HP 4200, 43 Service Manual bpl12625
5 Theory of operation
Contents
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
Basic operation of the printer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
Printer operating sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
Control system overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
Pickup and feed system overview . . . . . . . . . . . . . . . . . . . . . . . . . 68
Laser/scanner system overview. . . . . . . . . . . . . . . . . . . . . . . . . . . 68
Image formation system overview . . . . . . . . . . . . . . . . . . . . . . . . . 68
General descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
DC controller PCA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
Motor and fan control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
Power supply. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
Fuser-control circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
Fuser over-temperature protection. . . . . . . . . . . . . . . . . . . . . . . . . 72
High-voltage circuit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
Low-voltage circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
Overcurrent/overvoltage protection . . . . . . . . . . . . . . . . . . . . . . . . 75
Toner detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
Cartridge detection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
Cartridge memory. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
Laser/scanner assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
Laser/scanner control. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
Paper pickup system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
Paper pickup and feed block. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
Printing from tray 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
Printing from tray 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
Tray 2, 500-, 1,500-sheet feeder media size detection . . . . . . . . . 83
Lifter-driver operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
Multiple feed prevention. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
Media skew prevention . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
Fixing/delivery block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
Printer jam detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
Printer pickup delay jam from tray 1. . . . . . . . . . . . . . . . . . . . . . . . 87
Printer pickup delay jam from tray 2. . . . . . . . . . . . . . . . . . . . . . . . 87
Printer pickup stationary jam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
Printer delivery wrap jam when feeding regular media . . . . . . . . . 88
Printer delivery wrap jam when feeding non-regular media . . . . . . 88
Printer delivery delay jam. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
Printer door open jam. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
Printer residual media jam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
Printing from the 500-sheet feeder . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
500-sheet feeder pickup and feeding. . . . . . . . . . . . . . . . . . . . . . . 90
Printing from the 1,500-sheet feeder. . . . . . . . . . . . . . . . . . . . . . . . . . 92
1,500-sheet feeder pickup and feeding . . . . . . . . . . . . . . . . . . . . . 92
1,500-sheet feeder lifting mechanism . . . . . . . . . . . . . . . . . . . . . . 94
Q2431-90912 Chapter 5 Theory of operation 65
Envelope feeder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
Envelope feeder pickup and feeding. . . . . . . . . . . . . . . . . . . . . . . 96
Envelope feeder jam detection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
Envelope feeder pickup delay jam . . . . . . . . . . . . . . . . . . . . . . . . 97
Envelope feeder pickup stationary jam . . . . . . . . . . . . . . . . . . . . . 97
Duplexer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
Reversing and duplexer pickup. . . . . . . . . . . . . . . . . . . . . . . . . . . 99
Duplexer jam detection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
Stacker and stapler/stacker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
Stacker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
Stacker feed and delivery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
Stacker jam detection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
Stacker feed jam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
Stacker feed stationary jam. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
Stacker residual media jam. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
Stapler/stacker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
Stapler/stacker feed and delivery . . . . . . . . . . . . . . . . . . . . . . . . 107
Staple mode feed and delivery . . . . . . . . . . . . . . . . . . . . . . . . . . 109
Stapler unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
Stapler unit operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
Staple level detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
Stack mode feed and delivery. . . . . . . . . . . . . . . . . . . . . . . . . . . 116
Stapler/stacker jam detection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
Stapler/stacker feed jam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
Stapler/stacker feed stationary jam. . . . . . . . . . . . . . . . . . . . . . . 117
Stapler/stacker delivery jam . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
Stapler/stacker residual media jam . . . . . . . . . . . . . . . . . . . . . . . 117
Image-formation system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
Electrostatic latent-image formation . . . . . . . . . . . . . . . . . . . . . . . . . 120
Primary charging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
Writing the image . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
Developing the image . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
Transferring the image . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
Fusing the image. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
Cleaning the transfer charging roll er and photos en si tiv e drum . . 125
Print cartridge memory chip . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
Formatter system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
PowerSave . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
Resolution Enhancement technology. . . . . . . . . . . . . . . . . . . . . . . . 127
EconoMode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
Input/output. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
Parallel interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
Expanded I/O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
Flash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
Hard-disk accessory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
CPU. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
Printer memory. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
Read-only memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
Random-access memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
DIMM slots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
Firmware DIMM. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
Nonvolatile memory. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
Memory Enhancement technology . . . . . . . . . . . . . . . . . . . . . . . 129
PJL overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
PML . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
Control panel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
66 Theory of operation Q2431-90912
Introduction
This chapter presents an overview of the relationships between major components in the printer.
