Apple Laserwriter 12.640 Ps Service Manual

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LaserWriter 12/640 PS

Service Source

Overview

LaserWriter 12/640 PS

Overview About This Overview - 1

About This Overview

This overview briefly describes the servicing issues of the LaserWriter 12/640 PS, especially those that distinguish it from previous Apple laser printers.

Overview General - 2

General

The LaserWriter 12/640 PS is a 600 dpi, 12 ppm monochrome laser printer that is positioned as the workgroup laser printing solution for Mac OS and Windows users.

It features excellent print quality, cross platform support, and state-of-theart paper handling options.

LaserWriter driver 8.4 releases concurrently with this printer.

Overview Options - 3

Options

There are three options that are available for the Laser-

LW 12/640 PS Options

Writer 12/640 PS:

• Duplexer unit

• Face-up catch tray

• 500-sheet feeder (accommodates either standard or envelope cassette)

Each of the options can be added independently of one another so there are several configurations that you might encounter. The animation “LW12/640 PS Options” shows how the options go together.

Overview Paper Path - 4

Paper Path

The printer has a basic S-shaped paper path and multi-

LW12/640 PS Paper Path

purpose tray arrangement generally similar to the LaserWriter 16/600 PS.

On the exit side of the fuser assembly, however, there is a brand new function: the paper deflector guide. This solenoid-actuated guide switches paper between one of two paths. The inside path is the face-down delivery path that continues straight up to the delivery rollers and output tray. The outside path serves three purposes.

• Paper feed into the optional duplexer.

• Paper delivery to the optional face-up tray.

• If no options are attached, the outside path will feed paper directly out the rear of the printer.

Note

: The “LW 12/640 PS Paper Path” animation shows

the sequence of a duplex print job.

Overview Duplexer - 5

Duplexer

The most unique mechanical feature you will encounter is the optional duplexer unit. The term “duplex” denotes twosided printing.

The duplexer is an L-shaped feeder that cradles the underside and rear of the printer. When duplex printing is selected through software, the paper deflector guide is actuated and the sheet of paper diverts into the duplexer. There the paper is flipped and circulated back to the cassette feed roller in preparation for its second imaging cycle.

The drive train in the horizontal leg of the duplexer receives drive directly through an idler gear in the pickup drive assembly in the printer. The feed roller at the entrance to the duplexer also receives drive from the printer, but the other feed rollers in the vertical leg of the duplexer are driven by the duplexer motor.

Overview Sheet Feeder - 6

Sheet Feeder

An optional 500-sheet feeder is available. As with the LaserWriter 16/600 PS design, the sheet feeder fits squarely beneath the printer (or the duplexer, if present) to form a dual front-loading cassette arrangement. An envelope cassette is available for the sheet feeder only.

The feeder has a receptacle connector that mates with the printer and has its own circuit board. However, the feeder does not have its own motor. Instead it derives mechanical drive directly from the gear train above, either the pickup drive assembly in the printer or the intermediate gear train in the duplexer. There is a pickup roller and pickup solenoid in the sheet feeder.

Overview Face-Up Tray - 7

Face-Up Tray

The face-up tray is an option that attaches to the rear of the printer (or to the rear of the duplexer, if present).

This tray performs similarly to previous printers, except that output to this tray is selected through software, not by the setting of a lever. The presence of this tray is not sensed by the controller.

Optional

Face-Up Tray

Overview Duplex Scan Anomalies - 8

Duplex Scan Anomalies

After side one prints during a duplex job, the paper parks itself onto a horizontal shelf beneath the cassette. The duplexer motor then reverses direction. The trailing edge of the paper becomes the leading edge, and the paper exits the shelf and continues the path toward side-two printing.

Order of Scanning

Duplex documents stack in the face-down delivery tray in correct book order (with odd-numbered pages facing down). To achieve this, page two is imaged during the first pass and page one during the second pass.

Long Edge Binding

The reversing action of the duplexer flips the paper over

Long Edge Binding

Scan/Feed Directions

along its short axis. If you have chosen long edge binding in

Overview Duplex Scan Anomalies - 9

the software, the I/O controller flips the imaging data accordingly to counteract the reversal of the paper. The net effect is even-numbered pages scanned in reverse.

