What was found
AIRCRAFT MASTER CAUTION AND CABIN ABOVE 10,000 SEGMENT ILLUMINATED AND PAX OXYGEN MASKS DROPPED. EMERGENCY DECLARED, AIRCRAFT DESCENDED AND RETURNED TO DEPARTURE, UNEVENTFULLY. AIRCRAFT FLOWN ON MEL, UNPRESSURIZED. FOUND CLAMP AT SECONDARY HEAT EXCHANGER FAILED. CLAMP REPLACED, PRESSURIZATION SYSTEM CHECKED NORMAL, AIRCRAFT RELEASED TO SERVICE. (K)