HP 4550 Service Manual - Page 113

Toner density Maximum toner density is monitored and corrected if it is outside parameters to prevent variations in the toner density. Toner density can vary as a result of environmental conditions, a deteriorated photosensitive drum, or the amount of toner used. A pattern of toner patches with varying densities is laid on the photosensitive drum. The DC controller detects the density of these patches and determines the optimum developing bias for each color. The DC controller uses the density sensor (PS14), located in the drum drawer assembly, to read the density of the density detection pattern. PS14 contains a density detection circuit, a lightemitting diode (LED), and photodiodes 1 and 2. To read the density, the sensor emits light from the LED, reflects it off the density detection pattern on the photosensitive drum, and receives it with photodiode 2. Photodiode 1 receives the light directly from the LED. The signals from photodiodes 1 and 2 are then compared to determine density values. Toner density is monitored and corrected during the following occurrences:
when the power is activated
after a certain number of sheets are printed
when a toner cartridge is replaced
when the drum cartridge is replaced
when the printer wakes up after a standby mode of six hours or more Figure 5-12 Toner density sensor C7085-90921 Chapter 5 Theory of operation 111