As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
UPON TAKEOFF THE PILOT NOTICED A BURNED ODOR IN THE COCK-PIT. ALL CIRCUIT BREAKERS WERE IN, NOTHING TRIPPED. PILOT NOTICED THAT THE TRANSPONDER WAS DISPLAYING A "FAILURE" ON THE SCREEN. TURNED POWER OFF TO TRANSPONDER, AND CAME BACK AROUND FOR A SAFE LANDING. MX DETERMINED THAT THE BURNED ODOR WAS IN FACT COMING FROM THE TRANSPONDER.