Automatic Drum Current Adjustment - Ricoh FT5233 Service Manual

12 February 1992

8.4.2 Automatic Drum Current Adjustment

When SP mode #57 (auto drum current adjustment) is performed, the high
voltage supply for transfer corona turns on first. The I/O CPU detects the
drum current and compares it with the target current. If it is different, the I/O
CPU changes PWM value which is sent to IC137 (PWM timer) by an 8 bit
signal through data bus line. Then the PWM timer generates pulses with a
new width and send then to the high voltage supply board - CTBG. The new
drum current is fed back to the I/O CPU and it determines the final PWM
value by repeating this feedback routine. The determined PWM value is
stored in the RAM board through IC135 (main CPU).
Second, the high voltage supply for separation corona turns on and the I/O
CPU determines the proper PWM values for the separation (- ac and -dc) by
doing the same routine as for the transfer corona. These PWM values are
also stored in the RAM board.
For the charge corona output, this automatic drum current adjustment is not
applied because the drum current is determined by setting the charge grid
bias voltage. (The grid bias voltage is set by inputting a voltage correction
data for individual high voltage supply board -CTBG in SP mode #62.)
NOTE: This automatic drum current adjustment must be done with a new
drum, new corona wires, and a clean corona unit casing. The
production line adjusts the drum currents more precisely with a
special tool and the factory setting has wider margin against the
variation of each OPC drum characteristics than this adjustment
procedure. Therefore, the manufacturer recommends not to
perform this automatic drum current adjustment unless the high
voltage supply board is replaced.
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