What was found
UNCOMMANDED ND TRIM (FLIGHT 4650 - FILED CLOSED)ARRIVED AT AIRCRAFT AND FOUND PITCH TRIM INDICATION AT ND 3.4. INTERROGATED MDC AND FOUND NO PRESENT NOR HISTORIC FAULTS RELATED TO PITCH TRIM. TOOK PITCH TRIM THROUGH FULL TRAVEL ON BOTH CA AND FO SIDE. VISUALLY VERIFIED PITCH TRIM EICAS INDICATION MATCHED THE MECHANICAL POSITION OF THE HORIZONTAL STAB. VERIFIED STAB CHANNEL 1 AND CHANNEL 2 BOTH INDEPENDENTLY AND SIMULTANEOUSLY OPS CHECKED GOOD. AFTER FDR DOWNLOAD DATA REVIEW FOUND THAT PITCH TRIM STOPPED RESPONDING MIDFLIGHT. SUSPECT HORIZONTAL STABILIZER MOTOR CONTROL UNIT TO BE FAULTY. REMOVED AND REPLACED MCU IN REF TO CRJ 700 AMM 27-42-PERFORMED OPS CHECK OF MCU IN REF TO CRJ 700 AMM 27-61-05. OPS CHECK GOOD. REMOVED AND REPLACED CA SIDE PITCH TRIM SWITCH IN REF TO CRJ 700 AMM 27-41-05 OPS CHECK GOOD AT THIS TIME. PERFORMED OPS CHECK OF THE HORIZONTAL STABILIZER TRIM SYSTEM IN REF TO CRJ 700 AMM 27-40-00. AIRCRAFT REMAINS CAT II AUTHORIZED AT THIS TIME