What was found
AFTER APU WAS STARTED, APU AIR PLACED IN MAX COOL. ABOUT A MINUTE AFTER SELECTING AIR, A MUFFLED "POP" WAS HEARD, & A STRONG BURNING ODOR FILLED CABIN. NO SMOKE VISUALLY, AND ODOR CONTINUED TO INCREASE. APU SHUTDOWN, AND ODOR CLEARED. A VISUAL INSPECTION WAS DONE OF APU COMPARTMENT, AFT EQUIPMENT BAY, & APU EXHAUST, WITH NO VISUAL SIGNS OF FIRE. TROUBLESHOT TO A FAULTY TURBINE COMPRESSOR (ACM) ON CABIN PACK. REMOVED FAULTY ACM & INSTALLED REPAIRED ACM. OPS & LEAK CHECKED GOOD IAW MM 21-51-00.