HP LaserJet 4 Service Manual - Page 135

The DC bias of the developing cylinder is adjusted to change the force of attraction between the toner and drum by changing the print density setting from the control panel. A change in DC bias causes either more or less toner to be attracted to the drum, which in turn either increases or decreases the print density. The print density variation is approximately ±10% (see Figure 5-13). Both the primary and developer DC bias voltages are changed in response to the density setting. Figure 5-13 Developing Potentials Functional Overview 5-23