Canon PC920 Service Manual page 101

d. Switching the DC Bias Level
The machine switches the DC bias level between the copy image area (-625 V) and the non-
copy image area (-400 V).
The level of the DC bias is switched by the DC bias ON signal (serial signal) generated by the
DC controller PCB.
The microprocessor (Q900) on the composite power supply PCB varies the DC bias control
signal (PDC_PWM) to switch the level of the DC bias. (See Table 4-102.)
Reference:
The machine applies a DC bias over a non-copy image area (between sheets) so as to prevent
charging the photosensitive drum to a positive potential otherwise caused by the cleaning bias
from the transfer roller.
Main motor (M1)
Scanning lamp (LA1)
Primary DC bias
Primary AC bias
COPYRIGHT © 1999 CANON INC.

 
 
 
Copy Start key
ON
STBY INTR SCFW
-400V -625V
Figure 4-104
CANON PC800s/900s REV.0 AUG. 1999 PRINTED IN JAPAN (IMPRIME AU JAPON)
CHAPTER 4 IMAGE FORMATION SYSTEM
SCRV SCFW SCRV LSTR STBY
4-7