Easy laser printer maintenance and repair

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Tiie eiecirophoiographic printer E
An overview 23
What is elecfiophotographic‘? 2
Features and specifications 2
Power requirements 3
Interface compatibility 5’
Print capacity 4
Print characteristics 4
Reliability/life information 5
Environmental information 6'
Physical information 6
Typical assemblies 6
The ac power supply 6
The clc power supply 7
High-voltage power supply '7
Fusing assembly '7
Eraselamp assembly 8
Main motor 8
Writing mechanism 8
Scannenmotor assembly 8
Paper-control assembly 8
Main logic assembly 9
EP cartridge assembly .9
Control-panel assembly 9

2. 2
3
Typicai components i6
Mechanical parts 10
Gears J0
Pulleys 12
Rollers J8
Reducing friction 18
Electromechanical components J 4
Electrornagnetism J4
Relays J 5
Solenoids 16
Motors Z7
Passive components 18
Resistors 18
Reading resistors 20
Capacitors
Reading capacitors 24
Inductors 25
Active components 2'7
Diodes 27
Diode markings 82
Transistors 88
Transistor markings 87
Integrated circuits 88
Seiderrtg and test instruments 42
Small tools and materials 48
Hand tools 48
l/lat-erials -44
Soldering 45
Soldering bacl<gronnd 45
Irons and tips 45
Soldering 4'7
Surface~mount soldering i;8
Desoldering 50
l/Iultimeters 51
Measuring voltage 5,52
l/ieasuring current 52
Measuring frequency 54
Measuring resistance 54
Checkingacapacitor 55
Semiconductor checks 56
Logic probes 58
Oscilloscopes 59
Controls 60
Oscilloscope specifications 60

3. Oscilloscope startup procedures 6]
Voltage measurements 62
Time and frequency measurements 64
Service guideiiries 65
The troubleshooting cycle 65
Define your symptoms 66
identify and isolate 66
Repair or replace 6'7
Re-test 68
Gathering technical data 68
Electricity hazards 69
Static electricity '71
Static formation 72
Device damage '72
Controlling static electricity '78
Reassembly and disassembly hints '74
Housing disassembly '74
Electromechanical disassembly '74
Reassembly '75
Eiectrepiiutograpiiie teeitnolcgy 7'7
The electrophotographic approach 78
Cleaning '79
Charging 80
Writing 8]
Developing 82
Transfer 88
Fusing 84
Writing mechanisms 84
Lasers 85
LEDs 88
LCSs 89
The electrophotographic cartridge 90
Protecting an EP cartridge 91
Power supplies 98
Power supplies—~ac and linear dc 94
Transformers 94
The ac power supplies 96
Rectifiers 96
Filters 98
Regulators 99
Troubleshooting ac and linear dc supplies 102
Construction and operation of dc switching supplies 107

4. Troubleshooting dc switching supplies 1 J 0
High~voltage supply troubleshooting 1 J4
irnagefliurntatiuri system tin
System start-up problems J 1 '7
Laser-delivery problems Z Z 9
Fusing-assembly problems Z22
image-formation problems 124
R/ieeiiariieai systerns rs?
Paper problems Z88
Sensor and interlock problems J46
Resistive sensors Z46
Mechanical sensors J4'7
Optical sensors J48
Troubleshooting sensors and interlocks J48
SC&l'll1@l‘-1I10llO1‘/11183111-I‘fiOtOI‘ problems J 50
EP cartridge problems J 51
Tire eleetrunie euntrui paeirage E54
Communication J 55 ‘
Printer communication background 1 56
ASCll explained J 56
Control codes Z 5'7
1lumber systems J 5'7
Binary digits J 58
Communication links 159
Communication standards Z 60
Parallel communication Z 60
Serial communication J 6.8
lsolating the communication interface J 64
Troubleshooting a parallel interface Z 64
Troubleshooting a serial interface J 6'7
Memory J 69
Permanent memory Z 69
Temporary memory J '70
Troubleshooting memory 1'7]
Control panel 1'71
Sealed switches 1'72
Troubleshooting a control panel 1'78
Main logic J 78
Microprocessor operations 1'74
The system clock 1'75
ASIC operations 1'75
Troubleshooting main logic 1 '76

