Sharp AR-810 (serv.man12) Service Manual ▷ View online
AR-651/810 LASER & OPTICAL UNIT 8 - 5
(3) f
Ø lenses 1 and 2
These two lenses perform the following adjustment on the laser beams reflected by the polygonal mirror.
a. Uniform-velocity scanning
Since the polygonal mirror is rotating at a uni-
form velocity, the laser beam reflected from the
mirror scans over the drum surface at a uniform
angular velocity; namely, the pitch between the
dots on the drum is wider at both ends than at
the center of the scanning range. The f
Ø lenses
help to correct this difference, making all the
dot-to-dot pitches equal on the drum surface.
b.
Face tilt connection
The reflecting face of the polygonal mirror is tilted
slightly to one side against the perfect vertical.
Horizontal deviation of the laser light which is
caused by the tilt is corrected.
c. Sectional shape of laser beam
The shape of the laser beam spotted on the drum
is adjusted.
Wider
Narrower
Drum
Lens 1
Lens 2
Same intervals
Mirror plane is tilted.
(4) H-Sync detection PC board
The laser light which is started to be scanned from one of the reflected faces of the polygonal mirror is
reflected by the H-Sync detection mirror and goes into the PIN diode on the H-Sync detection PC
board. The primary scanning synchronizing signal is generated based on this reflected laser light tim-
ing.
The rudder sensor is attached to the H-Sync sensor to evenly space (42.3µm), by its detection value,
the intervals of the secondary scanning of the first and second beams.
Fig. 8.2-2
Fig. 8.2-3
Deviation
AR-651/810 LASER & OPTICAL UNIT 8 - 6
(5) Laser drive board
This control board has the following functions:
a. APC control function (adjusts disparity of the laser intensity caused by temperature)
b. Laser ON/OFF function
c. Function to generate the primary scanning synchronizing signals
(6) Slit glass
Slit glass is located where the laser beams are output from the laser optical unit, and it protects the unit
from dust.
AR-651/810 LASER & OPTICAL UNIT 8 - 7
Fig. 8.3-2 shows a block diagram of the semiconductive laser control circuit. The semiconductive laser
monitors the efficiency of the user drode and the laser itself is adjusted to initially output approx. 2.6mW
(395µW on the drum surface). The regulated voltage of the monitor output provides feedback to the laser
power comparison circuit. This monitor output voltage is compared with the laser power voltage set by the
control circuit for every scanning line. Laser driver circuit increases the forward current if the laser power is
insufficient and decreases the current when the power is excessive to maintain a constant optical output.
8. 3. Laser Diode
The laser used in this copier is a A1GaAs type semiconductive laser. It is generated in the single-horizontal
mode, and its wavelength is approx.780nm. This semiconductive laser outputs 15mW with standard DC
power supply. Laser intensity is controlled by using the output from the PIN diode for monitoring light output
in the semiconductive laser.
The relation between the current and optical output
of the semiconductive laser is as shown at right.
Light emission is started when the forward current
reaches or becomes greater than the threshold
current and outputs the monitor current which is pro-
portional to the optical output. The threshold current
and monitor current differs depending on the
semiconductive lasers due to their individual nature.
Optical output has to be adjusted to maintain a con-
stant value.
Since the optical output of the semiconductive la-
ser is decreased when the temperature of the laser
rises, APC control needs to be performed to maintain
a constant optical output.
Fig. 8.3-2 Block diagram
Threshold current
Optical Output (mW)
Low temp.
High temp.
Forward current
Monitor current
Current (mA)
Monitor output
Monitor effi-
ciency regula-
tion circuit
ciency regula-
tion circuit
Laser power
comparison
circuit
comparison
circuit
Laser driver
circuit
circuit
Semiconductive laser
Constant
optical output
optical output
Fig. 8.3-1
Regulation
Power source
AR-651/810 LASER & OPTICAL UNIT 8 - 8
(2) Remove 2 screws and take off the optical unit
fixing stay.
8. 4. Disassembly and Replacement
[A] Laser fan motor
(1) Remove the top right cover, feed side upper
cover and feed side center cover. Remove 1
screw and loosen 10 screws and take off the
plate cover.
(3) Disconnect 1 connector and remove 2 screws
to take off the laser fan motor.
Laser fan motor
Connector
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