What was found
ATB - IMMEDIATELY AFTER TAKEOFF, L ADC FAILED. FLAGS APPEARED ON CAPTAINS AIRSPEED, ALTIMETER AND IVSI. OVERSPEED WARNING OCCURRED EVERY FEW SECONDS THROUGHOUT CLIMB, LEVEL OFF, APPROACH AND TAXI-IN. SELECTED ALTERNATE ADC AND CAPTAINS RESTORED, BUT OVERSPEED WARNINGS CONTINUED. PERFORMED AIR TURN BACK. REMOVED AND REPLACED LT ADC IAW AMM 34-12-01-4.