5. APPENDTQES
A Typieai eieaning and inaintenanee E 78
R Qnnipreiiensiye trnublesiinuting guides E84
Q Vendors W2
Glcssary f%
index 268

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Laser printers (Fig. 1-1) have come a long way in the last few years. Not too long ago,
laser printers were expensive commodities restricted to the few serious businesses
that could afford them. However, the precision, speed, printing consistency, and im—
age quality offered by laser printers made them extremely attractive peripherals. As
computer designs continued to advance into the 1990s, laser printers also advanced
while printer prices plummeted. Today, laser printers are available for under $600
(US) and have become commonplace in homes and small businesses all over the
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8. 2 T/ize c?[8C7Z7i"'Op/‘2.i»OZOQ'i"CZ.QU/'Z»’ZlC? _];)'?""’Z1"lr'ZiZi6")"
world. This book shows you how modern laser printers work and explains how to
maintai.ii them effectively.
..-art verview
The intense competition between printer manufacturers has resulted in a stag-
gering variety of laser priiiter models—each with different sizes, shapes, and fea-
tures. ln spite of this physical diversity, every laser printer ever made performs the
same set of functions to 'f.l.'&l.l.SCl‘l.DO the output of a computer into some permanent
paper form. The process seems simple enough, right‘? in reality, however, it requires
a complex interacti.on of electncal, electronic, and mechanical parts all working to-
ether to make a practical laser printer. Stop for moment and consider some things
rat a laser printer must be capable of.
First, the laser printer can do nothing at all without a host computer to provide
data and control signals, so a conununication link must be established. To operate
with any computer system, the printer must be compatible with one or more standard
communication interfaces that have been developed. A printer must be able to use a
wide variety of paper types and thicknesses, which can include such things as en-
velopes and labels. it must be capable of printing a vast selection of type styles and
sizes, well. as gifaphics images, then mix those images together onto the same page.
The laser printer miist be fast. lt must communicate, process, and print infor-
riiation as quickly as possible. Laser printers must also be easy to use. Many features
and options are accessible with a few careful strokes of the control panel. Paper in-
put and output must be convenient. Expendable supplies such as toner should be
quick and easy to chaiige. Fiiilitll}/', laser printers must be reliable. They must produce
even and C.OlIl.SlSlI€1ill print over a long working life»-often more than 300,000 pages
(expendable items must be iI‘€plEtC€C;l more ;l’requently).
as-e*i.e
i it- -{*1 . -» 1 -- ..- T
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You are pi'obably woiidering why people use the term eLccti"ophotogmpitic
_']3"7"’Zl’7"Z.Zi€7"‘ when talking about laser printers. ln truth, electrophotographic (or EP) is a
broad l1€lf'll1 that refers to a 131‘.ilj1il€1I‘ that functions using the electrophotographic
proces Laser" printers are electrophotographic printers that use a laser beam to
write image data, but there are also LED (light-emitting diode) page printers that
use a bar of mici*oscopic LEDs, instead of a laser beam, to write image data. Both
laser and LED printers are electrophotographic printers (although laser-type print»
ers more conunon). You can learn much more about the electrophotographic
process and see how laser and LED printers work in this book. In this book, the
terms EP];>rr"2Irrz.te1", l(Tt88"'l"])7"Z-"12.-t€'?"‘, and LED ];>r/2'/utc/2" are interchangeable.
U3
nQ51? nRifleatnr seeifieatis
Make it point to know your laser printer specifications and features before you
begin any repair. The specifications and features give you a good idea what the

