What was found
DURING FLIGHT 3029, ATC REPORTED BOTH TRANSPONDERS INOPERATIVE ON THREE SEPARATE OCCASIONS, ON FLIGHT FROM GRF-MSP. TRANSPONDER LOCKED UP WITH ACT LIGHT ILLUMINATED. TCAS ALSO SWITCHED TO OFF MODE. ATC REPORTED LOST MODE C AND REGULAR RETURN ONLY WAY TO CLEAR WAS TO RUN TRANSPONDER TEST SWITCHING FROM 1 TO 2, NO EFFECT. MAINTENANCE REMOVED AND REPLACED TRANSPONDER CTL P/N 622-6523-213 SN/OFF 137D SN/ON 12761, REPLACED NR 1 TRANSPONDER P/N 622-9210-004, SN/OFF 5904, SN/ON 6145, TRANSPONDER SYSTEM OPS CHECKS GOOD IAW SF340 MM CHAP 34, FAR 43, APP E, PARA C C/W.