RPTR SETS UP ALT ALERTER FOR CLB TO FL350. ATC ADVISES TURB AT FL350. RPTR REQUESTS AND RECEIVES FL310. ON DSCNT BACK TOWARD FL310 RPTR FAILS TO SET ALT ALERTER TO FL310. WITH MANY DISTRS RPTR DSNDS 1300 FT BELOW FL310 BEFORE CTLR ALERTS PLT OF DEV.

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THIS IS A B737-300. WHILE CRUISING AT FL310; ATC CLRED US TO FL350. AT THE SAME TIME THEY ADVISED US OF TURB AT FL350 FOR ABOUT THE NEXT 80 MI. WE REQUESTED AND RECEIVED CLRNC TO MAINTAIN FL310. HOWEVER; WHEN WE WERE GIVEN THE FL350 CLRNC; I SET THE MCP ALT TO FL350 BEFORE RECEIVING THE MAINTAIN FL310. I DID NOT RESET THE MCP ALT TO FL310. SHORTLY AFTER THIS WE ENCOUNTERED LIGHT TURB. I DESELECTED VNAV TO SLOW OUR AIRSPD. AT ABOUT THE SAME TIME A FLT ATTENDANT CALLED. I ANSWERED; AND WHILE TALKING TO THE ATTENDANT ATC ASKED US TO CONFIRM OUR ALT. BY THIS TIME WE WERE ABOUT 1300 FT BELOW OUR ASSIGNED ALT. WE CLBED BACK TO FL310 WITH NO TFC CONFLICT; AND CONTINUED TO OUR DEST WITH NO FURTHER PROBS. THIS WAS A CASE OF 1) POOR PROC; 2) BEING DISTRACTED; AND 3) NOT MINDING THE STORE. RECOMMEND MORE EMPHASIS BE PLACED ON KEEPING ASSIGNED OR CLRED TO ALT IN THE MCP ALT WINDOW. IN THIS CASE WHEN I DESELECTED VNAV; IT REVERTED TO CTL WHEEL STEERING AS IT SHOULD. THE PROB; I DID NOT NOTICE IT BECAUSE OF THE FLT ATTENDANT'S CALL.