It also provides a general description of the following:
● Basic operation of the printer
● Power supply
● Laser/scanner assembly
● Image formation
● Paper pickup and feeding
● 500-sheet feeder operation
● 1,500-sheet feeder operation
● Envelope feeder
● Duplexer
● Stacker and stapler/stacker
Basic operation of the printer
Printer operation can be divided into four systems. The control system (which includes the power
supply and DC controller PCA), the pickup and feed system (which consists of various rollers
and transports the media through the printer, the laser/scanner system (which forms the latent
image on a photosensitive drum), the image formation system (which transfers a toner image
onto the print media), and.
Printer operating sequence
The operating sequence is controlled by a microprocessor on the DC controller PCA. The table
in this section describes the basic operating sequence from when the printer power is turned on
until the final printed page is delivered to an output bin. For information about the timing of the
basic operating sequence, see “HP LaserJet 4200 general timing diagram” on page 357 and
“HP LaserJet 4300 gene ral tim ing diagram” on page 358.
Table 30. Basic printer operating sequence
Period (sequence) Description
Waiting This is the period of time from when the printer power is turned on until the main
motor or drum motor (HPLaserJet 4300 only) begins to rotate. During this time
the transfer roller is cleaned and the microprocessor on the DC controller PCA
checks to determine if a print card ridge is installed in the printer.
Standby This is the period of time from the end of the waiting sequence until the print
command is input from the host computer, or from the end of the last rotation is
input from the sequence (described below) until a print command host computer,
or until the printer po wer is turned off. Th e m essa ge READY appears on th e control-
panel display.
Initial ro tation This is the period of time when th e photos ensit iv e drum is stab iliz ed to prepa re f or
printing.
Print This is the period of time from the initial rotation until control system detects the
page entering the printer (the p age is d etec ted by the top of pa ge sen so r (PS1 03).
Last rotation This is the period of time from the completion of the print job until the main motor
or drum motor (HP LaserJet 4300 only) stops. The final page of the job is
delivered to an output bin and the transfer roller is cleaned. If another print job is
immediately detected (sent by the host computer) the printer returns to the initial
rotation period. If no print jobs are waiting, then the printer returns to the standby
period.
Q2431-90912 Chapter 5 Theory of operation 67
Control system overview
The control system consists of the power supply and the DC controller PCA. It controls the
pickup and feed, laser/scanner, and image formation systems. The microprocessor on the DC
controller PCA controls the operating sequence of the printer.
When the printer power is in the standby sequence (see table 30 on page 67), direct current
power (dc voltage) is supplied to the DC controller PCA by the power supply . When the printer is
in the standby sequence (see table 30 on page 67) the microprocessor on the DC controller PCA
sends signals to turn on and off various solenoids, motors, and other printer components needed
to process and print the image data input by the host computer.
Pickup and feed system overview
The pickup and feed system consists of a motor, various rollers, and sensors that detect the
presence of media, transport the media into and through the printer, and deliver the media to an
output bin.
If during the transport process, the media does not reach specific sensors in a specified time, the
microprocessor on the DC controller PCA halts the motor and a jam message appears on the
control-panel display.
Laser/scanner system ov erview
The laser/scanner system forms a latent (or potential) image on a photosensitive drum according
to signals sent from the microprocessor on the DC controller PCA.
The main components of the laser/scanner assembly are the laser driver PCA, the scanner
motor and a six-sided mirror. The DC controller PCA sends image data signals to the laser/
scanner assembly. The laser/scanner PCA converts these data signals into a laser beam of light.
The laser beam of light is reflected by the six-sided mirror onto a photosensitive drum (in the
print cartridge) and a latent image of the image to be printed is created.
Image formation system overview
The image formation system uses toner in the print cartridge to transfer the latent image on the
the photosensitive drum to the media. Heat and pressure (from the fuser) are used to
permanently bond the toner image to the media.
The photosensitive drum (in the print cartridge) receives a uniform negative primary charge that
will be exposed to the laser beam of light.
