Brother HL-4200CN Service Manual page 109

7.4.2 Toner density control
The toner density must be kept constant to attain stable printing image. To keep the
toner density constant, the toner should be dispensed exactly by the quantity consumed
for the printing. This system is the PCDC. However, the system with only the PCDC
generates a difference from target toner density, thus requiring the system to correct this
error. This is the toner density control by the holder ASSY CTD. These two control
systems are altogether called the toner density control.
1) PCDC (Pixel count dispense control)
The toner quantity consumed in the developing process is calculated by counting the
video signals entered to the scanner ASSY. The video signal counting is made by
the charging and discharging of the capacitor in the CR circuit arranged in parallel to
the video signal line. The consumed toner quantity is calculated by the toner
dispense time. The Toner Motor in the holder toner cartridge ASSY is driven by the
amount of calculated toner dispense time to supply the toner into the developer.
2) ADC (Auto density control)
The patches of respective colors (yellow, magenta, cyan, and black) for the toner
density control are generated under specified potential condition, and transferred on
the transfer roll cartridge. The holder ASSY CTD measures this density. The
measured value is compared with reference value, and if the toner density is low,
the toner dispense quantity is increased at the next printing, or if the toner density is
high, the toner dispense quantity is reduced at the next printing. The toner dispense
quantity is calculated by the toner dispense time. This calculation is made for each
color.
ADC is made after the completion of printing, if either of the following conditions is
satisfied:
• When cumulative print count after the power on exceeds 16
• When a cleaning cycle is executed during continuous printing
3) Example of toner dispensation
The toner motor revolves for the duration of the specified time per 1-time
dispensation. The toner dispense time in 1) and 2) above is calculated with the
number of revolutions of toner motor. In the following description, this is called the
dispense count.
The dispense count calculated by the ADC is cancelled in the subsequent 8 prints.
If 16 dispense counts were calculated as a lack of toner by the ADC, the toner is
dispensed additionally to the dispense count calculated by the PCDC by 2 counts
(16(8=2) in the subsequent 8 prints.
Or, if 16 dispense counts were calculated as an excess of toner by the ADC, the
toner is dispensed by subtracting from the dispense count calculated by the PCDC
in the subsequent 8 prints. If the excess amount cannot be subtracted in 8 prints,
the remainder is subtracted in 9 and subsequent prints.
Total dispense counts
Correction amount by ADC +16
Dispense counts by PCDC
Correction amount by ADC -8
Total dispense counts
4 5 3 4
Even allotment in first 8 prints
2 3 1 2
1 2 0 1
Fig.3-46
III-45
2 2 3 4
0 0 1 2
0 0 0 0
3 2
3 2
2 2