Selenium Drum - Ricoh FT4430 Service Manual

DRUM

SELENIUM DRUM

Selenium has the characteristics of:
Being able to accept a high positive electrical charge in the dark. (The
1 .
electrical resistance of selenium is high in the absence of light.)
2 . Dissipating the electrical charge when exposed to light. (The conductivity
of selenium is greatly enhanced by exposure to light.)
3 . Dissipating an amount of charge in direct proportion to the intensity of the
light. That is, where a stronger light illuminates the selenium surface, a
smaller voltage remains on the selenium.
The sensitivity of selenium changes slightly with variations in the surface
temperature of the drum. (Under cool conditions, the drum may be
excessively charged; this will result in background or excessive image
density.) To prevent this, the CPU monitors the temperature variations around
the drum and changes the development bias accordingly. Also, during cold
periods, the drum heater warms the drum.
Drum sensitivity also depends on how long the drum has rested between
copy runs. The copier's CPU compensates for changes in drum sensitivity
due to rest time by changing the development bias. This prevents variations in
image density at the beginning of copy runs
The selenium drum used in this model has high sensitivity, good color
reproduction, and good reproduction of low contrast originals (pencil
originals, etc.)
2-1
16 April '88
.