I have an old transceiver. I have a problem where ocassionally the received signal will drop out when switching from transmit to receive. I've narrowed it down to the transmit/receive relay. When bench-testing the relay, it "clicks" over to receive, but shows a very high resistance. The contacts aren't visibly corroded or damaged. I have heard that the contacts may have to be cleaned - what's the best way to do this? Are there any cleaners that should be avoided?
Cleaning relay contacts is usually done mechanically. Use fine grade garnet paper (not generic sandpaper -- it can leave conductive dust behind), fold a strip in half and put it between the relay contacts, push the contacts together, and pull the paper out. Repeat until done.
To follow up on my comment to Pete's answer, the contacts probably have some oxidation (tarnish) on them, depending on the metal involved.
What you don't want to do is gouge up the contact surface with something that's too abrasive. You just want to clean the surface and restore conductivity. Rag based bond paper works well (a crisp US dollar bill, for instance). Insert the paper, gently hold the contacts together and swipe out the paper. Should only take a few swipes. If the contacts are plated over a base metal, any cleaning process will have a limited number of uses as the plating in these cases is very thin.