What was found
DURING INSPECTION AFTER MAINTENANCE, SMOKE/ODOR NOTICED IN AFT CABIN AREA. NOTICED SMOKE IN CEILING AREA AND REMOVED POWER FROM AIRCRAFT. DURING INVESTIGATION WITH POWER ON AIRCRAFT, FIRE OBSERVED IN OVERHEAD ELECTRONIC UNIT. FIRE EXTINGUISHED WITH HALON EXTINGUISHER. NO DAMAGE FOUND TO WIRING OR SURROUNDING AREA. SOURCE OF THE FIRE WAS TRACED TO THE TRANSFORMER IN THE OEU (PART NUMBER 285W0029-3). THE OEU WAS REPLACED. THE OEU IS THE SUBJECT OF BOEING SERVICE BULLETIN 777-23A0027 AND AD 97-23-16. THE AD REQUIRES REPLACEMENT OF THE 3 OEU WITH A NEW -5 UNIT. THE NEW -5 INCORPORATES A NEW TRANSFORMER.