What was found
DURING CLIMBOUT, AIRCRAFT WOULD NOT PRESSURIZE. CABIN CLIMBED AT SAME RATE OF CLIMB AS AIRCRAFT. FOLLOWED COCKPIT OPERATING MANUAL PROCEDURE. AFTER RESETTING CIRCUIT BREAKERS, SYSTEM BEGAN TO OPERATE INTERMITTENTLY. FLIGHT RETURNED TO MSP AND LANDED WITHOUT INCIDENT. REPLACED PRESSURE CONTROLLER AND AMPLIFIER, OPERATIONAL CHECK OK.