Xerox 6250N Service Manual - Page 45

Selective Control; Paper Pick Unless changed in printer setup, the default tray is Tray 2. Trays in the optional HighCapacity Feeder are identified as Tray 3 and Tray 4. The tray in the optional 500-sheet Feeder is identified as Tray 3. Laser Light Intensity Control The Laser Unit in the Phaser 6250 printer has four laser diodes: one each for yellow, magenta, cyan, and black. The diodes control circuits adjust the light intensity for each color automatically. Image data is transmitted to the laser diodes in the Laser Unit as digital signals. The laser diodes convert the image data from digital signals to optical signals. The laser assembly monitors and adjusts the light intensity of the laser beams to attain a stable electrostatic image. Variations in drum sensitivity may prevent the system from obtaining a proper electrostatic image. Process Control For stable printing, the parameters related to forming both the electrostatic and toner image must be continuously monitored and adjusted by the printer. Parameter correction and control over the entire printing process is called "process control". There are two main areas of control: ■ Bias Control ■ Toner Density Control To supplement these two controls, the following are provided: ■ High-Area Coverage Mode Control ■ Admix Mode Control ■ Toner Density Control (CTD) Sensor (ADC - Automatic Density Control) LED light density setting. Bias Control To attain stable image density, toner density is monitored for each color and the drum charging voltage and biasing DC voltages are adjusted according to the monitored results for each color. The bias control adjustment is made immediately before the start of printing, if any of the following conditions is satisfied: ■ First print produced after the power on. ■ Cumulative print count exceeds 16 since power on. ■ A cleaning cycle is executed during continuous printing. 2-6 Phaser 6250 Color Laser Printer Service Manual