As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CRUISE, CABIN RATE OF CLIMB RAISED QUICKLY TO 10,000 FT, FLIGHT ENGINEER RAN PNEUMATIC TO HIGH AND OPERATED THE PRESSURE CONTROL VALVE. THE CREW WAS PERMITTED PARTIAL CONTROL OF CABIN PRESSURE UNTIL ACFT DESCENDED BLOW FL200. THEY RETURNED TO DEPARTURE WITHOUT FURTHER INCIDENT. MX REPLACED CABIN RATE CONTROLLER. OPS CHECK GOOD IAW MM 21-31-2 AND 21-21-0. ACFT WAS RETURNED TO SERVICE.