What was found
ON APPROACH, DURING CONFIGURATION WHEN GEAR HANDLE WAS PUT IN DOWN POSITION, NOSE GEAR INDICATED DOWN AND LOCKED, BUT BOTH MAIN GEARS DID NOT INDICATE DOWN AND DID INDICATE UNSAFE ON BOTH SYS 1 AND 2. DISCONTINUED THE APPROACH AND CLIMBED TO 4000 FT. DID FEEL AN UNUSUAL AMOUNT OF VIBRATION. LEVEL AT 4000 FEET AND PRIOR TO STARTING QRH, ALL GEAR INDICATED DOWN AND SAFE ON BOTH SYS 1 AND 2. COMPLETED NORMAL LANDING AND TAXIED INTO GATE WITH NO ADDITIONAL ABNORMAL INDICATIONS. AN EMERGENCY WAS DECLARED. REMOVED AND REPLACED THE LANDING GEAR SELECTOR VALVE 3GA IAW AMM 32-31-12-04. OPS CHECKED NORMAL. LEAK CHECKED GOOD.