Good call on the AGC. I found some really weird voltages throughout the keyed AGC circuit leading all the way back to the flyback.
At one point my DMMs overload indicator was lighting up even though I only showed about 150 VDC. I switch to AC mode and the reading were off the scale.
That leads me to suspect something in the circuit where a pulse is picked off the flyback for the key pulse. Possibly a 120 pF 5,000 volt cap on the flyback frame.
I popped the plate cap off the HOT to kill the HV and test my theory. Bingo! We have sound
I'll have to disassemble the set again and go poking around for the culprit.