Brother MFC-9420CN Service Manual page 65

(2) Laser drive control circuit
Laser drive control circuit (LDC) consists of the video signal input circuit, laser drive
circuit, laser diode, output sensing circuit and output control circuit, as shown in Fig.3-34.
<Operation>
1) When the video signal is inputted, the laser drive control circuit has the laser diode
switched on and radiated according to the video signal.
2) Radiated laser beam is sensed by the photo detector (PD), and the detecting signal is
fed back to the output control circuit.
3) Output control circuit controls the radiation output to be constant, by comparing the
laser output default with the feed-back value transmitted from the output sensing
circuit.
4) Laser beam scanned by the scanner motor is sensed by the beam detector (PD), and
then, the beam detecting timing (BDT) signal will be outputted.
Fig. 3-34
3 - 3 3
OPC Belt
F-θLens
Scanner Motor
Confidential