Troubleshooting Image Defects - Brother HL-720 - B/W Laser Printer Service Manual

2.2

Troubleshooting Image Defects

Procedures to be followed in the event of specific image defects.
I-1 Light
Possible cause Step
Scanner 1
window
Poor contrast 2
Toner sensing 3
failure (toner
cartridge side)
Toner sensing 4
failure (printer
side)
Drum 5
connection
failure
Transfer 6
electrode
contact failure
Transfer failure 7
Developing bias 8
contact failure
High-voltage 9
power supply
failure
Panel sensor 10
PCB failure
Check
Is the scanner window dirt?
Is the contrast adjustment dial in the
center of the click position?
Is the problem solved when 4 or 5
pages are printed after the cartridge
is replaced with a full one?
Can printing be started with the
drum unit removed?
Is the drum shaft connected with
electrode when the drum unit is
installed?
Is there continuity between the
transfer roller shaft and the transfer
electrode on the external drum unit?
Is the problem solved when the
transfer roller is replaced?
Are the developing bias contacts
between the printer body and the
drum unit soiled?
Open the top cover and measure
the voltage at the electrode plate at
the right side face while printing
defeated.
Is the voltage correct?
Perform the same check as step 8
above.


Result Remedy
Yes Clean the scanner
window with a soft
dry cloth.
No Set it to the center
click position or
reasonable position.
Yes The wiper of the
toner cartridge is
defective. Replace
the toner cartridge.
Yes Toner sensor failure.
Check if the toner
sensor needs
cleaning and check
the toner sensor
connection.
No Clean contact
electrodes between
the printer body and
the drum shaft of
drum unit.
Yes Remove the transfer
roller and clean the
contact electrode.
Clean the external
electrode.
Yes Replace the drum
unit.
Yes Clean the terminals
of both sides.
No Check the harness
connection between
the high-voltage
power supply and
the panel sensor
PCB.
If the connection is
normal, replace the
high-voltage power
supply PCB.
No Replace the panel
sensor PCB.