The photosensitive drum is exposed to the laser beam and an electrostatic latent image is
created on the drum (this image is invisible to your eye) by the laser neutralizing specific areas of
the drum’s surface. When the areas exposed to the laser beam come in contact with toner, the
toner is attracted to them (now the image can be seen on the drum).
The transfer roller applies a positive charge to the back of the media. As the media passes the
photosensitive drum the toner image is attracted to the media and transfers from the drum to the
media.
The media then passes through the fuser where heat and pressure are applied to permanently
bond the toner to the media.
68 Theory of operation Q2431-90912
General descriptions
This section describes individual components found in the printer. Information is provided about
the following components.
● DC controller PCA ● 500-sheet feeder
● Power supply assembly ● 1,500-sheet feeder
● Pickup and feed assembly ● envelope feeder
● Laser/scanner assembly ● Duplexer
● Image formation system ● Stacker and stapler/stacker
DC controller PCA
The DC controller PCA controls the operation of the printer and its components. The DC
controller PCA starts printer operation when the printer power is turned on and the power supply
sends dc voltage to the DC controller PCA. After the printer enters the standby sequence (see
table 30 on page 67. the DC controller PCA sends out various signals to operate motors,
solenoids and other printer components based on the print command and image data sent by the
host computer. For a list of DC controller PCA connectors, see figure 233 on page 356.
HP LaserJet 4300 only
Figure 6. DC controller PCA block diagram
Q2431-90912 Chapter 5 Theory of operation 69
Motor and fan control
The HP LaserJet 4200 printer has three dc brushless motors. The main motor, the lifter driver
motor (inside of the lifter driver assembly), and a fan motor. The main motor is used for image
formation (rotating the photosensitive drum in the print cartridge) and paper pickup and feed.
The lifter motor raises the plate in the tray cassette. The fan motor rotates the fan blades.
The HP LaserJet 4300 printer has five dc brushless motors. The main motor, the print cartridge
motor, the lifter motor, and two fan motors. the main motor is used for paper pickup and feed.
The print cartridge motor rotates the photo sensitive drum (the photosensitive drum used in the
larger HP LaserJet 4300 print cartridge is heavier than the one used in the HP LaserJet 4200
printer). The lifter motor raises the plate in the tray cassette. Two fan motors rotate the left- and
right-side fans.
The DC controller PCA controls the operation of the motors and fans.
Table 31. Printer fans and motors
Motor names Purpose Type Rotation Speed Failure
detection
Motor Main motor (M101)
HP LaserJet 4200
Main motor (M101)
HP LaserJet 4300
Print cartridge
motor (M102)
HP LaserJet 4300
Lifter motor (M103)
HP LaserJet 4200
HP LaserJet 4300
Fan Left-side cooling
fan (FN101)
HP LaserJet 4200
HP LaserJet 4300
Drives the tray cassette
pickup roller, feed /
separation roller, tray 1
pickup roller, pretransfer roller,
photosensitive drum,
developing cylinder,
pressure roller, and
output delivery roller
Drives the tray cassette
pickup roller, feed /
separation roller, tray 1
pickup roller pretransfer rol ler , pres sure
roller, and output
delivery roller.
Drives the transfer
charging roller,
photosensitive drum,
and developing
cylinder.
Moves the tray cassette
lifting plate up and
down.
Cools the inside of the
printer
dc
Counter
motor
clockwise
dc
Counter
motor
clockwise
dc
Counter
motor
clockwise
dc
Counter
motor
clockwise
dc
NA 2-speed
motor
2-speed
(full and half)
2-speed
(full and half)
2-speed
(full and half)
1-speed Yes
(full and half)
Yes
Yes
Yes
Yes
Right-side cooling
fan (FN102)
HP LaserJet 4300
Cools the inside of the
printer.
dc
NA 1-speed Yes
motor
70 Theory of operation Q2431-90912
Po wer supply
The power supply consists of the fuser-control circuit, the high-voltage circuit, and the lowvoltage circuit. The fuser-control and high-voltage circuits control the temperature of the fuser
and generate high-voltage according to signals from the DC controller PCA. The low-voltage
circuit generates the dc voltages used by other components in the printer (for example the DC
controller PCA, the motors, and fans).
