Xerox Phaser 6200, Service Manual

Page: 325 / 386

Theory of Operation 307
Toner Density Control
Toner density must be kept constant to attain a stable print image. To keep the toner
density constant, the toner is dispensed at exactly the same rate it is consumed. There
are two systems to control toner density: the PCDC and CTD working in conjunction.
PCDC (Pixel Count Dispense Control) : The amount of toner consumed in the
developing process is calculated by counting the video signals entered into the Laser
Unit. The video signal count is made by the charging and discharging of the capacitor
in the CR circuit performed simultaneously to the video signal line. The consumed
toner quantity is calculated by the toner dispense time. The toner motor is then driven
by the calculated toner dispense time, supplying toner to the developer.
CTD (ADC Auto Density Control) : The patches of respective colors (yellow,
magenta, cyan, and black) for the toner density control are generated and transferred
by the transfer roller. The CTD (ADC) sensor measures this density. The measured
value is compared with the reference value and if the toner density is low, the
dispensed toner quantity is increased on the next print, or if the toner density is high,
the dispensed toner quantity is reduced on the next print. The dispensed toner quantity
is calculated by the toner dispense time. This calculation is made for each color.
S6200-051
Total dispense counts
Correction amount by ADC +16
Dispense counts by PCDC
Correction amount by ADC -8
Total dispense counts
2 3 1 2 0 0 1 2 3 2
Even allotment in first 8 prints
4 5 3 4 2 2 3 4 3 2
1 2 0 1 0 0 0 0 2 2