What was found
AIRCRAFT DEPARTED KTIX EMPTY HEADING TO KOPF. ON TAKEOFF/CLIMB, CAPTAIN NOTICED CABIN AIR SWITCH WAS OFF, CAPTAIN THEN TURNED IT ON. A FEW MOMENTS LATER CREW WERE GETTING HOT AIR & SMOKY RESIDUE IN COCKPIT AREA. THEY ELECTED TO RETURN TO KTIX, DECLARE AN EMERGENCY. LANDED SAFELY BACK AT TIX. MAINT INSPECTED ACFT & DID NOT SEE ANY OBVIOUS ISSUES WITH PRESSURIZATION SYS. NEXT MORNING MAINT RAN ENGS & ABLE TO DUPLICATE HOT AIR IN CABIN/COCKPIT AREA. COMPLETING TROUBLESHOOTING PROCEDURES IAW MM, LT ENG BLEED AIR MIXING VALVE WAS FOUND TO BE DEFECTIVE & REPLACED. EMERGENCY PRESSURIZATION VALVES REPLACED FOR PRECAUTIONARY PURPOSES. ENGS WERE RUN UP & FUNCTIONAL CHKS COMPLETED, NO DISCREPANCIES NOTED. A TEST FLT WILL BE COMPLETED BEFORE IT IS RETURNED TO SERVICE.