Canon PC980 Service Manual - Page 101

CHAPTER 4 IMAGE FORMATION SYSTEM 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. Copy Start key ON STBY INTR SCFW SCRV SCFW SCRV LSTR STBY Main motor (M1) Scanning lamp (LA1) Primary DC bias Primary AC bias -400V -625V Figure 4-104 COPYRIGHT © 1999 CANON INC. CANON PC800s/900s REV.0 AUG. 1999 PRINTED IN JAPAN (IMPRIME AU JAPON) 4-7