Your problem is not likely an electronic one but a mechanical one. There is a muting switch under the tonearm that is activated by the arm movement. It likelly needs its contacts cleaned. The service manaul is at https://beomanuals.com/manuals/Beocenter/Beocenter%207700/Beocenter_7700-Service_Manual.pdf. The part is 1567 in the parts list but the diagram marks it as 1557. You open it by removing screws in the rear and it hinges from the front.
