HP Color LaserJet 8500 Service Manual - Page 171

Mechanical systems Laser/scanner The laser/scanner unit scans the laser beam across the imaging drum. Video signals sent from the formatter are pulse-width modulated and converted to video data signals. The video data signals are then converted to two low-voltage differential signals (which suppress radio frequency emissions), and are sent to the laser driver PCA. There the voltage differential signals are converted to a single laser drive signal (the internal signal of the laser/scanner unit) in the laser driver PCA, which turns the laser diode on and off and generates the modulated laser beam. The modulated laser beam is aligned by the collimator lens and the cylindrical lens, becoming a parallel beam. The laser beam then strikes the scanning mirror that rotates at a constant speed. The beam reflected from the scanning mirror travels through the focusing lens and the reflecting mirror located in front of the scanning mirror, and is brought to a focus point on the imaging drum. The scanning mirror rotates at a constant speed, so the laser beam is scanned across the drum at a constant speed. The drum is also rotating at a slower constant speed, which allows the laser beam to form an image on the drum surface. EN Mechanical systems 169