For example, page 2 of a portrait oriented job images right to left from the bottom up. (This anomaly does not occur with short edge binding). For this reason, be aware that some skew or image cutoff symptoms may appear to be 180 degrees off from normally scanned pages.

Double-Page Batching

If there is enough buffer memory space available, the controller will image a print job four pages (two sheets) at a time. For example, the imaging order of an eight-page document would be pages 2, 4, 1, and 3 (with sheets one and two then being delivered), followed by pages 6, 8, 5, and 7 (with sheets three and four delivered). This batching

Sequence of

Double-Page Batching

feature maximizes throughput speed.

Overview Miscellaneous - 1 0

Miscellaneous

Fuser Pressure Roller

The fuser pressure roller provides an opposing surface to the fuser heater roller, the same function as in previous laser printers. However in the LaserWriter 12/640 PS, this pressure roller is seated in the chassis and is not a part of the fuser assembly itself.

Power Supply Assembly

The power supply assembly is a large multifunctional circuit board located in the lower section of the engine. In addition to the DC power supply functions that are common in previous engines, this assembly also houses some sensors and switches, as well as the high-voltage power supply and various other functions.

Overview Miscellaneous - 1 1

Beam Detection

Laser beam detection occurs outside of the laser/scanner assembly, in the start-of-scan (SOS) sensor located in the mirror assembly.

Two Shutters

There are two laser shutters, one inside the laser/scanner assembly and one covering the mirror assembly. Both are actuated with the closure of the top cover.

Two "Exit" Sensors

Unlike previous printers, there are two paper sensors downstream of the fuser assembly. The first is the fuser exit sensor that monitors the exiting of paper from the fuser assembly. The second is the delivery sensor that monitors

Overview Miscellaneous - 1 2

paper after it passes the deflection guide in route to facedown delivery on top of the printer.

One Fan

There is only one fan in this printer.

Support for Heavy Paper

The LaserWriter 12/640 PS supports throughput of 50 lb. paper for simplex (single-sided) print jobs. You must print from the multipurpose tray and choose face-up delivery when using 50 lb paper.

RAM Upgrade

The LaserWriter 12/640 PS comes with 4 MB of RAM in the righthand of two SIMM slots. This amount does not allow for

Overview Miscellaneous - 1 3

duplex printing nor does it enable PhotoGrade. Upgrades are required if you want either of these additional features.

No SCSI

There is no SCSI support in the LaserWriter 12/640 PS.

Startup Page

The startup page is a single-sided page with a layout similar to previous PostScript LaserWriter printers. The startup page does report the presence of a duplexer or sheet feeder but will not recognize the optional face-up tray.

Overview Miscellaneous - 1 4

Service Test Page

You may print a service test page by jumpering connector J101 on the I/O controller board. Consequently, unlike previous printers, the I/O controller board must be installed to run a service test page.

Overview Miscellaneous - 1 5

Top Cover Closure

The closing of the top cover initiates several more events than in previous printers. They are listed below and are described in detail in the “Principles of Operation” chapter.

• Actuation of cover interlock

• Opening of the laser shutter

• Opening of the mirror assembly shutter

• Latching of the top cover

• Pressing of the jam release plate

• Actuation of the toner cartridge latching mechanism

• Actuation of the fuser idler gear

Service Source

Principles of Operation

LaserWriter 12/640 PS

Principles of Operation Print Process - 1

Print Process

Print Process Overview

The print process consists of the stages shown below, during which a toner image forms on the drum surface, transfers and then bonds to the paper. Each sheet of paper passes through steps 4 to 6. During printing,the drum surface repeatedly cycles through steps 1 to 5, as well as cleaning stage 7.

Charging

Exposure

Xerographic Cycle

Cycle repeats for the next print

Development

Paper Enters

Transfer

Discharge

Cleaning

Paper Exits

Fusing

1 Charging

Places a uniform density of negative electric charge onto the drum surface.

2 Exposure

Scans the drum surface with a thin beam of laser light that switches on and off according to the video signal from the printer controller. This exposure forms an invisible electrostatic latent image on the drum surface.

3 Development

Attracts toner onto the electrostatic latent image on the drum surface, thus forming a visible toner image.

