Lexmark X 850 Owners Manual

General information1-29
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The Printhead also helps to clean and prepare the drum by scanning the surface of the drum at the beginning of 
each individual printer cycle. This action discharges a residual DC charge that may still remain on the Drum from 
the last print cycle.
Development
The toner contained within the PC Cartridge has a magnetic property that causes it to adhere to the Magnetic 
Roll. The Metering Blade spreads the toner into a very thin layer on the Magnetic Roll. Friction between...

1-30MFP Service Manual 7500-XXX
The toner adhering to the Magnet Roll is always in contact with the drum surface. When a less negative point on 
the drum (a discharged area) comes in contact with the more negative charged toner on the Magnet Roll, toner 
transfers from the Magnet Roll to that point on the drum. There is now a visible toner image on the drum surface. 
The image is called a developed image.
Magnetic
RollMetering
Blade
TonerHVPS
DRUM
-VDC
Drum Surface
Voltage Drum Surface
Image
Toner at...

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Transfer
As the paper travels between the Transfer Roll and the drum surface, the Transfer Roll applies a positive charge 
to the back of the printing paper. This positive charge transfers the negative charged toner image from the drum 
surface to the top surface of the paper. The toner image is now on the paper and the paper is now stuck to the 
drum surface due to the relative electrical differences between the negative electrical charge of the inner 
conductive layer...

1-32MFP Service Manual 7500-XXX
Cleaning 
The Cleaning Blade removes any toner that remains on the drum after the transfer process. The toner that the 
Cleaning Blade removes is collected inside the sealed PC Cartridge and reused.
Discharge
At both the start and the end of each individual printer cycle, the HVPS supplies the charge roll with an AC 
voltage that is used to electrically clean the drum. The AC voltage removes any residual DC charge that was left 
from the previous print cycle.
At the...

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Safety system diagram
PrintheadSwitch
(PC Cartridge
interlock)Switch
(Printer front
door interlock)Switch
(Printer left
door interlock)Fuser cooling fan
Dual drive motor ASM
Transport
motor
Printer engine card ASM
HVPS
LVPS Switch(main
power)Fuser unit ASM
Printhead
motor
Printhead motor
Transport motorFuser
cooling
fan
controlPC cartridge
motor
PC cartridge motor
Main heater lamp
Sub heater lamp
Sub heater lamp 

1-34MFP Service Manual 7500-XXX
Document scanning
The document scanning section of this machine consists of a scanner that reads a single-sheet document 
placed on the platen glass and a document feeder that can transport a multiple-sheet document and flip the 
sheets for two-sided scanning. 
This ADF employs a constant velocity transport system that scans images by feeding the document at a 
constant speed over the specified position (scan position) where the carriage (full/half rate carriage) of the...

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The full rate carriage and half rate carriage travel to read the document.
The exposure lamp is installed on the full rate carriage. As the full rate carriage travels, the document on the 
platen glass is scanned and exposed with the exposure lamp.
In conjunction with the full rate carriage, the half rate carriage travels half of the stroke of the full rate carriage.
The optical image of the exposed document is reflected by the scanner 1st mirror of the full rate carriage...

1-36MFP Service Manual 7500-XXX
First the pick roll moves down and presses the document on the document tray to enable document feed.
The pick roll moves down with the normal rotation of the pick roll position motor assembly and it moves up with 
the reverse rotation of the pick roll position motor assembly.
Upon completion of document feed, the pick roll returns to the normal (raised) position. 
Pre-feed
In the pre-feed step, a document sheet is fed from the Feed roll to the ADF transport roll assembly....

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Scan control
Scanning of the image illuminated with the exposure lamp of the full rate carriage is controlled by changing the 
feed speed according to the copy magnification. 
The document sheet stopping at the scan feed reference position is then fed to the scan position when the scan 
signal is sent from the main unit of the machine after the predetermined time lapse following the paper detection 
at sensor (ADF registration). Upon receipt of the scan signal, the ADF...

1-38MFP Service Manual 7500-XXX
Simplex document
For two simplex document sheets, feed is performed in the following sequence:
1.The first document sheet is fed to the ADF transport roll assembly. (see Pre-feed)
2.The document is fed to the ADF registration roll assembly, and then fed to the scan feed reference 
position. (see Pre-registration)
3.The document sheet is fed at the feed speed corresponding to the selected magnification, and the image 
on it is scanned with the exposure lamp at the scan...