Printhead Operation - Lexmark W820 Service Manual

4025-XXX

Printhead Operation

The MCU PWB supplies +5VDC to drive the Laser Diode semiconductor. The
semiconductor uses the +5VDC to generate a 5 milliwatt beam of invisible laser light.
As is characteristic of semiconductor devices, the Laser Diode can switch states very
rapidly. A switching circuit on the Laser Diode PWB switches the Laser Diode on and off
according to image data sent from the ESS Mother PWB.
Two lenses, an angled mirror, and another two lenses focus the laser beam onto the
rotating Polygon Mirror. The Polygon Mirror has twelve mirrored sides and rotates at
approximately 15,000 RPM. The Polygon Mirror reflects the laser beam back through the
two lenses and onto the Angled Mirror. The movement of the Polygon Mirror reflects the
laser beam in a sweeping motion, from side to side, across the Angled Mirror; with one
complete sweep for each mirrored side.
The Angled Mirror reflects the laser beam onto the Drum Mirror. The Drum Mirror is
aligned at an angle so the mirror reflects the sweeping beam down onto the surface of the
Drum. The combination of the rapid sweeping of the laser beam across the surface of the
charged Drum and the rapid switching on and off of the laser beam creates, on the Drum,
an invisible electrical image that corresponds to the screen image that was sent from the
host computer. Located at one corner of the Printhead, near the end of the Angled Mirror
is the Start of Scan Sensor (SOS) Mirror. The Start of Scan Sensor is located at the
opposite corner of the Printhead Assembly. Each sweep of the laser beam begins by
striking the SOS Mirror, reflecting the beam into the SOS Lens, which focuses the beam
onto the SOS Sensor. The SOS Sensor is electrically connected to the MCU PWB. When
the laser beam strikes the Sensor, the Sensor notifies the MCU that a new scan has
started.
1-48
Service Manual