What was found
SMOKE IN THE CABIN ON THE GROUND WITH THE APU RUNNING & BLEED AIR ON. DISCONNECTED BLEED AIR DUCT COUPLING & BORESCOPED APU INLET COMPRESSOR AREA & BLEED AIR PORT FOR OIL LEAK. NO OIL LEAKAGE NOTED AT THIS TIME. CONNECTED BLEED AIR COUPLING, LEAK CHECK NORMAL. WORK PERFORMED IAW CHAPTER 49-10-00 OF THE MM. POWERED UP ACFT TO RUN APU & SMELLED BURNT WIRING SMELL FROM COCKPIT. ISOLATED BURNED ODOR TO AFT CLOSET. GAINED ACCESS TO CABIN RECIRCULATION FAN. REMOVED CABIN RECIRCULATION FAN & OPS CHECKED. FOUND RECIRCULATION FAN BLOWING BURNED ODOR. INSTALLED REPAIRED EXCHANGE CABIN RECIRCULATION FAN IAW MM 21-22-03. OPS CHECKED NORMAL.