Ricoh FT5233 Service Manual page 48

12 February 1992 DRUM CHARGE
3. DRUM CHARGE
3.1 OVERVIEW
[A]
[B]
This copier uses a double corona wire scorotron system for drum charge.
Two corona wires are required to give sufficient negative charge on the drum
surface because of a rather high drum speed (180 mm/sec.). The stainless
steel grid plate [A] makes the corona charge uniform and controls the amount
of negative charge on the drum surface by applying the negative grid bias
voltage.
The high voltage supply board - CTBG [B] gives a constant corona current to
the corona wires (– 500 µA (– 5.6 kV)) and a constant bias voltage to the grid
plate (– 745 V: for image, – 500 V: for sensor pattern). The corona current is
controlled by the variable resistor in the circuit and the grid bias voltage is
controlled by a PWM (Pulse Width Modulation) system.
The transport fan located above the fusing unit provides an air flow to the
charge corona unit to prevent uneven built-up of negative ions that can cause
an uneven charge of the drum surface.
The air around the drum is exhausted outside of the copier by the exhaust
blower located on the rear side frame. The exhaust blower assembly has two
ozone filters (ceramic type and paper type) to absorb the ozone (O ) around
3
the drum generated by the corona discharge.
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