Sharp FO-3150 Service Manual page 88

FO-3150U/C
f. Laser scanning unit
This unit controls the laser beam power and laser beam scanning.
The control is performed with the signals inputted outputted to or from
the CPU and ASIC.
1) Signal functions
PMCLK
Clock signal for driving the scanning motor. (770Hz)
PMD
Scanning motor ON/OFF signal.
APCSTT
Used to start the laser beam generation circuit.
SYNC
This signal is outputted when the laser beam scanned by the laser
beam sensor signal is sensed by sensor (Photo diode D601).
Used for the left margin control.
VIDEO
This signal is used to control the laser diode emitting.
Not only when the laser beam is emitted to perform the LEND proc-
ess, but also when the laser beam is emitted as image data, ASIC
controls and the signal is outputted from video terminal.
Fig. 14
2) Laser beam power control
The laser beam power is controlled in the laser emitting unit PWB.
This circuit functions to keep the laser beam output power at a constant
level.
The laser beam output is monitored with photo diode LD601 for monitor.
When the laser beam output rises above the specified value, the imped-
ance of photo diode LD601 is decreased to decrease the monitor input
(3PIN) voltage of the laser diode control IC (IC701).
Then the laser diode (LTO28GS) drive voltage is decreased to decrease
the laser beam output to the specified level.
When the laser beam output is decreased below the specified level, the
contrary operation are performed.
5 – 15