HP LaserJet 9040/9050 Service Manual - Page 103

Pickup and feed system The pickup and feed system is divided into two blocks: pickup/feed and fuser/delivery. This printer contains two standard pickup sources (tray 2 and tray 3) and two delivery sources (face-up and facedown bins.) The pickup unit picks up single sheets from tray 2 or tray 3. The feed unit transports each sheet to the fuser unit. When the printer is turned on, the stack of print media is lifted by the lifter to where the paper pickup is enabled. When the DC controller receives a print command, the pickup roller starts rotating. This lowers the pickup roller to the paper surface. A sheet of paper is fed to the feed rollers and the preregistration roller. The pre-registration roller corrects the skew. After a short period of time, the sheet is transferred to the photosensitive drum. The image on the drum is transferred onto the sheet, which is then transported to the fuser. Media-size detection Because the printer can contain custom-size media in all trays, the printer has a custom/standard switch and two size-detection switches. The DC controller reads information from the two size detection switches in the trays to determine media width and length. If the switches are set to the custom size setting, the "X" and "Y" dimensions must be set in the control panel. The width-detection switches also sense the presence of trays: if all of the width-detection switches are off, the DC controller assumes that a tray is missing. Media-level detection The lifter that lifts the paper detects the media level in the trays. The lifter contains a lifter blocking plate. This plate shuts off two sensors in each tray, allowing the lifter position to be detected. This detected position determines the media level. Media level is based on approximate number of sheets in the tray. Depending on the lifter position, the sensors determine paper levels as full (200 or more sheets), high (100 to 200 sheets), middle (50 to 100 sheets), or low (less than 50 sheets). Multifeed prevention The printer utilizes the separation roller method for its multiple-feed-prevention mechanism. This method drives the feed rollers and separation roller in the reverse direction from each other. The separation roller contains a torque limiter. When a multifeed occurs, the torque limiter allows the top sheet to feed into the printer, but returns the additional sheet to the tray. Overhead-transparency detection The overhead transparency sensor emits light through the media path and reads light that is reflected back from a prism. When plain paper passes through the media path, no light is reflected back, but when a transparency passes through the media path, the sensor can read the reflected light, indicating the presence of a transparency. Fuser-wrapping-jam detection The printer contains a mechanism to detect wrapping jams in the fuser. The DC controller sends signals to the pressure roller bias generation circuit. The circuit applies the same bias values to the pressure roller and to the wrapping-jam-detection circuit. The jam-detection lever solenoid monitors the pressure roller bias through contact with the pressure roller. The wrapping-jam-detection circuit compares the monitored value to the applied value for differences in the values. When they differ, the circuit determines that a wrapping jam occurred, and signals the DC controller to stop the printer. Pickup and feed system 87