Ricoh FW780 Service Manual - Page 40

Rev. 5/95 2.2 CHARGE CORONA CIRCUIT Charge/Bias/Grid PP DC/DC Converter DC/DC Converter CN340-1 CN243 PSU -3 24V Va CN340-2 CN243-8 GND CN340-3 CN106 Main Board -A8 0/5 Charge CN340-4 CN340-6 CN106 -A9 -A10 Grid Drive 0/5 Grid Detect The PSU supplies +24 volts (VAA) to the charge/bias/grid power pack as the power supply source. Approximately 2 seconds after the copy process starts, the CPU drops CN106-A8 from 5 volts to 0 volts. This actuates the dc/dc converter within the power pack which applies a high negative voltage of approximately --5.0 kV to the charge corona wire. The corona wire then generates a negative corona charge. The grid limits the maximum corona charge to --860 volts. This ensures that the charge does not fluctuate and an even charge is applied to the drum surface. The copy grid voltage and charge voltage amounts can be adjusted using SP # --11 and SP #--10 respectively. The grid drive signal applied to CN340-4 is a pulse width modulated signal. As the width of the pulses applied increases, the strength of the grid charge also increases. The main board monitors the grid charge at CN106-A10 and controls the width of the drive pulses based on this feedback. SM 2-4 A163