Controlling The Transfer Roller Bias - Canon NP6621 Service Manual

E. Controlling the Transfer
Roller Bias
1. Outline
The machine uses a direct transfer method that
uses a roller; and, for this reason, it executes bias
control in a way different from copiers that rely on
corona charging.
The transfer roller bias may be any of three
types and has the following function and output
timing:
a. Transfer Bias
This bias corresponds to the transfer bias of
copiers that rely on corona charging. (constant
voltage of -3.5 kV to -6.0 kV DC)
b. Cleaning Bias
Since the machine uses a direct transfer method,
toner from the photosensitive drum can accumulate
on the transfer roller because of jams.
Copy Start key
Main motor (M1)
Transfer bias
Cleaning bias (+)
Reference bias (ATVC)
Static eliminator bias
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INTR SCFW
Figure 3-305
CANON NP6621 REV.0 FEB. 1998 PRINTED IN JAPAN (IMPRIME AU JAPON)
To remove such a build-up of toner, a positive
voltage (constant, DC) is applied, thereby returning
the toner from the transfer roller to the photosensitive
drum. The bias is applied as follows:
• during initial rotation after the Copy Start key
is pressed.
• part of the period in which the scanner is
moving in reverse.
• part of last rotation.
c. Reference Bias (ATVC)
Changes in the environment or deterioration in
the transfer roller can affect the resistance of the
transfer roller, possibly affecting the transfer
efficiency.
To suppress the changes in the image quality
caused by changes in the transfer efficiency, the
machine corrects the application voltage level of
the transfer bias.
The transfer ATVC bias (constant current, -190
µA DC) is a bias applied each time the Copy Start
key is pressed so that the aforementioned correction
value may be determined.
SCRV SCFW
OPERATIONS AND TIMING
SCRV LSTR STBY
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