HP LaserJet 4250 Service Manual - Page 170

At the beginning of each sweep, the beams strike the beam-detect mirror and, generating the beamdetect (BD) signal. The BD signal is sent to the dc controller PCA, where it is converted to an electrical signal that synchronizes the video data output for one sweep (two scan lines) and diagnoses problems that exist in the laser diode or scanner motor. Developing the image The developing process makes the latent electrostatic image a visible image on the drum. The developing unit consists of a metallic cylinder that rotates around a fixed magnetic core inside the toner cavity. Toner is a powdery substance made of black plastic resin bound to iron particles, which is uniformly attracted to the magnetic core of the cylinder. The toner particles obtain a negative surface charge by rubbing against the developing cylinder, which is connected to a negative dc supply. The negatively charged toner is attracted to the discharged (exposed, grounded) areas on the drum. An ac potential is applied to the developing cylinder to decrease the attraction between the toner and the magnetic core of the cylinder, and to increase the repelling action of the toner against the areas of the drum that have not been exposed to the laser beam. This ac potential improves density and contrast. Figure 5-39 Developing the image The print-density control in the print-quality menu adjusts the dc bias of the developing cylinder by changing the force of attraction between the toner and drum. A change in the dc bias causes either more or less toner to be attracted to the drum, which in turn either increases or decreases the print density. Both the primary and developer dc-bias voltages change in response to the density setting. 156 5 Theory of operation