Brother MFC-9420CN Service Manual page 252

I-1 Background
Phenomenon
Background is smeared due to toner spread as shown in
Main Causes
(1) Too small charging amount in the development process.
(2) Insufficient contact of the developer roller's bias pole.
(3) Life or failure of the belt cartridge PU.
(4) Failure of the HV power supply unit PU (HVU).
OPC belt
Countermeasures
(1) Replace the toner cartridge PU.
(2) Confirm if the developer bias pole is deformed or not.
(3) Replace the belt cartridge PU.
(4) Replace the HV power supply unit (HVU).
print sample [1] of Fig.9-1.
Fig. 9-2
9 - 2 5
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