Canon GP215 Service Manual page 116

3. OPERATIONS AND TIMING
4. Correcting the Drum Surface
Potential
The drum surface potential fluctuating in rela-
tion to changes in the machine internal tempera-
ture is corrected by varying the primary charging
roller bias voltage. The machine internal temper-
ature is detected by the thermistor mounted to the
rear plate of the machine, and the data is sent to
the CPU on the DC controller PCB.
The CPU n the DC controller PCB subjects the
data to computation, and the results are sent to
the composite power supply PCB in serial com-
munication mode.
The composite power supply PCB, in
response, uses the data to determine the primary
charging roller bias (DC) value.
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COPYRIGHT © 1996 CANON INC.
5. Resistance of the Photosensitive
Drum
The photosensitive drum can be thought of as
a type of resistor. The resistance of the photosen-
sitive drum tends to decrease because of
changes over time.
To measure the resistance, the surface of the
photosensitive drum is charged to an even poten-
tial at a specific voltage (DC + AC). The CPU on
the composite power supply PCB samples the
current of the charges being applied to find out
the resistance of the photosensitive drum. The
result is converted into a density correction curve
to exert control for stable generation of good qual-
ity images, and this mode of control is called
automatic density correction curve selection
(AGC).
The timing at which measurements are taken
is as follows:
• During initial rotation after a press on the
Copy Start key
• During initial rotation after opening/closing
the front door (except for jam removal pur-
poses)
• During initial rotation after turning on the
power switch on the control panel
• During post rotation after generating 500
copies/printouts
CANON GP215/200 REV.0 JULY 1996 PRINTED IN JAPAN (IMPRIME AU JAPON)