Xerox DocuPrint N2025 Service Manual page 285

The laser light beam is modulated according to the Video signal from the Controller:
switched on when the video signal denotes a black pixel point and off when it denotes a
white pixel point. This is known as a "write black" system.
The laser beam shining onto the drum surface generates electron-hole pairs in the photo-
conductive layer by exciting electrons to the conduction band and thereby creating holes
in the valence band. The electrons are attracted by the electric field and move toward the
inner conductor of the drum to flow into it. The holes move toward the outside surface of
the photoconductive layer and recombine with the negative charge (electrons) on the sur-
face, decreasing the negative charge there. The resulting less negative charged parts on
the drum surface form an invisible electrostatic latent image.
Figure 4 Exposure
3. Development (1/2)
Initial Issue
DocuPrint N2025/N2825
This step attracts toner over the electrostatic latent image on the drum surface to form a
visible toner image. Figure 5
This printer uses non-conductive magnetic toner that is easy to transfer to plain paper. In
the Print Cartridge, the toner is supplied from the toner compartment to the Magnet Roll.
The toner is attracted to the surface of the Magnet Roll by the magnetism of the Magnetic
Roll. The CMB (Charging and Metering Blade) spreads the toner on the Magnetic Roll
into a very thin layer with a silicone rubber blade. The toner is given a negative electric
charge by the friction with the silicone rubber. As the Magnet Roll rotates, the toner is
transported to the gap between the drum and the Magnet Roll to contact the drum surface
at the nip.
The Mag. Roll is covered with a thin, semi-conductive sleeve. A DB (Development Bias)
voltage is applied to the Conductive Sleeve from the HVPS Figure 6. The DB voltage is a
negative DC voltage with an AC voltage superimposed. The DB voltage retains the Mag-
netic Roll at a DC electric potential between the electric potential of the areas on the drum
surface with a full negative charge and that of the areas on the drum surface with the
decreased negative charge with respect to the conductive layer of the drum (i.e., the
latent image), thereby generating an electric field that attracts the negative charged toner
to the areas on the drum surface with the decreased negative charge and repels it from
the areas wit the full negative charge, in the gap between the drum and the Magnet Roll.
The "Knob Volume Deve" on the HVPS varies the DB voltage slightly acting as a fine tun-
ing control over the amount of development that takes place.
In the presence of this development bias field, the toner transfers selectively to those
areas with the decreased negative charge.
The AC voltage agitates toner particles, making toner transfer easier and causing extra
toner on the drum surface to migrate back to the Magnet Roll.
The invisible electrostatic latent image on the drum surface is thus developed into a visible
toner image.
02/2000
6-53
General Procedures / Information
GP 5.1