What was found
DEN - FLT 122 - CAPTAIN REPORTED HE WAS UNABLE TO PRESSURIZE THE AIRCRAFT IN CLIMB-OUT. RETURNED TO FIELD AT DEN. THE FLIGHT LANDED WITHOUT FURTHER INCIDENT. MAINTENANCE FOUND THE RT PACK FROZEN AND REPLACED THE RT 35 DEGREE TEMP SENSOR (PN B7040-2), S/O VALVE (PN 321942-1-1), AND CONTROLLER (PN 607510-3). RAN ENGINES AND FOUND LT PRSOV BACKING DOWN AT HIGH POWER WHICH LED TO REPLACEMENT OF LT HIGH STAGE REGULATOR (PN 107484-5), PRSOV REGULATOR (PN 107492-2), AND PRECOOLER CONTROL VALVE (PN 3289562-5). RAN ENGINES, SYSTEM CHECKS OK. (X)