What was found
FLT 3707. NR 2 ENGINE ROLLED BACK ON TAKEOFF, MAX N1 WAS 84 PERCENT, ENGINE FAIL LIGHT ON. CKED VIA'S AND HAD NR 2 ENG N1 FAILED MSG -PROBABLE CAUSE ENG 2 TACHOMETER. REPLACED NR 2 ENG FAN SPEED SENSOR/N1 TACH PER AMM 77-11-01-2 OPS CKS NRML, RAN ENG TO T/O PWR PER JRH ALL IND NRML AND WITHIN LIMITS NOTE: NEW SENSOR FAILED AND WAS REPLACED WITH WRI 77-15 3467, HAS FLOWN GOOD WITH 2ND SENSOR.