[Arran]

 I think that I may have the service folder for one of these, or at least for the Canadian version, I will have a look. However I don't think that it is particularly complex electrically to work on, if it has not been tampered with or altered.
  I would say that the cartridge is probably a write off if it is a crystal type, the idler wheel may be alright to use if the rubber isn't rotted, though sometimes the motor shaft will leave a divot in the rubber causing a bump, bump, bump, sound if there is no neutral setting for the changer. Even so the rubber may even out if it is played enough, if not sometimes there are certain rubber "O" rings or belts that can be used to replace the old tire.
Regards
Arran