Brother MFC8420 Service Manual page 238

I-11 Faulty registration
Possible cause Step
Excessive paper 1
load
Print paper 2
3
Rear regist 4
sensor position
incorrect
I-12 Poor fixing
Possible cause Step
Print paper 1
Toner sensing 2
failure
(When printing
is faint.)
Fixing unit 3
thermistor failure
Low-voltage 4
power supply
PCB failure
Check
Is the paper loaded in the
paper tray more than 27mm
high?
Is the specified weight of the
recommended paper being
used?
Is the first printing position
within ±1mm of the tolerance
specification?
Is the position of the rear
registration sensor normal?
Check
Is thick paper of more than
43lb being used?
Is the problem solved by
replacing the drum unit or the
toner cartridge?
Is the thermistor fitted
correctly?
Is the problem solved by
replacing the low-voltage
power supply PCB?
6-43
MFC-8420/8820D/8820DN, DCP-8020/8025D/8025DN
Result Remedy
Yes Instruct the user to keep paper
loads below 27mm in depth.
No Recommend to use the
specified types of paper.
Yes Adjust the Y offset by using the
utility software supplied.
No Reposition the sensor to the
correct position.
Result Remedy
Yes Recommend to use the
specified types of paper.
Yes 1) Toner is empty.
2) The toner sensing is
defective. Clean the toner
sensor.
3) If the wiper in the toner
cartridge is broken, replace
the toner cartridge with a new
one.
No Fit the thermistor correctly.
Yes Replace the low-voltage power
supply PCB.
SERVICE MANUAL