kind, CM was the one and only product than proved reliable as long-term protection against audio connection issues.