Thanks for your email. I'll answer it here in case it helps others too.
Glad to hear in your email that after a couple of days being left switched on, that it's now making some sounds.
The model 77 dates from 1967. It's a transistor model, no tubes, and the electronics inside will be pretty basic. Nothing high tech at all. The motor you can hear is the rotary speaker going round. They call it 'Theatrical Tremolo' on the 77. Apart from that I can't tell you much. I've not seen a 77 close up but I have played the model 91, which sort of replaced the 77 and it sounded pretty good at the time. That was back in 1971, when the organ was new.
If the only problems are just a few odd missing notes, it might need no more than a little bit of TLC, a service and a clean up inside. Anything more than that might cost more than the organ is worth. Play it a little, see if those notes come back to life.