Brother MFC8420 Service Manual page 231

CHAPTER 6 TROUBLESHOOTING
I-2
Possible cause
Corona failure
(contact failure)
Drum unit failure
Toner cartridge
failure
High-voltage
power supply
PCB failure
Main PCB
failure
Engine PCB
failure
Dark
User Check
(1) Check the paper used meets the recommended paper specifications.
(2) Check the machine's environment. High temperature and high humidity
conditions can increase the amount of background shading.
(3) Clean the corona wire with the wire cleaner.
(4) Try installing a new toner cartridge or drum unit.
Step
1 Are the charge electrodes
between the machine and the
drum unit dirty?
2 Is the problem solved after
replacing the drum unit?
3 Is the problem solved after
replacing the toner cartridge?
4 Is the connections of the
connector between the high-
voltage power supply PCB and
the engine PCB secured
correctly?
5 Are there any disconnected
connectors?
6 Are there any disconnected
connectors?
Check Result
6-36
Remedy
Yes Clean both electrodes.
Yes Replace the drum unit
with a new one.
Yes Replace the toner
cartridge with a new
one.
Yes Replace the high-
voltage power supply
PCB.
No Replace the main
PCB.
No Replace the engine
PCB.
Ground
contacts
(3)