Got a chance to look at the schematics of the unit (thanks Carl!).

Does the "INT" LED illuminate when the VFO selector is set to "internal"?

If so, do you have a way to look at the VFO's output (pin 5 of J701?) Any signal?

Also check Pin 3 of the same connector - you should see 6VDC there when the VFO selector is set to "internal". Absence of voltage means the VFO won't operate.