What was found
ITH- FLIGHT 3729 FLIGHT ATTENDANT REPORTED THAT WITH THE BLEEDS ON A BURNING SMELL WAS PRESENT AND CABIN WAS VERY HOT WITH TEMPERATURE GAUGE IN COCKPIT IN MIDDLE OF GREEN. MAINTENANCE INSPECTED ACM, PACK AND TRIM VALVES, INSPECTED BLEEDS FOR LEAKS, NONE NOTED. GROUND RAN WITH CREW WITH APU AND ENGINE BLEEDS ON. SYSTEM OPERATED NORMAL. SUSPECT CONDENSER MIXER FROZEN UP IN FLIGHT. AIRCRAFT RETURNED TO SERVICE.