What was found
ON TAKEOFF CREW REPORTED SMELL OF SMOKE IN THE COCKPIT, THEY RETURNED TO BASE WITHOUT INCIDENT. THEY ALSO REPORTED THE CABIN WOULD NOT DEPRESSURIZE IN THE GROUND. UPON INVESTIGATION, WE FOUND A AFT LH CABIN HEATING DUCT HAD BURST CAUSING THE PLASTIC VACUUM LINE FOR THE OUTFLOW VALVE HAD MELTED WHICH SUPPLIES VACUUM TO THE OUTFLOW VALVE TO OPEN. REPLACED THE HEATING DUCT AND VACUUM LINE. GROUND OPS CHECK GOOD.