Canon PC720 Service Manual - Page 63

oscillation/amplitude

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MWAIOPERATIONS AND TIMINGKM 2. Operation a. Turning On and Off the DC/AC Bias When the Copy Start key is pressed, the microprocessor on the DC controller PCB instructs the microprocessor on the composite power supply PCB to turn on the DC/AC bias. The microprocessor on the composite power supply PCB drives the main transformer (T101) while sending the AC bias on signal to the oscillation/amplitude control circuit. The AC bias is supplied to the primary charging roller through the amplifier and the transformer (T508) by the pulse signals generated by the oscillation/amplitude control circuit. b. Controlling the DC Bias to a Specific Voltage The DC bias is controlled to a specific voltage by the microprocessor on the composite power supply PCB. The microprocessor on the composite power supply PCB constantly monitors the DC voltage. The microprocessor serves to maintain the DC bias voltage to a constant value by varying the DC bias control signal (PDCPWM) based on measurements of voltage taken. c. Controlling the AC Bias to a Specific Current The AC bias is controlled to a specific current by the oscillation/amplitude control circuit. The oscillation/amplitude control circuit checks the current of the AC component and changes the amplitude based on measurements of current taken so as to maintain the AC bias current constant. d. Switching the DC Bias Voltage Level The copier switches the level of the DC bias voltage according to whether or not copy images are being formed. The DC bias voltage is switched by the microprocessor on the composite power supply PCB by varying the DC bias control signal (PDCPWM) according to the DC bias switching timing signal sent by the microprocessor on the DC controller PCB. during copying: -625 V other: -400 V Copy Start key ON STBY INTR SCFW SCRV LSTR STBY Main motor (M1) Scanning lamp (FL1) Primary charging roller DC bias Primary charging roller AC bias P e-heatn -400V -625V -400V Figure 3-307 COPYRIGHT O 1994 CANON INC. CANON PC720/740/750/770 REV.O AUG.1994 PRINTED IN JAPAN omput AU JAPON) 3 - 23

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MWAIOPERATIONS
AND
TIMING
KM
2.
Operation
a.
Turning
On
and
Off
the
DC/AC
Bias
When
the
Copy
Start
key
is
pressed,
the
microprocessor
on
the
DC
controller
PCB
instructs
the
microprocessor
on
the
compos-
ite
power
supply
PCB
to
turn
on
the
DC/AC
bias.
The
microprocessor
on
the
composite
power
supply
PCB
drives
the
main
trans-
former
(T101)
while
sending
the
AC
bias
on
signal
to
the
oscillation/amplitude
control
cir-
cuit.
The
AC
bias
is
supplied
to
the
primary
charging
roller
through
the
amplifier
and
the
transformer
(T508)
by
the
pulse
signals
gen-
erated
by
the
oscillation/amplitude
control
circuit.
b.
Controlling
the
DC
Bias
to
a
Specific
Voltage
The
DC
bias
is
controlled
to
a
specific
voltage
by
the
microprocessor
on
the
com-
posite
power
supply
PCB.
The
microproces-
sor
on
the
composite
power
supply
PCB
constantly
monitors
the
DC
voltage.
The
microprocessor
serves
to
maintain
the
DC
bias
voltage
to
a
constant
value
by
varying
the
DC
bias
control
signal
(PDCPWM)
based
on
measurements
of
voltage
taken.
c.
Controlling
the
AC
Bias
to
a
Specific
Current
The
AC
bias
is
controlled
to
a
specific
current
by
the
oscillation/amplitude
control
circuit.
The
oscillation/amplitude
control
cir-
cuit
checks
the
current
of
the
AC
component
and
changes
the
amplitude
based
on
mea-
surements
of
current
taken
so
as
to
maintain
the
AC
bias
current
constant.
d.
Switching
the
DC
Bias
Voltage
Level
The
copier
switches
the
level
of
the
DC
bias
voltage
according
to
whether
or
not
copy
images
are
being
formed.
The
DC
bias
voltage
is
switched
by
the
microprocessor
on
the
composite
power
supply
PCB
by
varying
the
DC
bias
control
signal
(PDCPWM)
according
to
the
DC
bias
switching
timing
signal
sent
by
the
micropro-
cessor
on
the
DC
controller
PCB.
during
copying:
-625
V
other:
-400
V
Copy
Start
key
ON
Main
motor
(M1)
STBY
INTR
SCFW
SCRV
LSTR
STBY
P
e-heatn
Scanning
lamp
(FL1)
-400V
-625V -400V
Primary
charging
roller
DC
bias
Primary
charging
roller
AC
bias
Figure
3-307
COPYRIGHT
O
1994
CANON
INC.
CANON
PC720/740/750/770
REV.O
AUG.1994
PRINTED
IN
JAPAN
omput
AU
JAPON)
3
-
23