4 Transfer

Transfers the toner image from the drum surface to the paper.

5 Discharge

Enables the paper to come off the drum surface.

6 Fusing

Fixes the toner image to the paper.

7 Cleaning

Cleans the drum surface of any residual toner.

Principles of Operation Print Process - 2

Print Process Diagram and Paper Path

Laser Beam

1 Charging 2 Exposure

6 Fusing

Fuser Roller

BCR

7 Cleaning

Drum

Cleaning

Blade

3 Development

CM Blade

Attraction Roller

Paper

4 Transfer5 Discharge

Pressure

Roller

Cleaning Blade

Fuser Roller

Pressure Roller

Detack Saw

Mirror Assembly

Transfer Roller

Toner Cartridge

CM BladeBCR

Laser/Scanner Assy

Drum Detack Saw

Transfer Roller Attraction Roller

Principles of Operation Print Process - 3

Print Process Description

1 Charging

During this stage, the drum surface receives a uniform negative charge.

Note

: The drum surface is made of a photoconductive material and the inner part of the drum is made of a conductive material. This negative charge is actually placed onto the outside surface of the photoconductor layer.

The charge is imparted by the bias charge roller (BCR) within the toner cartridge. The BCR is a conductive roller in contact with the drum surface and is supplied with a discharge voltage by the high-voltage power supply (HVPS). The discharge occurs through the small gaps between the BCR and the drum. The discharge voltage is an AC voltage with a negative DC voltage superposed. The negative DC voltage is about -330 V. The AC voltage varies within the range of about 1.6 to 2.0 kVp-p with the discharge current since the AC current is regulated to be constant.

Note

: Before this stage, the drum surface will have been given a uniform negative charge relative to the inner conductor of the drum (see “Cleaning”).

High-Voltage

Power Supply

(HVPS)

Bias Charge Roller (BCR)

Negative Charge Positive Charge

Drum

Photoconductor Layer

Conductor Layer

Principles of Operation Print Process - 4

2 Exposure

During this stage, the laser beam scans across the negatively charged drum surface. The laser beam is emitted from the laser diode in the laser/scanner assembly. This diode is

stationary. Its beam reflects off the face of a rapidly rotating scanning mirror, through a correction lens, and outward off a stationary mirror which directs the beam onto the surface of the drum.

The laser beam is modulated according to the video signal from the printer controller, switched on when the video signal denotes a black pixel and off when it denotes a white pixel.

The laser beam that strikes the drum surface generates electron-hole pairs in the photoconductor layer, exciting electrons to the conduction band and thereby creating holes in the valence band. The electrons are attracted by the electric field and move toward the inner conductor of the drum to flow into it. The holes move toward the outside surface of the photoconductor layer and recombine with the negatively charged electrons at the surface to decrease the negative charge at the surface. The resultant less-negatively charged parts (parts whose potential has risen relative to the inner conductor) in the drum surface form an invisible electrostatic latent image.

Electron

Photoconductor

Layer

Conductor Layer

Laser Light

Electron-hole pair

Recombination

Decrease of

Negative Charge

Principles of Operation Print Process - 5

3 Development

During this stage, the electrostatic latent image on the drum surface attracts toner and forms a visible toner image.

The LaserWriter 12/640 PS uses nonconducting magnetic toner that readily transfers to plain paper. The toner is supplied from the toner compartment to the attraction roller positioned close to the drum within the toner cartridge. Because of its slight magnetism, the toner is attracted to the surface of the attraction roller and is spread evenly into a very thin layer by the charging and metering blade (CMB). This blade is made of silicone rubber. As it comes into contact with the toner, the resulting friction imparts a negative charge onto the toner particles.

The attraction roller is covered with a thin conductive sleeve. A development bias (DB) voltage is applied to the conductive sleeve from the HVPS. The DB voltage is a negative DC voltage with an AC voltage superposed. The DC voltage is about -250 V and the AC voltage is about 1.7 kVp.

Since the attraction roller is kept at a potential of about -250 V DC relative to the conductor layer of the drum, the parts of the drum surface with the full negative charge have a lower electric potential than the attraction roller, and the parts with decreased negative charge have a higher electrical potential.

