> The power to operate a transponder may have come from a Hydraulic Motor Generator (some 767's have three of them, one on each hydraulic system).
Qavion; actually some 767's have one HMG (now called HDG by the new Boeing books). The HMG comes on-line automatically (after a 10-15 second delay) with the loss of both left and right AC buses. To power the HMG the center hydraulic system's ADP (Air Demand Pump) is signaled to operate (the selector must be in Auto).
The HMG powers 7 buses. The Left (#1) transponder is powered, as is the Captain's flight instrument bus for a full EFIS compliment.