What was found
AFTER TAKEOFF, CREW DISCOVERED ACFT WOULD NOT PRESSURIZE AND AIR CONDITIONING SYS WOULD ONLY PRODUCE HEAT. RETURNED TO DEPARTURE. LANDED WITHOUT INCIDENT. UPON ARRIVAL, DISCOVERED CIRCUIT BREAKER FOR RT MODULATION VALVE WAS TRIPPED. RESET TRIPPED BREAKER AND PERFORMED AN OPS CHECK OF SYS. CIRCUIT BREAKER REMAINED SET AND SYSTEM OPS CHECKED GOOD. ACFT CONTINUED ON TO DESTINATION WITHOUT FURTHER INCIDENT. ON ARRIVAL, ACFT INSPECTED AND AN OPS CHECK OF THE SYS WAS PERFORMED IAW MM. RT MODULATION VALVE CIRCUIT BREAKER REMAINED SET AND SYS OPS CHECKED GOOD. ACFT RETURNED TO SERVICE.