Where does the transponder get power from on the 767?
It depends on the individual aircraft. On some of our old 767's, we used to have the Left ATC powered only by the 115Vac Left Main Bus and on the rest of the 767 fleet, it was powered by the 115Vac Left Main Bus with backup from one of the HMGs (Hydraulic Motor Generators). You would need to look at the wiring diagrams/schematics for that particular aircraft.