As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
APPROXIMATELY 1 HOUR INTO THE FLIGHT THE THE TRIPLE TACH (NR, N2 FOR BOTH ENG) DECREASED TO ZERO WITH NO OTHER INDICATIONS. REMOVED AND REPLACED TRIPLE INDICATOR IAW AMM 62-41-00, 4-3. MOC SUCCESSFULLY COMPLETED IAW RFM AND USAACE CKLIST.