A B737-300 CREW; AFTER LEVELING AT FL350; EXPERIENCED A LOSS OF CABIN PRESSURE; AND INITIATED AN EMER DSCNT TO 7000 FT AND A RETURN TO DFW.

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THIS WAS THE ACFT'S FIRST REVENUE FLT AFTER UNDERGOING A HVY MAINT CHK IN TAMPA. DURING PREFLT; SOME MINOR MAINT WAS REQUIRED ON SOME PREVIOUS MCO'S AS WELL AS SOME NEW WRITE-UPS. PREFLT; TAXI; TKOF; CLB; AND LEVELOFF WERE NORMAL. A FEW MINS AFTER LEVELOFF AT FL350; THE MASTER CAUTION LIGHT AND THE OVERHEAD LIGHT ON THE MASTER CAUTION ENUNCIATOR PANEL ILLUMINATED. THE PAX OXYGEN 'ON' LIGHT ON THE OVERHEAD PANEL WAS FOUND TO BE ILLUMINATED. CABIN ALT WAS FOUND TO BE APPROX 15000 FT; WITH THE CABIN CLBING AT APPROX 300 FPM. THE AUTO FAIL LIGHT ON THE PRESSURIZATION CTL PANEL WAS ILLUMINATED. THE CABIN ALT WARNING HORN FAILED TO SOUND AT 10000 FT CABIN ALT. THE STANDBY LIGHT ON THE PRESSURIZATION CTL PANEL WAS NOT ILLUMINATED UNTIL THE PRESSURIZATION CONTROLLER WAS SWITCHED TO THE STANDBY MODE. AN EMER WAS DECLARED; AND ALL APPLICABLE CHKLISTS WERE PERFORMED. DURING DSCNT; A RETURN TO DFW WAS COORDINATED WITH ATC. AN ATTEMPT WAS MADE TO CONTACT THE DISPATCHER THROUGH ATL RADIO; BUT WE WERE UNABLE TO CONTACT ATL RADIO. AFTER LEVELING OFF AT 7000 FT; THE R PACK AIR MIX VALVE WAS OBSERVED TO BE IN THE WARM POS AND WOULD NOT RESPOND TO TEMP CTL INPUTS. THE L PACK WAS PUT IN HIGH AND THE R PACK WAS SHUT OFF IN ORDER TO CTL CABIN TEMP. DURING THE RETURN TO DFW; THE APCH; LNDG; AND TAXI WERE NORMAL. THE DFW CHIEF PLT MET THE CREW AT THE GATE AND CLRED US TO CONTINUE THE TRIP. SUPPLEMENTAL INFO FROM ACN 564711: AFTER LEVELING AT FL350; THE AUTO MODE OF THE CABIN PRESSURIZATION CONTROLLER FAILED. AN EMER WAS DECLARED AND EMER DSCNT AND AUTO PRESSURIZATION FAILURE CHKLISTS WERE ACCOMPLISHED.