Hit 120 will not mute

Hello everyone, I have a nsm hit 120 fitted with a 35M amplifier, the jukebox as been working perfect untill a few weeks ago when i noticed interferance from the selector buttons and from the contacts as the carriage moved along the rack, at this time the amp was muting, after a few days the amp now fails to mute at all. I have replaced the amp with a second hand unit but the problem is still there, could this be a control unit problem? Everything else works fine. I would be very grateful for any help with this problem, also does anybody know where i might get a service manual in English.
Many thanks, Jeff.