What was found
AIRCRAFT PERFORMED AN UNSCHEDULED LANDING AS A CONSEQUENCE OF PRESSURIZATION FAILED. CAPTAIN REPORTED THAT DURING FLIGHT AT 35000 FEET, CABIN PRESSURE INCREASED AND DECREASED 500 FEET, AFTER THAT CONDITION WAS SELECTED PRIMARY AND SECONDARY SYSTEM AND AFTER 10 MINUTES PRESSURE WAS LOST AND "CABIN ALT" LIGHT TURNED ON. CONTROLLERS WERE CHECKED AND FAIL WAS DETECTED ON CONTROLLER NR 2 AND PRESSURE VALVES WERE DETECTED DIRTY. PRESSURE VALVES WERE CLEANED AND TESTED IAW MM 21-32-5, ALL CORRECT. DURING OVERNIGHT SERVICE, CABIN PRESSURE CONTROLLERS NO. 1 AND 2, AND PRESSURE VALVES, WERE TESTED IAW MM 21-32-00, ALL CORRECT.