Brother MFC-9420CN Service Manual page 291

I-11 Vertically Staggered Image
Phenomenon
Printed image staggered in the vertical direction as shown in
Main Causes
(1) Shock or vibration is applied to the machine.
(2) Failure of the laser ASSY PU : Vibration from the rotation of scanner motor.
Laser ASSY PU
Cover C
Front cover unit PU
Countermeasures
(1) Do not apply shock or vibration to the machine body.
(2) Replace the laser ASSY PU with a new laser ASSY PU.
print sample [11] of Fig.9-1.
Fig. 9-12
9 - 3 5 Confidential