Vintage Synth Repair

So did I get this right:

At first the machine seems to be completely locked up?

Then after a while the controls (buttons, LED's and so on) start to work
but there's still no sound?

There's one thing to keep in mind: The processors of the voice boards need
their reset pulse, too. They are supplied with that pulse from the main
processor board. So break out the oscilloscope and measure the situation on
pin 9 of all three processors.
If you have the service manual the reset procesure and the components are
described on page 2b.