[Solamar]

Ok Let's try the grid cap of the 77 IF amp tube. Can you hear the 400cy tone when connected there? If you can measure the voltage on pin 2 and 4 of the 6A7. If you can't measure pins 3 and 2. All of these pins should have some HV on them. If not you may have a bad (open) IF transformer.

Frequency generator connected to grid cap of my IF amp tube (it's a 78) - Yes, I hear the 400cy tone (tuned to 550kc) and can peak it with trimmer 4.  Voltage at 6A7 pins 2 and 4 is 110 vdc.  Voltage at 6A7 pins 2 and 3 is 186 vdc