HP Laserjet 4 Manual

Removing and Replacing the Transfer Roller  
The Transfer Roller is the black sponge-rubber roller located
inside the printer just to the front of the black plastic Paper Feed
Guide (see Figure 4-9). To remove and replace the Transfer
Roller, follow these steps.
NoteDo not handle the new Transfer Roller
unnecessarily.  Oils will build up on the roller and
cause print quality problems.
Figure 4-9 Replacing the Transfer Roller
Preventive Maintenance   4-15 

Removing the Transfer Roller   
1Turn OFF and unplug the printer.
2Open the printer’s top cover.
3Remove the Toner Cartridge.
4Use the hook inside the top cover to remove the Transfer
Roller as follows. Hook the left side of the chrome rod beside
the sponge roller, and lift the roller out.  
5Discard the used roller.
Replacing the Transfer Roller   
1Slide the right end of the new roller (the end without the
gear), onto the right slot. Press the rod into the slot.
2Lower the gear end of the roller into...

5
Functional Overview
Introduction
This section of the manual presents a functional overview of the
printers. It provides the Service Representative with an
understanding of the printing process. Refer to the block diagram
of Figure 5-1 to see the relationship of the parts as a whole. 
This chapter discusses the following printer functions:    
•DC Controller System
•Formatter System
•Image Formation System
•Paper Feed System
•Power System
•Basic Sequence of Operation
Functional Overview  5-1 

DC Controller System
The DC Controller PCA is responsible for coordinating all
activities involved in the printing process. The DC Controller
enables the drive circuitry for the laser beam, and coordinates
print data from the Formatter PCA with paper size, laser beam
motion, the high voltage system, fuser temperature, and motors.
The DC Controller also shares machine status information with
the Formatter PCA so that proper diagnostic messages are
displayed on the control panel. 
The DC Controller...

DC Controller Loads
Figure 5-1 DC Controller Loads
Functional Overview  5-3 

Serial Data Communication        
The printer communicates with and controls the Paper Feeder,
the Lower Cassette Assembly, and the Envelope Feeder through
serial data lines SLI and SLO. In addition, the DC Controller
uses information encoded on SLI from the High Voltage
Transformer to monitor the toner level.
Communicating with the various subassemblies of the printer
through serial data lines allows many of the functions of the
processor on the DC Controller to be off-loaded to local State
Machines on...

Solenoids      
Solenoids are controlled by the DC Controller.  Each solenoid
drives a specific roller (see Table 5-1). The DC Controller outputs
+24B VDC to energize the solenoids at the proper time. When
either of the doors are open, the +24B VDC is removed and the
solenoids are disabled.     
Solenoid Description PCA
SL1 PC tray (Tray 2) Pickup
Roller Clutch SolenoidPaper Control PCA (J704) 
SL2 Multi-purpose (MP) Tray
(Tray 1)  Pickup Roller
Clutch SolenoidPaper Control PCA (J702)
SL3 Lower Cassette...

Photosensors    
Photosensors, monitored by the DC Controller, are responsible for
detecting the presence of paper, paper motion, and timing. (See
Table 5-2.) Photosensor PS2 senses paper in the PC tray;
photosensor PS4 senses paper in the MP tray. The message PC
LOAD or MP LOAD is displayed when either the PC tray or the MP
tray is selected as the primary tray and is out of paper. PS6 and
PS7 (respectively) sense the presence of paper in the Lower
Cassette and the Envelope Feeder. Paper must be loaded...

Sensor Description PCA
PS1 Input/Registration Sensor - detects paper in
pickup areaPaper Control
PCA (J605)
PS2 PC tray (Tray 2) Paper-out Sensor - detects the
presence of paper in the PC tray.Paper Control
PCA (J605)
PS3 Output/Fuser Exit Sensor - detects paper in
fuser.DC Controller
(J208)
PS4 MP Tray (Tray 1) Paper Out Sensor - detects
the presence of paper in the MP tray (Tray 1).Paper Control
PCA (J701)
PS5 MP Tray (Tray 1) paper end - detects the trailing
edge of paper fed from the MP tray (Tray...

Microswitches         
All microswitch conditions are monitored by the DC Controller
over serial data lines SLI and SLO. The State Machines on the
individual control PCAs, (for example the Paper Control PCA)
convert the switch condition to a digital logic level, and encode
that information on the data stream for the DC Controller to use.
SW601 (Door Open Sensing switch) monitors the top cover.  If the
top cover is open, the printing process stops and the 12
PRINTER OPEN message is displayed.  SW601...