Ricoh FT8680 Service Manual page 34

PROCESS CONTROL
1.3.7 Vd Correction
Exposure
Glass
Vd Pattern
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The drum potential just after the black pattern (Vd Pattern) is exposed (Vd:
Dark Potential) tends to lower during drum life due to a decrease in the
drum's capacity to carry a charge.
To check the actual Vd, the first scanner moves to the return position and the
Vd pattern (Black) stuck on the bottom side of the exposure glass bracket is
exposed on the drum.
The main PCB measures Vd through the drum potential sensor and adjust it
to a target value by adjusting the charge power pack output.
On the other hand, there is a change of the drum residual voltage (Vr), so
that the target Vd voltage is compensated as follows:
Target Vd Value: Vd = Vr + 770 V - 20 M
NOTE: M is the setting for the Latent Image Adj. in the SP mode.
The adjusted charge power pack output control value (PWM value) is kept in
memory until the next initialization.
If the target value cannot be achieved within the charge power pack output
control range, the copier indicates the system error on the CRT screen. The
copier tries the Vd check again after the error reset. Even if the second Vd
adjustment fails, no SC condition is displayed. The previous power pack
output control data will be used in this case.
Vd
Drum
Potential
Dark
2-10
31 July 1995
Vr
Original Density
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Light