Brother MFC-9420CN Service Manual page 260

I-9 Black Line
Phenomenon
Fine black line appears in the printed image as shown in
Main Causes
(1) The toner cartridge PU blade is deformed.
(2) The OPC belt surface is damaged.
(3) Foreign particles (paper dust, etc.) are stuck in between the cleaning blade and the OPC belt.
(4) Foreign particles adhere to the perimeter parts of the OPC belt and the transfer belt, and
consequently contact the toner image formed on the belt.
Belt cartridge PU
Countermeasures
(1) Replace the toner cartridge PU with a new toner cartridge PU.
(2) Replace the belt cartridge PU with a new belt cartridge PU.
(3) Clean the perimeter of the mounting area of the OPC belt and transfer belt.
print sample [9] of Fig.9-1.
Top cover PU
OPC belt cartridge PU
Toner cartridge PU
(K, Y, M, C)
Laser ASSY PU
Paper tray PU
Fig. 9-10
9 - 3 3
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OPC belt
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