What was found
AFTER DEPARTING, CLIMBIMG THRU 4000 FOR 10,000 FT COCKPIT SEEMED WARM, CREW CHECKED DUCT TEMP ON COCKPIT SIDE & IT WAS CLIMBING UNCONTROLLED IN THE AUTO POS. SELECTED MAN/FULL COLD & TEMP KEPT INCREASING. DUCT O'H ANNUNCIATOR CAME ON & DUCT TEMP HIT 350, & COULD NOT CONTROL THE TEMP IN MANUAL MODE. AN EMERGENCY WAS DECLARED & TURNED BACK TO DEPARTURE. CRW COMPLETED COCKPIT DUCT OVER TEMP CHECK LIST. UPON DECENDING INTO VFR CONDITIONS, REGAINED CONTROL OF TEMP IN MANUAL MODE & FINALLY STABILIZED AT 53 DEGREES. UNDECLARED EMERGENCY AND PROCEEDED BACK TO DESTINATION. TEMP REMAINED STABLE IN THE MANUAL MODE & FLIGHT COMPLETED WITH OUT INCIDENCE. MX WORKING WITH THE MFG WAS UNABLE TO DUPLICATE DISCREPANCY. LT & RT TEMP CONTROLLERS SWAPPED. TEMP CONTROL OPS OK IN AUTO & MANUAL. ACFT RETURNED TO SERVICE.