Ricoh FT7770 Service Manual page 57

DRUM
1. Selenium Drum
Selenium has the following important characteristics.
1. It can accept a high positive electrical charge in the dark. (The electri-
cal resistance of selenium is high in the dark.)
2. It will dissipate the electrical charge on its surface when exposed to
light. (Exposure to light greatly increases the conductivity of selenium.)
3. The amount of charge dissipation is in direct proportion to the intensity
of the light.
surface, a smaller voltage remains on the selenium.
The sensitivity of selenium changes slightly with variations in the tempera-
ture around the drum.
charged, resulting in
drum has an internal
That is, where stronger light is directed to the selenium
Under cool conditions, the drum may be excessively
overtoning of the copy image.
heater which warms the drum if it gets too cool.
2-1
30
April, '89
To prevent this, the