A B737-300 FLC EXPERIENCES A LOSS OF CABIN PRESSURE CLBING THROUGH FL190. THE FO HAD FORGOTTEN TO TURN ON THE ENG BLEED AIR FOR THE CABIN PRESSURIZATION SYS.

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ACFT DEPARTED DEN ON NIGHT FLT TO SFO. TKOF WAS CONDUCTED IN SOP BLEEDS OFF TKOF CONFIGN. WX WAS A CONCERN WITH TSTMS ON COURSE SO RADAR WAS TUNED FOR TSTM AVOIDANCE. CAPT (PF) HANDLED FLYING AND ATC COM WHILE FO LEFT PRIMARY FREQ TO CALL COMPANY WITH OUTBOUND CALL ON RADIO #2. PRIOR TO LEAVING FREQ; NOW AT ABOUT 10000 FT MSL; FO SECURES APU. AROUND 15000 FT MSL; FO RPTS BACK ON RADIO #1 AFTER CALLING COMPANY. ABOUT FL200; THE CABIN ALT WARNING HORN SOUNDS. FO SILENCES HORN; CREW DISCOVERS CABIN ALT AT 10000 FT MSL AND CLBING. ALL CAUTION AND WARNING LIGHTS ARE NOT ILLUMINATED. CAPT CALLS FOR COCKPIT CREW TO DON OXYGEN MASKS AND ESTABLISH CREW COM. PRESSURIZATION PANEL SWITCHED TO ALTERNATE AND MANUAL TO CLOSE OUTFLOW VALVE WITHOUT SUCCESS. CABIN CONTINUES TO CLB. FO DIRECTED TO QRH FOR 'LOSS OF PRESSURIZATION' CHKLIST WHILE CAPT REQUESTS LOWER ALT FROM ARTCC FOR PRESSURIZATION PROB. AT 14000 FT MSL; THE PAX OXYGEN LIGHT ILLUMINATES INDICATING OXYGEN MASKS IN CABIN DEPLOYED. CAPT REQUESTS RETURN TO DEN AND FURTHER DSCNT. IN TURN; CAPT DISCOVERS BLEEDS/PACKS LEFT IN TKOF CONFIGN. NEVER RECONFIGURED DURING CLB CHKLIST AFTER FLAP RETRACTION. FO MISSED CLB CHKLIST ITEM 'BLEEDS/PACKS CONFIGURED.' APU SECURED WITHOUT CHKING BLEEDS/PACK CONFIGN. AFTER HORN SOUNDED; BOTH CREW MEMBERS MISSED BLEEDS/PACKS IN WRONG POS; CREW FOCUSED ON PRESSURIZATION PANEL. CREW FAILED TO DISCOVER INCORRECT BLEEDS/PACKS UNTIL AFTER MASKS DEPLOYED. HUMAN FACTORS INCLUDE SOME FATIGUE AFTER 3 1/2 HR LAYOVER. FO COMMUTES FROM DETROIT TO DENVER AND HAD PREVIOUSLY SHOWN POOR PERFORMANCE AT THE LATE STAGES OF THE DAY. RECOMMEND COMMUTE DAY PRIOR TO TRIPS. PHYSICAL FACTORS: DARK NIGHT; OVERHEAD PANEL LIT DIMLY. TSTMS AND AVOIDANCE A CONCERN AND OCCUPIED TIME. SUPPLEMENTAL INFO FROM ACN 403145: COMPANY POLICY STATES THAT ALL TKOFS OUT OF DENVER ARE DONE WITH ENG BLEEDS OFF. AT 1000 FT MSL; THE CAPT CALLED FOR THE 'FLAP UP; CLB CHKLIST.' AS I DID IN THE PAST; I EXECUTED THE CHK BY A FLOW PATTERN FIRST. WHILE DOING THAT; WE WERE TOLD BY TWR TO SWITCH OVER TO DEP CTL. THIS CAUSED ME TO STOP IN THE MIDDLE OF DOING MY CHK IN ORDER TO RESPOND TO THE TWR CTLR AND CALL DEP CTL. BY THE TIME I WAS DONE TALKING TO DEP CTL; WE WERE AT 10000 FT; THE ALT WHEN I NORMALLY CALL THE COMPANY TO RETURN THE DEP TIMES. ON THE WAY BACK TO DENVER; THE CAPT NOTICED THAT THE ENG BLEEDS WERE NOT RESET TO THE 'ON' POS; INDICATING THAT THE CABIN NEVER PRESSURIZED. I THEN REALIZED THAT I DID NOT FINISH THE CLB CHKLIST BECAUSE RESETTING THE ENG BLEEDS TO 'ON' POS IS AN ITEM ON THE CLB CHKLIST. (INDICATED ON THE CHKLIST AS ENG BLEEDS -- CHK.) WE THEN RETURNED WITH A NORMAL LNDG AT DENVER. BECAUSE THE PAX OXYGEN SYS WAS USED AND COCKPIT OXYGEN WAS USED; THE ACFT WAS GNDED. WE SWITCHED PLANES AND COMPLETED THE FLT TO SAN FRANCISCO. BECAUSE I ALLOWED MYSELF TO GET CAUGHT UP IN DOING THE RADIOS AND POSSIBLY FATIGUE; I THINK IS THE REASON I FORGOT TO FINISH THE CHKLIST. IN OTHER WORDS; POOR RADIO AND CHKLIST MGMNT.