What was found
BLEED AIR: NOTICED A SLIGHT BURNING SMELL ON TAKEOFF, WENT AWAY QUICKLY. SHORLTY BEFORE LEVELING OFF AROUND FL230, THE BURNING SMELL CAME BACK AND WAS STRONGER. TURNED OFF AC FROM AUTO TO MAN, DIDNT MAKE A DIFFERENCE. HEAT EXCHANGER TEMP WENT TO AROUND 200F. HALFWAY BACK TO LAS THE SMELL WENT AWAY, THEN CAME BACK EVEN STRONGER ON APPROACH AND TAXI IN, ON THE TAXI IN, BLEED 2 FAIL CAS DISPLAYED. REMOVED AND REPLACED #2 FAN AIR VALVE WITH A MODIFIED VALVE IAW MM 36-1-04. OPS AND LEAK CHECKS GOOD.