Gestetner A294, Service Manual

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LASER EXPOSURE
SM 2-35 A294/A295
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2.4.3 AUTO POWER CONTROL (APC)
There are four LD drivers (IC408, IC409, IC422 and IC423) on the LDB (Laser
diode Drive Board). The LD drivers control the laser diodes on the 4-channel laser
diode array. Even if a constant electric current is applied to the laser diode, the
intensity of the output light changes with the temperature. The intensity of the
output decreases as the temperature increases.
In order to keep the output level constant, IC410 and IC411 monitor the current
passing through the photodiode (PD). Then they increase or decrease the current
to the laser diode as necessary, comparing it with the reference levels (V
REF
). To
apply APC separately to each laser diode using only one photo diode, the monitor
current is sent in turn to each laser diode.
This auto power control is done just after the machine is turned on and during
printing while the laser diode is active.
The reference levels are adjusted on the production line. Do not touch the variable
resistors on the LDB in the field.
O U T
S E N S E _ I N
W R I T E _ L E V E L
O U T
S E N S E _ I N
W R I T E _ L E V E L
O U T
S E N S E _ I N
W R I T E _ L E V E L
O U T
S E N S E _ I N
V
R E F
W R I T E _ L E V E L
V
R E F
V
R E F
V
R E F
+
–
IC410
IC411
PD Charge
IC408
IC409
IC422
IC423
Analog SW
Analog SW
LDA
L D D
IC406
IC407
IC421
IC420
O U T
O U T
O U T
O U T
W R _ P U L S E
W R _ P U L S E
W R _ P U L S E
W R _ P U L S E
+
–
B G A V D
O U T
O U T
O U T
O U T
L V D S
Image Data
Clock
D/A
Converter
L D B
A294D999.WMF
CÓPIA NÃO CONTROLADA
CÓPIA NÃO CONTROLADA