Service manual

2. Laser diode automatic power control
The laser diode automatic power control (APC) is performed by the IC801 to make the laser diode
emit the laser beam at a constant intensity. The APC is performed prior to the image formation
(Initial APC) and between lines (between line APC).
When the CNT0 and CNT1 input to the IC801 from the CPU are “L” and “H” respectively, the
sample hold circuit on the IC801 enters the sample mode (mode in which the APC is performed).
At the same time, the laser diode (LD) emits the laser beam.
The intensity of the laser beam emitted from the laser diode is detected by the photo diode
(PD) and is fed back to the sample hold circuit on the IC801.
The IC801 controls the output current of the drive circuit to make the fed back laser inten-
sity become equal to the target laser intensity set in the IC801.
After a specified period of time, when the CNT0 becomes “H”, the laser diode is turned OFF
compulsively and the sample hold circuit enters the hold mode. The IC801 converts the con-
trolled laser intensity to voltage and stores it in the C803.
3. Horizontal sync control
The CPU controls the horizontal sync control based on the HORIZONTAL SYNC INPUT signal
(/BDI) sent from the BD sensor in the laser scanner.
The CPU turns CNT0 and CNT1 into "L" and "H" respectively, and the laser diode emits the
laser beam at the specified timing based on the /BDI signal.
The laser beam passes through collimator lens and cylindrical lens. Then the laser beam is
scanned by the four-faced mirror to pass through the BD lens on the scanning line and enters
the BD sensor.
When the BD sensor detects the laser beam, the BD sensor generates the HORIZONTAL SYNC
signal (/BD) based on the /BDI signal to send it to the interface controller.
4. Image masking control
The CPU performs the image masking control in order to prevent laser beam emission in the
non-image area other than the unblanking period.
The laser beam emission enable period of the laser diode is controlled by masking the /VDO
and VDO with CNT0 and CNT1 in the logic circuit on the IC801.
The CPU sets the CNT0 and CNT1 “H” in the image masking area other than the unblanking
period so that the laser diode will not be turned ON/OFF when the /VDO and VDO are sent to
the IC801 from the interface controller.
In image unmasking area, the IC801 turns ON/OFF the laser diode based on the /VDO and
VDO by setting CNT0 “H” and CNT1 “L.”
CHAPTER 2
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