HP LaserJet 4 Service Manual - Page 153

The formatter then sends VDO data to the DC controller. The DC controller translates the VDO signal into a VIDEO OUT signal (VDOUT), which modulates the laser diode ON and OFF, creating intermittent bursts of laser light that are reflected off the scanner mirror and onto the photosensitive drum to form a latent electrostatic image. Upon receipt of a BD signal from the DC controller, VDO data is sent from the formatter for each scan line of print data on a page. Before the next sheet of paper arrives, the voltage on the transfer roller switches from the nominal standby voltage to a positive high voltage, to prepare to transfer the toner image from the drum to the paper. The process repeats for each successive page as necessary. If a failure occurs during this period, the following errors may be displayed: 41 ERROR This error occurs when a sheet of paper is fed too soon or when an unstable BD or TOP signal is received (see "41.x ERROR" message in Chapter 7). 50 SERVICE This error occurs when the fusing roller fails to rise to the correct temperature in the required time (see "50 SERVICE") in Chapter 7). 51 ERROR This error occurs when the BD signal is not received by the DC controller (see "51 ERROR" in Chapter 7). 52 ERROR This error occurs when the scanner motor fails (see "52 ERROR" in Chapter 7). Functional Overview 5-41