What was found
AT 39,000 LOSS PRESSURIZATION, PROCED FOR EMERGENCY DESCENT. RETURNED TO MIAMI. AT ARRIVAL AIRCRAFT WILL NOT ACCEPT EXTERNAL POWER. FOUND CLAMP TO NR 3 T/C DUCT HAD FAILED, CAUSING DUCT TO COME LOOSE. RESECURED DUCT WITH NEW CLAMP AND PERFORMED OPS CHECK OF PRESSURIZATION SYSTEM. OPS CK'D NORMAL PER MM 21-30-01. REPAIRED WIRE IAW 21-51-00 AND REMOVED AND REPLACED EXTERNAL POWER RELAY. CHECK NORMAL IAW MM 24-40-0.