HP LaserJet 4 Service Manual - Page 273

Table 7-22 Black Pages with Horizontal White Strips Possible Cause Laser/Scanning Assembly or DC Controller PCA. Action Replace the Laser/Scanning Assembly followed by the DC Controller PCA, if necessary (see Figure 7-12 for locations). Table 7-23 Image Skew Possible Cause Action Input/Registration Sensor (PS1). The Paper Pickup Roller positions the leading edge of the paper at the Input/Registration Sensor (PS1). If this sensor is defective it may not sense the presence of the paper reliably (on time). Replace the Input/Registration Sensor. Skew tolerance for the printer is 1.5mm over 260mm page length. Drive Gears. If any gear within the gear drive train becomes excessively worn, or is excessively dirty, erratic paper movement would result, leading to image misregistration. Closely examine the gears along the paper path for wear and contamination. Clean and inspect the gears. Replace any defective gear. Paper Tray. The paper tray may be preventing the paper from freely moving through the printer. Verify that the tray is loaded correctly. Verify that the paper tray is seated fully into the printer. Switch trays to verify that the problem follows the tray. Replace the paper tray. Paper. The surface of the paper may be too smooth for the pickup roller to move the paper through the paper path. Change paper. Try paper known to be good. Paper too heavy? See Paper Specifications in Chapter 2. Troubleshooting 7-43