Figure 7. Power supply block diagram
Fuser-control circuit
The fuser-control ci rcuit c ontro ls the f user ’s components. The two fuser heaters provide the high
temperatures which cause the toner to be permanently bonded to the media. The fuser
thermistor is used to monitor the fuser temperatures. The thermal switch detects abnormally high
fuser temperatures and interrupts the supply of voltage to the fuser if the temperature is
determined to be too high.
Fuser heaters
Fuser thermal switch
Fuser thermistor
Figure 8. Fuser components
Q2431-90912 Chapter 5 Theory of operation 71
Fuser over-temperature protection
The fusing heater safety circuit is located on the power supply and constantly monitors the fusing
temperature.
To protect the fuser from excessive temperatures, the printer has the following three protective
functions:
● The CPU monitors the voltage of the thermistor. If the fuser temperature reaches
240° C (464° F) or higher, the CPU turns off the relay (RL101) to interrupt the power
to the fusing heater.
● If the temperature of the fusing heater continues to rise abnormally and the
temperature of the thermistor (TH1) exceeds about 250° C (482° F), the relay 1
(RL101) opens up to cut off the power supply to the fusing heater.
● When the temperature of the heater exceeds about 250° C (482° F), the thermal
switch (TP1) is turned off to cut off the power supply to the fusing heater
.
Figure 9. Fuser over-temperature protection circuit block diagram
72 Theory of operation Q2431-90912
High-voltage circuit
The high-voltage circuit produces the voltage biases that are applied to the primary charging
roller, the developing cylinder, the transfer charging roller, and the pressure roller.
Figure 10. High-voltage circuit block diagram
The primary charging voltage (bias) applies a uniform negative charge to the photosensitive
drum in the print cartridge. There are two types of primary charging biases. The primary charging
dc negative voltage and the primary charging ac bias. Both biases are generated by the highvoltage circuit on the power supply. These biases are superimposed on one another and then
applied to the primary charging roller which will transfer the biases to the drum. The laser/
scanner assembly generates the electrostatic image on the primary charged photosensitive
drum. See “Image formation system overview” on page 68. The electrostatic image cannot be
seen until toner is deposited on the drum.
Q2431-90912 Chapter 5 Theory of operation 73
The developing voltage (bias) causes the toner to adhere to the electrostatic image that the
laser/scanner assembly created on the photosensitive drum. There are two types of developing
biases. The developing dc negative bias and the developing ac bias. Both biases are generated
by the high-voltage circuit on the power supply. These biases are superimposed on one another
and then applied to the primary charging roller which will transfer the biases to the drum. The
biased developing cylinder picks up toner particles and deposits them onto the electrostatic
image on the photosensitive drum. The image is now visible on the drum.
The transfer voltage (bias) allows the toner image on the photosensitive drum to transfer to the
media. There are two types of developing biases. The transfer dc positive bias and the dc
negative bias. Both biases are generated by the high-voltage circuit on the power supply.
Transfer dc positive bias is applied to the transfer roller during the toner transfer process.
Transfer dc positive bias is applied to the transfer roller during the transfer roller cleaning
process. The dc positive bias attracts the toner to the media (this transfers the toner image on
the photosensitive drum to the media). The dc negative bias is used to clean residual toner off of
the transfer roller.
The fuser voltage (bias) prevents toner on the media from sticking to the fuser’s pressure roller.
For the HP LaserJet 4200 there is one type of fuser bias. A dc positive bias is generated by the
sub high-voltage circuit on the power supply . The dc positive bias is applied to the pressure roller
in the fuser.
For the HP LaserJet 4300 there are two types of fuser biases. The fuser dc positive bias and a
dc negative bias. Both biases are generated by the sub high-voltage circuit on the power supply .
The dc positive bias is applied to the pressure roller in the fuser. The dc negative bias is applied
to the fixing film in the fuser.
Low-voltage circuit
The low-voltage circuit converts the ac power from the power source (the wall receptacle the
printer’s power cord is plugged into) into the direct current voltage (vdc) used by printer
components (like the motors and fans). The ac voltage is converted into +24 vdc, +5 vdc, and
+3.3 vdc. The +24 vdc voltage is supplied to printer components like the main motor, laser/
scanner assembly motor, solenoids and clutches. The +5 vdc voltage is supplied to the laser/
scanner assembly. The +3.3 vdc is supplied to the sensors and the DC controller PCA.