Therefore, the negatively charged toner on the attraction roller is pulled only to those areas of the drum with the decreased negative charge (since the attracting force of the positive charge on the conductor layer of the drum is greater than the repelling force of the negative charge on the drum surface, the toner is attracted in spite of the repelling force of the negative charge). As the toner adheres, the negative charge of those parts increases because of the negative charge of the toner, lowering the electric potential and increasing the repelling force of those parts to decrease the resultant attracting force.

The AC voltage agitates the toner particles on the attraction roller surface and facilitates toner release. The invisible electrostatic latent image on the drum surface is thus developed into a visible toner image.

HVPS

Development Bias

Exposure

Charging

Conductor

Layer

Drum

Scanner

Irradiated Part of the Drum Surface

Development

CM Blade

Attraction

Roller

Photoconductor

Layer

Negative Charge Positive Charge Negatively Charge Toner Non-Charged Toner

Principles of Operation Print Process - 6

3 Development (cont.)

The change of the electric potential of the drum surface while the drum surface passes through the charging to the development steps

0 V

Development Bias

About -250 V

Bias Charge

About -330 V

Conductor Layer

of the Drum

The electric potential of the parts of the drum surface struck

by the laser beam (the areas with decreased negative charge) rises above the electric potential of the attraction roller.

The electric potential of the parts with decreased negative charge

lowers as the toner is attracted.

The degree of inclination of the line indicates the strength

of the force and the direction of inclination indicates the

direction of the force exerted on the toner particles.

Drum Surface

Attracting

Repelling

Attraction Roller

¿ Charged ¡ Exposed ¬ Developed

Principles of Operation Print Process - 7

3 Development (cont.)

The diagram below illustrates the change of the potential as the drum surface passes from the charging through the development stages:.

Drum Surface Voltage

1 Charging

2 Exposure

3 Development

Electrostatic Latent

Image (Invisible)

-V

About -233V Bias Charge

About -250V Development Bias

Toner Image

(Visible)

Principles of Operation Print Process - 8

4 Transfer

During this stage, the toner transfers from the drum to the paper. The bias transfer roller (BTR), referred to herein simply as the transfer roller, applies a

positive charge to the back side of the paper as it feeds between the roller and the drum. The toner image transfers from the drum surface to the front side of the paper due to this positive charge on the back side of the paper.

The paper is attracted by the negative charge induced in the inner conductor layer of the drum by the positive charge of the paper and sticks to the drum surface.

Laser Beam

1 Charging 2 Exposure

3 Development

Drum

HVPS

Transfer Roller

4 Transfer

Drum

Conductor Layer Photoconductor

Layer

Paper

Transfer Roller

Negative Charge Positive Charge Negatively Charged Toner

Principles of Operation Print Process - 9

5 Discharge

During this stage, the paper comes off the drum surface. The detack saw applies a negative charge to the back side of the paper to neutralize the

positive charge previously applied by the transfer roller. The force which attracts the paper to the drum surface disappears, and the paper releases naturally from the drum due to gravity.

Laser Beam

1 Charging 2 Exposure

3 Development

Drum

HVPS

5 Discharge

4 Transfer

Paper

Detack Saw

Positive Charge Negatively Charged Toner Non-Charged Toner

Principles of Operation Print Process - 10

6 Fusing

During this stage, heat and pressure fix toner to the paper. The toner image transferred from the drum surface to the paper is easily dislodged. To fix

the image to the paper, the paper is passed between the fuser assembly and a pressure roller. The fuser roller applies the heat necessary to bond toner to paper and also supplies rotational force to help pull the paper evenly through. The pressure roller, though neither heated or mechancially driven, provides the surface against which the pressure and heat can be evenly applied.

Service Source

Speciﬁcations

LaserWriter 12/640 PS

Specifications Introduction - 1

Introduction

Specifications information for this product can be found in this chapter and also in the Spec Database, which you can access in one of three ways:

Ñ Launch it directly by double-clicking the Apple Spec Database runtime alias at the top level of the Main Service Source CD.

Ñ Select "Apple Spec Database" from the Service Source drop-down main menu.

Ñ Click the Acrobat toolbar icon for the database, which is near the right end of the toolbar with the letters "SP."

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