HP 2500 L Service Manual

EN 4 Theory of operation69
Photosensors and switches 
Figure 17.Photosensors and switches
Figure 18. Photosensors and switches
1
23
4
5
6
7
Table 17. Photosensors and switches
Reference Description
1 Tray 1 sensor
2 Roller-engaging sensor
3 Developing-rotary-engaging sensor
4 Fuser-delivery sensor
5 Front fuser detection sensor
6 Door-open detection switch
7 Power switch 

70Theory of operation C9706-90926
Solenoid and clutches
Figure 19.Solenoid and clutches
Figure 20. Solenoid and clutches
1
2
3
4
Table 18. Solenoid and clutches
Reference Description
1 Developing-rotary-stopper solenoid
2 Tray 1 pickup solenoid
3 Roller-engaging clutch
4 Registration clutch 

EN 4 Theory of operation71
Printed circuit assemblies
Figure 21.Printed circuit assemblies
Figure 22. Printed circuit assemblies
123
4
5
78
96
Table 19. Printed circuit assemblies
Reference Description
1 Developing-rotary/toner-level detection PCA
2 Transfer-belt home-position detection PCA
3 Density-detection PCA
4 Waste-toner-detection PCA
5 Registration-detection PCA
6 Dc controller
7 Sub high-voltage power-supply PCA
8 High-voltage power-supply PCA
9 Power supply 

72Theory of operation C9706-90926
Image formation system
The image formation system, which forms a toner image on media, consists of several 
interdependent systems:

electrostatic latent-image-formation system

developing system

transfer block

fuser

ITB cleaning block

imaging drum (with ITB)

photosensitive drum
Figure 23. Image formation system 

EN 4 Theory of operation73
Image formation process
Laser printing requires the interaction of several different technologies including electronics, 
optics, and electrophotographics to provide a printed page. Each process functions 
independently and must be coordinated with the other printer processes. Image formation 
consists of the following five processes:
1Latent-image formation
2 Developing
3 Image transfer
4 Fusing
5 Transfer-belt cleaning
The five processes are divided into twelve steps, which...

74Theory of operation C9706-90926
Latent image formation stage
Step 1: primary charging
A dc bias is applied to the primary charging roller, which transfers a uniform negative potential to 
the photosensitive drum.
Figure 25. Step 1: primary charging
Step 2: laser-beam exposure 
The laser beam scans the photosensitive drum to neutralize negative charges on parts of the 
drum. An electrostatic latent image is formed on the drum where negative charges were 
neutralized.
Figure 26. Step 2: laser-beam exposure 

EN 4 Theory of operation75
Developing stage 
The developing cylinder comes in contact with the photosensitive drum to deposit toner onto the 
electrostatic latent image.
NoteThe charges on the exposed area on the drum are shown as positive in figure 27. The charges 
are actually negative, but they are more positive than the charges on the developing cylinder.
Figure 27. Developing block
Step 3: developing
Toner acquires a negative charge through friction from the developing cylinder and the blade. 
When...

76Theory of operation C9706-90926
Transfer stage
Step 4: primary transfer 
The primary transfer roller, to which a dc positive bias is applied, imparts a positive charge on the 
transfer belt. The negatively-charged toner on the drum is transferred to the positively-charged 
transfer belt. This procedure is repeated for each color (yellow, magenta, cyan, and black).
The dc positive bias is increased for each successive color to make sure that all of the colors 
adhere to the transfer belt.
Figure 29....

EN 4 Theory of operation77
Step 5: secondary transfer
The secondary transfer roller, to which a dc positive bias is applied, imparts a positive charge to 
the print media. When the print media comes in contact with the transfer belt, the toner is 
transferred to the print media.
NoteWhen secondary transfer is complete, a dc negative bias is applied to the secondary transfer 
roller to prevent toner on the transfer belt from adhering to the secondary transfer roller.
Figure 30. Step 5: secondary transfer...

78Theory of operation C9706-90926
Fusing stage
Step 7: fusing
The dc negative bias applied to the fusing film strengthens the holding force of the toner on the 
print media and prevents the toner from scattering.
Figure 32. Step 7: fusing
Step 8: roller charging (auxiliary ITB cleaning roller)
During secondary transfer, not all of the toner on the transfer belt is transferred to the print 
media. Toner that remains on the belt is called  “residual toner. ”
The auxiliary transfer belt cleaning roller, to...