Brother MFC8420 Service Manual page 233

CHAPTER 6 TROUBLESHOOTING
I-4 All black
Possible cause
Corona failure
Harness
connection
High-voltage
power supply
PCB failure
Main PCB
failure
CCD unit
User Check
(1) Clean the corona wire of the drum unit.
(2) The drum unit may be damaged. Install a new drum unit.
Step Check
1 Is the corona wire dirty?
2 Is the corona wire broken?
3 Are the charge electrodes
between the machine and the
drum unit dirty?
4 Is the laser unit connected to
the main PCB correctly?
5 Is the problem solved after
replacing the high-voltage
power supply PCB?
6 Is the problem solved after
replacing the main PCB?
7 Is the problem occurring on
CCD replacement?
Result
Yes Clean the corona wire
with the wire cleaner.
Yes Replace the drum
unit.
Yes Clean both electrodes.
No Connect the harness
between the laser unit
and the main PCB
correctly.
Yes Replace the high-
voltage power supply
PCB.
Yes Replace the main
PCB.
Yes Replace the CCD.
6-38
Ground
Remedy
contacts
(2)
(3)