As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
THE PILOT DETECTED AN ELEC BURNING SMELL IN THE COCKPIT ON APPROACH. ONCE ON THE GND, HE CALLED IN THE MECHANIC TO INSPECT THE ACFT. FOUND THAT THE TRANSPONDER HAD BEEN THE SOURCE OF THIS SMELL & WAS INOPERATIVE. PLACED THE TRANSPONDER ON ORDER & DEFERRED THE SYS IAW THE AMC BK117 SERIES MINIMUM EQUIPMENT LIST. THIS DEFERRAL REMAINS IN USE AT THE TIME OF THIS SUBMISSION AWAITING A REPLACEMENT TRANSPONDER & THE SUBSEQUENT RE-CERTIFICATION.