What was found
AIR TURNBACK, DURING FLIGHT AT 37,000 FEET, CABIN ALTITUDE WARNING HORN SOUNDED ALERTING THE CABIN ALTITUDE WAS 10,000 FT. DECLARED EMERGENCY LANDING. CABIN ALTITUDE NEVER GOT ABOVE 10,000 FT AND THE DESCENT WAS NORMAL. INSPECTED ALL DOOR PRESS SEALS, PRESS DUCTINGS, C/W BITE CHECK ON PRESS CONTROLLER, PRESSURIZED ACFT AND PERFORMED LEAK CHECK. NO LEAKS FOUND AND PRESS SYSTEM OP CHECKED GOOD. LATER AT HOU, REMOVED AND REPLACED PRESS RATIO AUTO CONTROLLER PER MM.