RPTR ERROR ADMITTED IN RAISING GEAR HANDLE AND NOT INSURING THAT THE GEAR PINS WERE IN PLACE. ACFT DAMAGED.

Narrative

INADVERTENTLY RETRACTED NOSE GEAR ON A BOEING 737-300 ACFT WHILE PERFORMING AN OPERATIONAL CHK ON AN ACFT SYS DURING MAINT. GEAR RETRACTED DUE TO THE REMOVAL OF THE GEAR PIN LOCKS BY THE PREVIOUS MAINT CREW; AND BY ME NOT PHYSICALLY CHKING THE GEAR AREA BEFORE PLACING THE GEAR HANDLE INTO THE UP POS AFTER TAKING OVER THE MAINT OP. FAILURE TO PERFORM EACH STEP OF THE OPERATIONAL CHK AND PHYSICALLY CHKING THAT THE GEAR PINS WERE NOT REMOVED BY SOMEONE ELSE BEFORE ACCOMPLISHING THE PROC. BOEING 737- 300 ARRIVED WITH 'B' SYS HYD FAILURE AND LOSS OF HYD FLUID. REPLACED ENG DRIVEN HYD PUMP; CHANGED FILTERS; RAN ENGS AND ALL ACFT HYD SYS INCLUDING PINNING THE LNDG GEAR AND OPERATING GEAR LEVER UP AND DOWN WITH NO FLUID LOSS FOUND. MAINT CREW COULD NOT FIND ANY FLUID LOSS AND THEREFORE WOULD NOT RELEASE THE ACFT FOR SVC. I TOOK OVER THE TROUBLE-SHOOTING OF THE ACFT SYS AND REPLACED A COMPUTER CARD AND A LNDG GEAR TRANSFER VALVE. PARTS OF THE OPERATIONAL CHKS WHICH FAILED PRIOR TO THE VALVE CHANGE PASSED PER THE MAINT MANUAL CHK AND UPON COMPLETION OF THE CHKS AND BECAUSE FLUID LINES TO THE LNDG GEAR HAD BEEN OPENED; I WANTED TO VERIFY POSITIVE PRESSURE ON THE DOWN LOCKS OF THE GEAR AND CYCLED THE GEAR HANDLE TO ACCOMPLISH THIS. UPON RAISING THE GEAR HANDLE THE ACFT NOSE SETTLED ONTO THE RAMP; DAMAGING THE NOSE GEAR DOORS. THE PREVIOUS MAINT CREW REMOVED THE GEAR PINS PRIOR TO LEAVING EVEN THOUGH THE ACFT WAS NOT RELEASED FOR SVC. MY FAILURE TO PHYSICALLY CHK THE GEAR TO BE SURE THE GEAR PINS WERE STILL INSTALLED CAUSED THE INCIDENT.