What was found
WHILE SITTING ON THE RAMP, AIRCRAFT POWERED UP WITH THE APU RUNNING WITH THE AIR ON AND THE DOOR CLOSED, FOR ABOUT 20 MIN. STARTED GETTING A BURNING SMELL, HAD HINTS OF ELECTRICAL, NOT APU BLEAD AIR OR OUTSIDE EXHAUST. SHUT THE AIRCRAFT DOWN IMMEDIATELY CORRECTIVE ACTION: REMOVED HEPA CABIN AIR FILTER P/N:9914836-1, S/N:1594. INSTALLED NEW CUSTOMER SUPPLIED HEPA CABIN AIR FILTER P/N: 9914836-1, S/N:1567. REF CE 680A AMM 21-70-10. OPS CHECK GOOD, NO DEFECTS NOTED