Apple LaserWriter 12, 640 PS User Manual
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Service Source
LaserWriter 12/640 PS
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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 - 10
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 - 11
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 - 12
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 - 13
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 - 14
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 - 15
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
K
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.
1
Charging
2
Exposure
Xerographic Cycle
Cycle repeats for the next print
3
Development
Paper Enters
4
Transfer
5
Discharge
7
Cleaning
Paper Exits
6
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)
AC
DC
CR
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
AC
DC
Development Bias
2
Exposure
1
Charging
Conductor
Layer
Drum
Scanner
Irradiated Part of
the Drum Surface
3
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
a:
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
b:
lowers as the toner is attracted.
c:
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.
b
a
¬
¿
Drum Surface
Attracting
c
c
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
-V
-V
About -233V
Bias Charge
0
0
About -250V
Development Bias
Toner Image
(Visible)
0
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
TR
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
DS
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.
K
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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