HP Laserjet 4 Manual

Conditioning Stage  
After the drum is physically cleaned, it must be conditioned. This
process consists of applying a uniform negative charge on the
surface of the drum with the primary charging roller, located in
the toner cartridge. The primary charging roller is coated with
conductive rubber with an AC bias applied to erase any residual
charges and maintain a constant drum surface potential. In
addition, a negative DC bias is applied by the charging roller to
create a uniform negative potential on...

Writing Stage  
At the writing station, the laser beam is used to discharge the
drum’s negative potential by focusing laser light onto areas of the
photosensitive drum. This creates a latent electrostatic image,
which later is developed into a visible image.
Laser light is produced by a small modulated laser diode. The
beam created by the diode shines onto a rotating polygonal
mirror. As the mirror rotates, the beam reflects off the mirror and
sweeps the page from left to right (see Figure 5-10).
A set...

At the beginning of each sweep (before the beam reaches the
drum), the beam is reflected off the beam detect mirror. The
momentary pulse of light is sent to the DC Controller, where it is
converted to an electrical signal used to synchronize the output of
data (
VDO) for one sweep (scan line). This pulse, referred to as
the Beam Detect signal (
BD), is used to diagnose problems with
the laser diode or scanner motor also.
After the writing station, the drum surface has an invisible
(latent) electrostatic...

Developing Stage  
The developing station develops the latent electrostatic image
into a visible image on the drum. The developing unit consists of
a metallic cylinder that rotates around a fixed magnetic core
inside the toner cavity. Toner is a powdery substance made of
black plastic resin bound to iron particles, which is attracted to
the magnetic core of the cylinder. A rubber blade “brushes” the
toner on the developing cylinder to a uniform thickness.
The toner particles obtain a negative surface...

The DC bias of the developing cylinder is adjusted to change the
force of attraction between the toner and drum by changing the
print density setting from the control panel. A change in DC bias
causes either more or less toner to be attracted to the drum,
which in turn either increases or decreases the print density. The
print density variation is approximately ±10% (see Figure 5-13).
Both the primary and developer DC bias voltages are changed in
response to the density setting.
Figure 5-13 Developing...

Transferring Stage   
At the transferring station, the toner image on the drum’s surface
is transferred to the paper. A positive charge applied to the back
of the paper by the transfer roller causes the negatively charged
toner on the drum’s surface to be attracted to the page.  
The small radius of the drum and the stiffness of the paper peel
the paper away from the drum. The static charge eliminator also
helps separate the paper from the drum. The static charge
eliminator weakens the attractive forces...

Fusing Stage    
At the fusing station, the toner is fused into the paper by heat
and pressure to produce a permanent image. The paper passes
between a heated fusing roller and a soft pressure roller. This
melts the toner and presses it into the paper. To keep the toner or
paper from sticking, the upper fusing roller is covered with a
non-stick, “Teflon-type” sleeve. The fusing roller contains an
halogen quartz lamp that provides heat for the fusing process.
The fusing roller temperature is monitored by...

Paper Feed System  
NoteThe LJ 5 printer designators differ as follows.
LJ 4 / 4 PlusMultipurpose (MP) Tray Paper Cassette (PC) Tray Lower Cassette (LC) Tray
LJ 5Tray 1Tray 2Tray 3
The following paper paths are described in this section of the
manual:
•PC Tray (Tray 2) to Output Trays
•MP Tray (Tray 1) to Output Trays
The paper feed system is responsible for picking paper from
either the PC tray (Tray 2) or MP tray (Tray 1) and delivering it
to the image formation system, feeding it into the fusing...

The Paper Feed Assembly performs horizontal registration of the
print media through the Oblique Rollers. As the print media
passes through the Oblique Rollers, the media is moved
uniformly to the left side of the print path. The movement
ensures that each print image is oriented properly on the
horizontal axis.
Figure 5-16 Input Paper Feed System Diagram
Functional Overview  5-27 

Figure 5-17 Paper Feed Assembly (Sectional View)
Part Description
PS1Leading Edge Registration Sensor
PS2PC Tray (Tray 2) Paper Out Sensor
PS3Exit Sensor
PS4MP Tray (Tray 1) Paper Out Sensor
PS5MP Tray (Tray 1) Paper End Sensor
SL1Cassette (Tray 2) Pickup Roller Clutch Solenoid
SL2MP (Tray 1) Tray Pickup Roller Clutch Solenoid Table 5-6
5-28 Functional Overview