Figure 11. Low-voltage circuit block diagram
74 Theory of operation Q2431-90912
Overcurrent/overvoltage protection
If a short-circuit or other problem on the load side causes an excessive current flow or generates
abnormal voltage, the overcurrent/overvoltage protection systems automatically cut off the
output voltage to protect the power supply circuit.
If the overcurrent or overvoltage protection system are activated and the power supply circuit
does not generate dc voltage, it is necessary to turn the power off, correct the problem, and then
turn the printer on again.
The circuit has two fuses (FU1, FU2), which break and cut off the output voltage if overcurrent
flows through the ac line.
Toner detection
To monitor the toner level, the printer uses two plate antennas and a toner level circuit in the
high-voltage power supply circuit (see figure 10 on page 73). Toner level detection is performed
by the DC controller PCA which monitors the output signal of this circuit. The signal is fed back to
the DC controller PCA from the antennas during the wait and standby operating periods (see
table 30 on page 67). The DC controller PCA detects toner level from 1 percent to 100 percent. If
the toner is detected as being low, a message will appear on the control-panel display (see
“Alphabetical prin ter mess ag es ” on page 258).
Cartridge detection
The presence of the cartridge is detected using information stored in plate antenna 2 and the
print cartridge memory tag (see “High-voltage circuit block diagram” on page 73). The DC
controller PCA detects the presence (or lack of) the print cartridge during the wait operating
sequence ((see table 30 on page 67).
Cartridge memory
This memory is built-in EEPROM in the cartridge, so that the printer is capable of detecting the
cartridge conditions.
Read/write of the cartridge memory is performed by the memory controller board through the
antenna unit. The cartridge information read by the memory controller is updated by the DC
controller PCA and written to the memory. The read/write of the memory is implemented when
the memory controller board receives a command from the DC controller PCA. The DC controller
PCA instructs the memory controller to perform read/write at the following timing.
Reading timing
● When the power is turned on
● When the door is closed
● When the DC controller PCA receives a command from the formatter
Writing timing
● When printing is completed
● When the DC controller PCA receives a command from the formatter
The memory data sent from the memory controller also contains the error status that has
occurred during read/write operation. When error status is sent, the DC controller PCA attempts
to read the operation four times. If the error status is not cleared after the operation, the DC
controller PCA determines one of the following error conditions: sub-CPU failure, memory data
abnormality, or memory access abnormality.
Do not remove the toner cartridge when the top cover interlock is overridden. Cartridge memory
will be disabled.
Q2431-90912 Chapter 5 Theory of operation 75
Laser/scanner assembl y
The laser/scanner produces the latent electrostatic image on the photosensitive drum in the print
cartridge.The main components of the laser/scanner assembly are the laser driver PCA, the
scanner motor, various mirrors, and the focusing lenses.
Scanner motor PCA
Scanner motor
Scanner mirror
Focusing lens
BD mirror
Mirror
Figure 12. Laser/scanner assembly
The laser scanner uses two laser diodes to scan two lines simultaneously producing high speed
laser scanning. After receiving the print command from the host computer, the DC controller
PCA activates the scanner motor which rotates the six-sided scanner mirror. The laser driver
PCA emits light from the two laser diodes according to signals from the DC controller PCA. The
two laser beams strike the six-sided scanning mirror and are directed through the focusing
lenses and down onto the photosensitive drum. The modulated laser beams generate the latent
electrostatic image on the photosensitive drum according to the image data signals received
from the DC controller PCA.
BD PCA
Photosensitive drum
(inside the print cartridge)
Laser beams
76 Theory of operation Q2431-90912
Laser/scanner control
The laser/scanner control circuit on the laser driver PCA turns the laser diodes on an off
according to image data signals received from the DC controller PCA. The DC controller PCA
sends image data signals VD01/VD01,VD02, and /VD02 and the laser control signals CNT0,
CNT1, and CNT2 to the logic circuit on the laser driver PCA. The laser control signals control
laser emission, automatic power control (APC), horizontal synchronization control, and image
mask control.
