Cause: The music center can produce two types of humming sounds: (i) a hum which in
fact is a 'pilot tone' that is automatically recorded onto the tape during RECORD mode, and
audible in PLAYBACK mode at the end of a recorded track, and (ii) an intermittent hum resulting
from a failure of the head-switching mercury relay. It's the Type 2 hum that is relevant
Remedy: If one of
the mercury relays fails, the only solution is to replace the failed mercury relay with a modern,
conventional one. (See also 'Trouble with relays').