HP Laserjet 4 Manual

Replacing Fuser Assembly Parts    
 The following procedures describe the removal and replacement
of major Fuser Assembly Components (Fusing Roller, Thermistor,
Thermoswitch, and Heater Lamp).  When performing these
procedures, note the order in which the parts were removed and
their orientation. 
CautionHandle the Heater Bulb by the ceramic ends.  Do
not touch the Heater Bulb, or allow it to become
contaminated.  If contamination occurs, clean the
bulb thoroughly with isopropyl alcohol.
6-47 Removal and...

Fuser Assembly Configuration 
Figure 6-40 provides an overview of the orientation of the major
Fuser Assembly components.  Before beginning the fuser
disassembly procedures, become familiar with the location and
orientation of the various Fuser Assembly parts (see Table 6-1).
Figure 1-40 Fuser Assembly 
Callout Assembly Callout Assembly 
1 Exit Sensor Flag 5 Fusing Roller
2 Thermistor 6 Plug to Interconnect PCA
3 Thermoswitch 7 Pressure Roller
4 Heat Lamp
Table 1-1Fuser Assembly Configuration
6-48...

Removing the Fuser Roller Heat Lamp 
1Laser Jet 4 only: Remove the (2) latches that hold the right
side cover. (There is a gear exposed on the right side of the
fuser, see Figure 6-41, callout 1.)
2Release the (7) latches on the LaserJet 4 or the (5) latches on
the LaserJet 4 Plus that hold the wire cover to the Fuser
Assembly. These latches can be accessed through slots on top
of the fuser.
(continued on next page)
Figure 1-41 Fuser End Cap Removal (LaserJet 4 only)
6-49 Removal and Replacement 

3LaserJet 4: With a screwdriver, release the latches by first
pressing to clear the lower latch, then lifting to release the
upper latch (see Figure 6-42a).
LaserJet 4 Plus: With a screwdriver, release the latches by
first pressing to clear the lower latch, then lifting to release
the upper latch. Repeat until all (4) latches are free, then
rotate the cover to release the latch on the side (see Figure
6-42b and 6-42c).
4Release the two end latches (see Figure 6-42b). 
NoteSix of the seven latches are...

(continued on next page)
Figure 1-42b Fuser Cover End Latches (LaserJet 4 Plus)
Figure 1-42c Fuser Wire Harness Cover Removal  (LaserJet 4
Plus)
6-51 Removal and Replacement 

5Remove the (1) Heat Lamp lead (thermoswitch screw)(see
Figure 6-43 callout 1).  
6Press the thermoswitch tab towards the paper exit flag to
release the tab (see Figure 6-43 callout 2).  Slide the
Thermoswitch off the fuser frame (see Figure 6-43, callout 3). 
7LaserJet 4: Remove the cable retainer from the fuser frame,
and free the heat lamp lead from the cable retainer. Note the
orientation of the cable retainer.
LaserJet 4 Plus/5: Remove the (2) screws from the end cover
(see Figure 6-43, callouts 4...

NoteLaserJet 4 and 5 only: Note the orientation of
the bushing on the left side of the Fusing Roller. 
8Squeeze the lamp connector to release the slip-on connector
locking tab from the right side of the heat lamp (see Figure
6-44, callout 1). 
(continued on next page)
Figure 1-44 Gear Side Heat Lamp Connector
6-53 Removal and Replacement 

NoteBefore removing the gear cover, note the routing
of the wires in the gear cover slots.
9Remove the (2) screws from the right side gear cover (see
Figure 6-45, callouts 1 and 2). Note the wavy washer on the
(1) screw (callout 1) touching the copper contact. Remove the
cover.
10Remove the Heat Lamp (see Figure 6-45, callout 3), from the
roller. Pull the lamp out the side of the assembly.  
Figure 1-45 Fuser Gear Cap Removal 
6-54 Removal and Replacement 

Fuser Assembly Thermoswitch Removal 
NoteSix of the seven latches are accessed from the top
of the wire cover. These latches are beneath the
felt pad. The slot on the front left side of the wire
harness cover is the best place to begin removing
the cover.
1Remove the Wire Harness Cover as shown in Figure 6-42a
(LaserJet 4) or Figure 6-42b and c (LaserJet 4 Plus).
2Remove the (2) screws from the Thermoswitch connector (see
Figure 6-46, callout 1).
3Release the thermoswitch latch from the paper sensor...

Thermistor Removal
1Remove the Wire Harness Cover as shown in Figure 6-47.
2Remove the (2) screws from the thermoswitch connector (see
Figure 6-47, callout 1).
3Release the thermoswitch latch from the paper sensor flag
side of the Fuser Assembly (behind the side cover, see Figure
6-47, callout 2).
4Slide the thermoswitch to the gear side of the fuser and
release it from the fuser frame (callout 3).
NoteWhen installing the thermoswitch, make sure the
sensing surface contacts the fusing roller
uniformly....