What was found
LAX INFLIGHT T285/17APR - CREW REPORTED STRONG ELECTRICAL SMELL IN AFT CABIN FOLLOWED BY SMOKE ALARM SOUNDING IN AFT LAVATORY. RETURNED TO LAX AND LANDED WITHOUT INCIDENT. FOUND OIL IN APU AIR DISCHARGE DUCT. REPLACED APU DUE TO OIL LEAK. ACCOMPLISHED CONTAMINATION REMOVAL. REPLACED AIR CYCLE MACHINES AND PRIMARY AND SECONDARY HEAT EXCHANGERS. SYSTEM GROUND CHECKED NORMAL OPERATION.