I want to home brew an external DDS VFO for my Drake Tr-4 I understand all the DDS and Arduino stuff. That is easier on my end. I am trying to figure out how Drake implemented their external VFO with the TR-4 through the built in connector. Please see the attached schematics. It looks like when the external VFO is in receive and transmit mode (thus replacing the built in VFO) there is no change to any of the pins other than the signal going out pin 6 due to the V1 filament turning on. Am I misunderstanding the switch diagrams? This pin 6 is attached directly to the internal VFO Q2 buffer through R148 (middle of schematic). I am not seeing how the internal VFO gets “turned off” when the external is being used. The signal goes in to the grid of V1b. Anyone know how this works? I need to be able to tell the TR-4 to use the external signal on pin 6 of the connector (middle of the second schematic, top left of the first schematic). Thanks.