Brother MFC-9420CN Service Manual page 283

I-3 Jitter
Phenomenon
Uneven optical density appears periodically in the horizontal direction on the printed image as
shown in print sample [3] of Fig.9-1.
Main Causes
(1) Failure of main motor.
1) Irregular rotation of the drive motor.
2) Failure of the OPC drive gear ASSY PU.
3) Variation of OPC belt running speed due to above reasons.
(2) Failure of the belt cartridge PU.
(3) Failure of the main drive gear ASSY PU.
(4) Failure of the developer drive ASSY PU.
Countermeasures
(1) Replace the OPC drive gear ASSY PU with a new OPC drive gear ASSY PU
(2) Replace the belt cartridge PU with a new belt cartridge PU.
(3) Replace the main drive gear ASSY PU with a new main drive gear ASSY PU.
(4) Replace the developer drive ASSY PU with a new developer drive ASSY PU.
Main Motor PU
Developer Drive Assy PU
Fig. 9-4
9 - 2 7 Confidential