HP LaserJet 4 Service Manual - Page 280

Drum Rotation Functional Check The photoconductive drum, located in the Toner Cartridge, must rotate for the print process to work. The photoconductive drum receives its drive from the Main Motor gear train. To verify whether the drum is rotating: 1 Open the printer's Top Cover Assembly. 2 Remove the Toner Cartridge. Open the Toner Cartridge's drum shield (cover) to view the photoconductive drum. 3 Mark the gear with a felt-tipped marker. Note the position of the mark. 4 Install the EP Toner Cartridge, and close the Top Cover. The 1-second startup sequence should rotate the drum enough to move the mark. Open the printer, and inspect the drum. Verify that the mark moved. If the mark did not move, inspect the gear train to ensure that it is meshing with the Toner Cartridge gears. If the drive gears appear functional, and the drum does not move, replace the Toner Cartridge. Note This test is especially important if refilled Toner Cartridges were used. 7-50 Troubleshooting