As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED. THE CREW REPORTED A STRONG ELECTRICAL BURNING SMELL THROUGHOUT THE AIRCRAFT WHEN PACKS TURNED ON WITH POWER TRANSFER FROM EXTERNAL POWER TO APU. MAINTENANCE ACCOMPLISHED MCM CARD 21-96-01 AND NOTED ODOR FROM RECIRCULATION FAN. REMOVED AND REPLACED THE RECIRCULATION FAN IAW AMM 21-20-12. OPERATIONAL CHECK NORMAL.