Brother MFC8420 Service Manual page 220

M-5 Insufficient output from high-voltage power supply unit
Possible cause Step
High-voltage 1
contact
High-voltage 2
power supply
PCB
M-6 Fixing heater temperature failure
Possible cause Step
Poor thermistor 1
harness contact
Blown thermal 2
fuse
Thermistor 3
failure
Halogen heater 4
lamp failure
M-7 Laser Unit failure
Possible cause Step
Harness 1
connection
failure (1)
Harness 2
connection
failure (2)
Check
Do any of the terminals on the
high-voltage contacts have dirt
or contact burns?
Check the connections of the
connector between the high-
voltage power supply and the
engine PCB are secured
correctly.
Check
Is the contact of connector
CN7 on the engine PCB good?
Remove the fixing unit and
measure the resistance of the
thermal fuse. Is it open circuit?
Is the thermistor installed
properly?
Remove the fixing unit and
measure the resistance of the
halogen heater lamp. Is it
open circuit?
Check
Is the LD harness connector
on the main PCB secured
correctly?
Is the polygon motor connector
on the engine PCB secured
correctly?
6-25
MFC-8420/8820D/8820DN, DCP-8020/8025D/8025DN
Result Remedy
Yes Clean the terminals.
Yes Replace the high-voltage
power supply PCB
No Reconnect the connector
between the high-voltage
power supply and the engine
PCB.
Result Remedy
No Reconnect the connector.
Yes Replace the fixing unit.
Yes Replace the fixing unit.
No Reinstall the thermistor
properly.
Yes Replace the halogen heater
lamp.
Result Remedy
No Reconnect the connector
securely.
Yes Replace the laser unit.
No Reconnect the connector
securely.
Yes Replace the laser unit.
SERVICE MANUAL