What was found
THE NR 2 PACK OVERHEATED WITH SMOKE ENTERING THE CABIN WHILE LEVELING OFF AT FL 280. THE AIRCRAFT WAS RETURNED TO DUS, WHERE IT LANDED WITHOUT INCIDENT. TROUBLESHOOTING FOUND THE NR 2 ENGINE PRSOV WOULD NOT SHUT OFF ELECTRICALLY, CAUSING THE NR 2 PACK TO OVERTEMP AND TRIP. THE NR 2 PRSOV WAS LOCKED OUT MANUALLY AND THE NR 2 PNEUMATIC SYSTEM AND NR 2 PACK WERE PLACARDED INOPERATIVE. THE ENGINES WERE RUN AND ALL OPERATION CHECKED GOOD.