Canon PC300 Service Manual page 88

MECHANICAL SYSTEM : See
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A. Cartridge
i. Outline
The copier is a cartridge type, in that its photosensitive drum, primary charging roller, devel-
oping assembly, and cleaner are designed as a single entity.
Developing blade
Primary charging roller Light-blocking shutter,
Photosensitive
Cleaning blade
Drum cover shutter Developing cylinder
Figure 4-501
a. Drum Cover Shutter
The photosensitive drum is susceptible to light; if exposed to strong light, it will develop
residual memory, causing white spots or black lines on the copies.
To avoid such problems,
you must not open the drum cover shutter unless it is absolutely necessary. The drum cover
shutter is designed to open when the cartridge is inserted into the copier and the copier's top
unit is closed.
b. Light-Blocking Shutter
The absence of a cartridge inside the copier allows light to enter the inside of the copier
from the opening created for exposure.
The light-blocking shutter is provided to protect the
photosensitive drum against intrusion of such light.