Xerox Phaser 6121MFP Service Manual page 68

Theory of Operation
Developer Roller Drive
2-26
4. To attract toner to the drum, a developing bias voltage (Vb) that includes both
DC (-) and + AC components is applied to the developing roller during
development. The AC component is applied only during development. The
developing bias voltage (Vb) supplied by the High Voltage Power Supply is
applied to the developing roller at the contact point shown below.
The Developer Roller is driven by the Developing Motor and intermediate gears.
Developing Roller
Developing Roller Drive Gear
Bias Contact Point
Developing Motor (M3)
Phaser 6121MFP Service Manual
s6121mfp-283
Intermediate Gear
s6121mfp-128