What was found
FLIGHT 4264, CREW REPORTED THAT CLIMBING-OUT OF 27,600 FL CABIN PRESSURE WAS LOWER THAN CHART ON 21-24. INCREASED TO STAY ON CHART. AT 3000FL CABIN ALT LOST. INITIATED AN EMERGENCY DESCENT, DECLARED AN EMERGENCY AND COMPLIED WITH AOM VOL 1 CHECKLIST AND DIVERTED TO DEPARTURE. LANDED WITHOUT INCIDENT. THE AIRCRAFT WAS REMOVED FROM SERVICE. TROUBLESHOT IAW FIM 21-31-00, HAD DIFFICULTY WITH THE MANUAL CONTROLLER. REMOVED AND REPLACED THE MANUAL CONTROLLER IAW AMM 21-31-02. OPS CHECKED GOOD. NO FURTHER FAULTS FOUND. DISCREPANCY CLEARED. OUTFLOW VALVES CHECKED GOOD IAW AMM 21-31-00. AIRCRAFT RETURNED TO SERVICE.