A B737-200 OUTFLOW VALVE FAILED TO CLOSE AFTER TAKEOFF CAUSING A MASTER CAUTION LIGHT FOR AIR CONDITIONING/PRESSURIZATION. FLT RETURNED TO DEP ARPT.

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ON DEP OUT OF ZZZ; SHORTLY AFTER TAKEOFF WE NOTICED THAT THE CABIN WAS CLBING AT THE RATE OF THE ACFT CLB. SHORTLY THEREAFTER; THE MASTER CAUTION LIGHT CAME ON FOR AIR CONDITIONING/PRESSURIZATION. WE LEVELED OFF AT 8000 FT MSL AND RAN THE ABNORMAL CHKLIST. WE WERE UNABLE TO CTL THE OUTFLOW VALVE IN EITHER AUTO OR MANUAL AND THEREFORE RETURNED TO ZZZ. THE OUTFLOW VALVE WAS IN AN OPEN POSITION AND WOULD NOT RESPOND TO THE POSITION SWITCH INPUTS. WE MADE A NORMAL LNDG AND ON TAXI IN; TRIED TO POSITION SWITCH AGAIN AND IT WORKED NORMALLY. CONTRACT MAINT CHKED THE OUTFLOW VALVE AND IT APPEARED OK. WE AGREED TO START UP AND CHK IT OUT AND RETURN TO ZZZ1 IF PRESSURIZATION WAS OPERABLE. IT WAS; AND WE FLEW TO ZZZ1 WITHOUT INCIDENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE CABIN ALT WARNING HORN DID NOT SOUND BECAUSE THE CABIN REMAINED BELOW 10000 FT. THE OUTFLOW VALVE REPLACEMENT AT THEIR FINAL DEST APPEARS TO HAVE CORRECTED THE PROBLEM. RPTR STATED THAT THIS ACFT HAD SEEN EXTENSIVE SVC IN AFRICA UP TO ABOUT 1990 AND SINCE HIS ACR HAS BEEN OPERATING IT THEY HAVE EXPERIENCED NUMEROUS ANOMALIES. IN THIS PARTICULAR CASE THEY SUSPECT ICE ON EITHER THE OUTFLOW VALVE ITSELF OR ON THE ELECTRICAL CANNON PLUG TO THE OUTFLOW VALVE. THE ACFT WAS EXPOSED TO FREEZING PRECIPITATION EARLIER ON THE GND AND THE RPTR BELIEVES THAT SOME COULD HAVE ACCUMULATED IN THE OUTFLOW VALVE AREA.