That hidden connector is for the remote control receiver module IIRC...
These machines are a bird's nest of wiring harnesses and connectors. The wires are mostly solid core, crimped into IDC connectors. They are tin plated rather than gold, and can become unreliable with age in my experience. Be careful if you start disturbing things inside.
A bit more info is required here. Has the machine ever worked properly in your possession, or have you just acquired it in this state? Is it the same with known-good recordings (vs. recordings made now)? Does it also affect E-E sound (I think it does from what's been said so far)? Answers to these questions will help to narrow things down - they are complicated machines, and it would be easy to lose yourself in a rabbit hole
