What was found
ON FLIGHT 3531 CREW REPORTED DURING APU START ALL ELECTRONICS & RELAYS WERE CLICKING ON & OFF & THERE WAS A BURNING SMELL. AFTER DE SELECTING THE GPU THE SMELL WENT AWAY GPU SHOWED 28.4 VOLTS. THE ACFT WAS REMOVED FROM SERVICE. POWERED ACFT ELEC SYS DOWN & INSPECTED ELEC COMPARTMENT BY DVI & FOR BURNING SMELL, NONE WAS NOTED. RE-SET ELEC POWER & STARTED APU SYSTEMS, OPERATING NORMAL IAW AMM 24-31-00. THE ACFT RETURN TO SERVICE.