HP 4300 Service Manual - Page 141

Writing the image The laser/scanner contains two diodes in the laser unit. During the writing process, the modulated laser diodes project two beams onto the rotating six-sided scanning mirror. As the mirror rotates, the beams reflect off the mirror, through a set of focusing lenses, through a slot in the top of the toner cartridge, and onto the photosensitive drum. The beams sweep the drum from left to right, discharging the negative potential wherever the beams strike the surface. This creates a latent electrostatic image, which later is developed into a visible image. Laser Beam Unexposed area Exposed area Figure 52. Writing the image to the photosensitive drum Because the beams are sweeping the entire length of the drum and the drum is rotating, the entire surface area of the drum can be covered. The speed of the scanner motor (which turns the scanning mirror) and the speed of the main motor (which turns the drum) are synchronized, and each successive sweep of a beam is offset by 1/1200th of an inch. The beams can be turned on and off to place a dot of light every 1/1200th of an inch. This is how the printer achieves its true 1200 by 1200 dpi resolution. After the writing process, the drum surface has an invisible (latent) electrostatic image. At the beginning of each sweep, the beams strike the beam detect mirror and PCB, generating the beam detect (BD) signal. The BD signal is sent to the DC controller PCA, where it is converted to an electrical signal this is used to synchronize the output of video data for one sweep (two scan lines) and to diagnose problems with the laser diode or scanner motor. Q2431-90912 Chapter 5 Theory of operation 121