HP 4550 Service Manual - Page 122

Note While the toner cartridge is facing the photosensitive drum, light from the light emitter diode passes through the light guide in the carousel and enters the detection port of the toner cartridge. The light passes through the cartridge when the toner around the detection port is scraped off by the cartridge stirring function and is detected by the light receiver of the toner level sensor (PS12) (see Figure 5-23). The printer measures the time it takes for the toner to cover the port again so that light cannot pass through. This measurement determines the amount of toner remaining in the toner cartridge. When the light passing through the cartridge exceeds a specified time before being interrupted by toner, the DC controller warns the formatter that the toner life is low. The TONER LIFE LOW message also appears on the control panel. When light passes through the cartridge all the time without interruption, the printer is stopped by the DC controller, and the formatter is notified that no toner is left in the cartridge. A TONER OUT message also appears on the control panel. The stirrer is also used to supply toner smoothly to the sleeve. The toner cartridge motor (M5) rotates the stirrer and the toner feed roller. Both the stirrer and the toner feed roller stir the toner in the toner cartridge. Figure 5-23 Toner level detection/toner cartridge detection Note The Color LaserJet 4550 series has a continuous toner level sensor (CTLS) that was not available in the 4500 series. The continuous toner level sensing printed circuit assembly (PCA) is mounted on the right side of the printer. The sensor allows the printer to track toner usage and to display toner levels on the Supplies Status page. Laser/scanner operation The DC controller signals the laser driver to activate the laser diode that generates the laser beam. The laser beam is aligned by the collimator lens and the cylindrical lens (see Figure 5-24). The beam then strikes the six-faced prism mirror located atop the scanner motor, while the mirror is rotating at a constant speed. The beam is then reflected and brought to a point to maintain a consistent round dot (focused) on the drum surface by the focusing lens. At the beginning of the scan line, the beam first strikes the beam detect (BD) mirror and then the BD PCB. After the initial strike, the beam strikes the drum surface. With the drum rotating at a constant speed, the laser beam successively scans across the drum's surface, exposing the image. 120 Theory of operation C7085-90921