Canon NP6621 Service Manual page 103

2. Turning ON/OFF the Developing Bias
DC Component and Controlling to
Constant Voltage
The microprocessor on the composite power
supply PCB generates the BPWM signal (pulse
signal) under the control of the microprocessor
master (Q301) on the DC controller PCB, thereby
turning ON the secondary side of the main
transformer (T4) and applying the developing bias
DC component.
The application voltage is read as the BIAS
signal (analog signal) from the developing DC bias
voltage detection circuit while the developing bias
DC component is being applied and is used to
control the output voltage to a constant level.
3. Turning ON/OFF the Developing Bias
The microprocessor on the composite power
supply PCB generates the ACBIAS signal (pulse
signal) under the control of the microprocessor
master (Q301) on the DC controller PCB. The
signal causes the developing bias AC component
generation transformer (T3) to turn ON, and the
sum of the AC component of the developing bias
and the DC component is applied:
• Application frequency:
• Application voltage:
4. Controlling the Voltage Level of the
Developing Bias DC Component
The machine changes the voltage level of the
DC component of the developing bias to suit the
following:
COPYRIGHT © 1998 CANON INC.
about 2400 Hz
about 1300 Vp-p
CANON NP6621 REV.0 FEB. 1998 PRINTED IN JAPAN (IMPRIME AU JAPON)
OPERATIONS AND TIMING
• Setting of the Copy Density key.
• Output of the AE sensor
• Setting of the copy density correction volume
(VR850)
The machine exerts control so that the voltage
of the developing bias DC component varies
between -80 and -560 V as a result of varying the
output of the BPWM signal.
To meet the increases in the light area potential
(VL) caused by the deterioration of the photosensitive
drum, the machine is equipped with a copy density
correction volume. You may turn the volume (VR851/
860) to adjust the DC bias potential for each increase
in VL, thereby ensuring fogging-free images.
(to light)
VR850
Developing bias
DC component
-500
-400
-100
F1 (dark)
Figure 3-309
(to dark)
Lighter
Medium
Darker
5 8 (light)
Copy density
3–47