Figure 13. Laser control circuit block diagram
Laser emission control is simply turning the laser diodes on and off. Automatic power control is
used to limit the amount of light that is emitted from the laser diodes. Horizontal synchronization
control is used to determine the starting position for the images horizontal direction. Image mask
control is used to avoid laser beam emission on the non-imaging areas of the drum (about 5mm
down the vertical edges and 8mm at the top and bottom)
Q2431-90912 Chapter 5 Theory of operation 77
Paper pickup system
The paper pickup and feed system consists of various kinds of pickup and feed rollers that are
driven by the printer’s motor(s). The printer uses tray 1 (the manual feeding tray) and a cassette
in tray 2 as media sources. The printed media is delivered to either the rear output bin (straight
through printing) or the top output bin (the default destination). Two additional 500-sheet feeders
and one 1,500-sheet feeder can be added to the printer. These accessories are discussed
further along in this chapter.
Media is detected in tray 1 by the tray 1 paper sensor (on the tray 1 pickup assembly; PS105).
The media is detected in tray 2 by the tray 2 paper sensor (PS101). The paper size sensor
(PS106) and the paper size switch (SW102) detect the media that is loaded in the tray 2
cassette.
All of the rollers in the printer are driven by two motors, a clutch, and a solenoid which are
controlled by the DC controller PCA (for the HP LaserJet 4300 has three motors). See “Motor
and fan control” on page 70.
The pre-feed, top of page, and fuser assembly delivery sensor (PS102, PS103, PS108) detect
arrival and passing of media along the paper path. If the paper does not reach or pass these
sensors within a specific amount of time the microprocessor on the DC controller PCA halts the
printer functions and a jam error message will appear on the control-panel display. See
“Alphabetical printer messages” on page 258 and “Numerical prin ter mess ag es ” on page 274.
For information about the location of printer switches, sensors, and motors see “Printer switches
and sensors” on page 336 and “Printer motors and fans” on page 337.
Figure 14. Printer paper pickup and feed block diagram
78 Theory of operation Q2431-90912
The paper pickup and feed system is divided into two blocks. The paper pickup/feed block, and
the fuser/delivery block.
Fuser/delivery block
Figure 15. Paper pickup/feed and fuser/delivery block diagram
Paper pickup and feed block
Paper pickup/feed block
The printer functions that take place in the pickup/feed block are cassette media size and
presence detection, media entering the paper path from tray 1 or tray 2, Lifting of the tray 2
paper plate, multiple feed prevention, and page skew correction. For information about the
locations of switches, sensors, and motors in the pickup/feed block, see “Printer switches and
sensors” on page 336 and “Printer motors and fans” on page 337.
When the print command is received from the host computer by the DC controller PCA it turns
the main motor (M101) power on. This motor will drive the tray 2 pickup, feed, and separation
rollers to rotate. For the HP LaserJet 4300, the print cartridge motor power also is turned on. The
laser/scanner motor power is turned on.
The DC controller PCA then activates the feed clutch (CL101) to rotate the feed roller. The tra y 2
pickup solenoid is activated (SL101) and the pickup arm descends. The pickup roller touches the
media and a sheet is fed into the printer. The separation roller prevents multi-sheets of media
from being fed all at one time.
As the pre-feed sensor (PS102) detects the media, the DC controller PCA turns off the clutch
which stops the media. When the DC controller PCA detects that the laser/scanner is ready it
activates the feed clutch again. The feed roller moves the media further into the printer. Page
skew is corrected by the registration shutter and the media is transported to the fuser/delivery
block (feed belt, fuser, and delivery output bin).
For information about the timing of these operations, see “HP LaserJet 4200 general timin g
diagram” on page 357 and “HP LaserJet 4300 general timing diagram” on page 358.
Q2431-90912 Chapter 5 Theory of operation 79
Printing from tray 1
The presence of paper in tray 1 is detected by the tray 1 paper sensor (PS105).
When the DC controller PCA receives the print command, the printer starts the initial rotation
phase. (This consists of main motor warm-up, scanner motor warm-up, high-voltage control
sequence and fuser warm-up.) When the initial rotation phase ends, the tray 1 pickup solenoid
(SL102) is activated.
The cam rotates, the paper tray lifter rises, and the media comes in contact with the tray 1 pickup
roller. At the same time, the tray 1 pickup roller rotates twice and a sheet of media in tray 1 is
picked up. The separation pad prevents unnecessary sheets from feeding with the first sheet.
The sheet then reaches the registration assembly, where its skew is corrected. Then it goes
through transfer, separation, and fusing stages; passes through the delivery unit; and is
delivered to the output bin.
