What was found
(CAN) FLT 146 AFTER DEPARTURE, CABIN ALTITUDE WARNING HORN SOUNDED. CREW EXECUTED CABIN ALTITUDE WARNING / RADID DEPRESSURIZATION CHECKLIST & CABIN PRESSURE STABILIZED AT 12,000 FT. AIRCRAFT DESCENDED TO 26,000 FT, FLT ATTENDENTS DONNED PORTABLE OXYGEN MASKS & CHECKED ON PASSENGERS. ONCE A/C AT 26,000 FT CREW SWITCHED PRESSURE CONTOLLER TO STANDBY AND THE CABIN SLOWLY DESCENDED TO 10,000 FT. A/C CONTINUED ON & LANDED W/O FURTHER INCIDENT. MAINT PERFORMED A/C PRESSURIZATION TEST IN ACCORD WITH MM & ELECTED TO CHANGE CABIN PRESSURE CONTROLLER. SYS GROUND CHECKED SERVICEABLE. A/C RETURNED TO SERVICE.