What was found
CONTROL CABIN TEMP BECAME UNCONTROLLABLE DURING CLIMB THROUGH FL260 ABOUT 20 MINUTES AFTER TAKEOFF. SUPPLY DUCT TEMP ROSE 44 DEGREES C AND STABILIZED. PLACING CONTROL CABIN TEMP CONTROLLER TO OFF, HAD NO IMPACT. LT PACK DISCHARGE TEMP WAS 37 DEGREES C, RIGHT PACK 6 DEGREES C. PLACED TRIM AIR SWITCH TO OFF. OVER THE NEXT 2 HOURS WITH TRIM AIR OFF AND TEMP CONTROLLER AT FULL COLD THE SUPPLY DUCT TEMP SLOWLY DROPPED TO 30 DEGREES C WITH BOTH PACKS AT 24 DEGREES C. RAN AWAY AGAIN TOP OF DECENT. ACFT GROUNDED. PERFORMED FIM TASK 21-61-861 AND FOUND THE CONTROL CABIN ZONE TEMPERATURE SENSOR FILTER CLOGGED WITH DUST. CLEANED THE CONTROL CABIN ZONE TEMPERATURE SENSOR FILTER IAW AMM TASK 21-61-06-100-801. ZONE TEMPERATURE CONTROLLERS BITE CHECK GOOD FIM 21-62-801.