Note If paper is removed from tray 1 just before it is picked, the tray 1 pickup roller might continue to
rotate up to six times and a jam will be detected.
Figure 16. Tray 1 timing diagrams
80 Theory of operation Q2431-90912
Separation pad
Tra y 1 pic kup roller
Tra y 1 pic kup solenoid
Cam
Lifter
Figure 17. Tray 1 pickup
Q2431-90912 Chapter 5 Theory of operation 81
Printing from tray 2
When the DC controller PCA receives print command, the main motor (M101) and scanner
motor start rotation. When the main motor reaches its prescribed speed, the feed roller clutch
(CL101) and tray 2 pickup solenoid (SL101) are activated. (The tray 2 pickup roller, tray 2 feed
roller, tray 2 separation roller, and paper feed rollers are driven by the main motor rotation.)
The tray 2 pickup roller, activated by the pickup solenoid, rotates once and picks up the media in
the tray. The unnecessary sheets are removed by the separation roller and the media is fed to
the pre-feed sensor (PS102).
The sheet then reaches the registration assembly, where its skew is corrected. Then it goes
through transfer, separation, and fusing stages; passes through the delivery unit; and is
delivered to the output bin.
Figure 18. Tray 2 timing diagrams
82 Theory of operation Q2431-90912
Tray 2, 500-, 1,500-sheet feeder media size detection
Media size in the cassette are detected by three switches. The switches are active after the
cassette is placed in the tray 2 feeder. (this also applies to the optional 500- and 1,500-sheet
feeder). The DC controller PCA microprocessor detects the size and presence of the media by
the combinations of the switches.
Table 32. Tray 2 and 500-sheet feeder media size switch settings
Paper size Media size switch setting
Upper Center Lower
No cassette installed
A4
LTR
B5
A5
EXE
LGL
UNV
Off Off Off
Off Off On
Off On Off
Off On On
On Off Off
On Off On
On On Off
On On On
Table 33. 1,500-sheet feeder media size switch settings
Paper size Media size switch setting
Upper Center Lower
No cassette installed
A4
LTR
LGL
Off Off Off
On Off On
Off On On
On On Off
The tray 2 cassette can detect the media size using the switches describe above, however the
user can define the media size for the tray using the control-panel (see “Paper Handling menu”
on page 42). In this case the printer may not correctly detect the media size if the user’s defined
size does not match the tray settings.
To prevent a false size detection, the printer measures the time it takes for the media to pass
from its leading edge to its trailing edge and determines the media size that was fed from the
tray. When the measured size differs from the user’s defined size or from the media size
switches, a message will appear on the control-panel display (see “Alphabetical printer
messages” on page 258).
Q2431-90912 Chapter 5 Theory of operation 83
Lifter-driver operation
The lifter driver keeps the media stack surface at a specific level in order to have a stabilized
pickup operation regardless of the size of the media in the tray 2 cassette. The DC controller
PCA operates the lifter driver motor (M103) for 50 seconds. The motor stops when the paper
stack position sensor (PS107) detects the media. If the paper stack position sensor does not
detect any media within 8 seconds after the lifting operation has begun, the DC controller PCA
determines there has been a lifter driver motor failure and a message appears on the controlpanel display (see “Alphabetical printer messages” on page 258 or “Numeric al printer
messages” on page 274). The DC controller PCA stops the lifting operation if the paper stack
position sensor detects the absence of the tray 2 cassette.
Multiple feed prevention
The printer uses the separation roller in tray 2 to prevent multiple-feeding. Normally, the
separation roller rotates in the same direction as the feed roller. The separation roller is equipped
with a torque limiter, but because the force of the feed roller exceeds that of the torque limiter, the
separation roller is actually driven by the feed roller.
If multiple sheets of media are picked up, however, the low friction force between the sheets
weakens the rotational force from the feed roller to the separation roller. Consequently, the
torque limiter takes control of the separation roller, and rotates the separation roller in the reverse
direction, which removes the extra sheets.
Feed roller
Pickup roller
Media
Normal
Driving force transmitted from the feed roller
Separation roller
Driving force transmitted from the motor through the torque limiter
Multiple feed
Figure 19. Multiple feed prevention
84 Theory of operation Q2431-90912
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