Of f Topic Short-Wave Radio Maintenance Question

An audio friend who is not computer literate has Short-wave radio which uses silver conductors which touch a cylinder to gain access to the frequency (my apologies; this is a layman's description second hand), and the conductors have deposits. He thinks he can access the silver points to clean them, but wants to know what might be used to clean the silver part without damage.

Any repair experts comment? Much appreciated.