What was found
AFTER PLYING TAKEOFF POWER A FAINT ODOR BEGAN COMING INTO THE CABIN. AROUND 2000' AGL BURNING SMELL BECAME VERY STRONG. RETURNED TO APA FOR LANDING. FREON AIR WAS OFF. DURING ENG RUNS, OPERATED PRESSURIZATION & AIR CONDITIONING SYS AT BOTH HIGH & LOW ENG POWER SETTINGS. CHECKED COALESCE SOCK IAW MM 21-70-00. CHECKED ALL LIGHTING, DISPLAYS & AC GENERATOR, OPERATIONS CHECKS ALL GOOD, NO DISCREPANCIES NOTED.