FLC OF MLG RETURNED AND LANDED DUE TO THE FAILURE OF THE MAIN LNDG GEAR TO RETRACT AND THE DOORS TO CLOSE.

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WE TOOK OFF FROM PIT ON FLT FOR BNA. AFTER TKOF; THE LNDG GEAR WOULD NOT RETRACT. SOME GEAR DOORS OPENED AS EVIDENCED BY 'GEAR DOOR LIGHT' AND WIND NOISE; BUT THE GEAR REMAINED DOWN. WE TRIED TO RECYCLE THE GEAR; WITH NO CHANGE. WE ANNOUNCED TO DEP CTL WE WOULD RETURN TO PIT. WHILE BEING VECTORED FOR RETURN; WE GOT OUT THE FLT MANUAL AND WENT THROUGH THE ABNORMAL PROCS. WE ELECTED TO HAVE THE EMER EQUIP STAND BY AND TO COME TO A COMPLETE STOP ON RWY 28L IN CASE WE HAD AN UNSAFE GEAR. AFTER LNDG; MAINT INSTALLED THE GEAR PINS AND WERE ABLE TO RETRACT THE GEAR DOORS. SO WE TAXIED BACK TO A GATE. SOME MINOR SHEET METAL DAMAGE OCCURRED ON LNDG TO THE MAIN GEAR DOORS BECAUSE THEY WERE NOT UP AND LOCKED AT THE TIME OF LNDG. THERE ARE SOME POSSIBLE FACTORS THAT MIGHT HAVE PREVENTED THIS INCIDENT. WE WILL KNOW MORE WHEN MAINT DETERMINES THE CAUSE OF THE GEAR PROB. THEIR INITIAL FINDING WAS THE L SPOILER BYPASS VALVE WAS IN THE BYPASS POS. I'M NOT SURE. THIS ACFT HAD JUST BEEN RELEASED FROM MAINT. THE #2 TIRE AND BRAKE ASSEMBLY HAD BEEN CHANGED. HYD FLUID WAS ADDED AND THE BRAKE SYS BLED.