9. Fca.tw"es cmd specifications 3
printer can do, which might help you to test it more thoroughly during and after your
repair. A listing of specifications is usually contained in an introductory section of
the printer instruction manual or at the end in an appendix. If you do not have a copy
of the printer documentation on hand, the manufacturer can often fax a copy of the
specifications directly to you. Remember that there is no standard format for listing
printer specifications. The format is up to the preferences of each manufacturer. Re-
gardless of how the specifications are listed, you will most often find the following
subjects: power requirements, interface compatibility, print capacity, print charac-
teristics, reliability/life information, environmental information, and physical infor-
mation. Each of these specifications has some importance, so you should be familiar
with them in detail.
Puwer requirements
As with any electrical device, a printer requires power to function. Voltage, fre-
quency, and power consumption are the three typical specifications that you will find
here. Domestic U.S. voltage can vary from I05 to I30 Vac (alternating-current volts)
at a frequency of 60 Hz (hertz). European voltage can range from 210 to 240 Vac at
50 Hz. Many current laser printers have a power selection switch that toggles the
printer between 120 and 240 V operation. Power consumption is rated in watts (W).
Depending on the particular model, laser printers can use up to 900 W during print-
ing. However, most models use an automatic power-down mode that shuts down the
major power-consuming components after the printer is idle for several minutes.
Chapter 6 discusses the operation and repair of laser printer power supplies.
interface cninnatihiiity
A printer is a pc’r'2Ip/teral device. That is, it serves no purpose at all unless it can
communicate (or Z/nterfctce) with a computer. A communication link between
printer and computer can be established in many different ways, but three interface
techniques have become standard: RS-232, Centronics, and IEEE 488. Only a prop-
erly wired and terminated cable is needed to connect the printer and computer.
Printer communication and troubleshooting are discussed in chapter 9.
RS-282 is a serial interface used to pass binary digits (or bits) one at a time be-
tween the computer and printer. Serial links of this type are very common, not just
for printers, but for other serial communication applications such as modems and
simple digital networks. RS-232 is popular due to its high speed, physical simplicity,
and its ability to handle data over long distances.
Ceittrom'cs is the standard for parallel communication. Although one used ex-
clusively by printers, parallel communication has become popular for other peripher-
als such as parallel-port tape drives and portable CD-ROIVI (compact disc read-only
memory) drives. Centronics is a de facto standard, so it is not officially endorsed by
standards organizations such as the IEEE (Institute of Electrical and Electronic En-
gineers), EIA (Electronic Industries Association), or CCITT (International Consulta-
tive Committee for Telephone and Telegraph). Instead of passing one bit at a time,
Centronics interfaces pass entire characters from the computer to the printer as sets
of bits. Centronics is popular because of its functional simplicity. Although parallel

10. 4 The electrop/totog?/up/1'20 ]J7”Z7’?/Z6’?
connections require more interconnecting signal wires than an RS-232 cable, the
hardware required to handle parallel information is simpler.
[EEE 488, also known as GPIB (general-purpose interface bus), is an official
IEEE standard for parallel communication. It is not as widely used as Centronics or
RS-2-32, but GPIB supports network and bidirectional communication between in-
struments. The GPIB technique Was originally developed by Hewlett-Packard Goin-
pany, where it is still widely used in their line of printers and plotters.
Print capacity
Pi'"mt co;-pdc/tip is a generic term including several different laser printer specifi-
cations that outline what a printer can do. One of the most common print capacity
specifications is p7"’Z’7”?,l§ speed, which is measured in pages per minute (ppm). Inex-
pensive EP printers work at 4 ppm, but 8 to 10 ppm printers are available. Next, you
must be concerned with '1/'esoZ.ut'io"r"2., which is the number of individual dots that can
be placed per linear inch. Typical EP printers offer 300 >< 300 dpi (dots per inch) res-
olution (300 lines per inch at 300 dots per linear inch, or 90,000 dots per square inch).
A resolution this high is adequate for most business and personal graphics. The cur-
rent generation of laser and LED printers is capable of 600 >< 600 dpi resolution.
You might find a section on paper specrfijcdtions. Although dot matrix and ink
jet printers are very flexible in accepting a wide variety of paper thicknesses and fin-
ishes, the paper used in laser printers must fall within certain weights and finishes if
the EP process is to work correctly. In most cases, standard letter-size, Xerography-
grade paper (16-24 pound bond) will work. The paper also should have a plain fin-
ish. Shiny or gloss-finished papers will cause problems with the EP process. Most
laser printers will handle envelopes, transparencies, and labels. Before choosing
such materials, be certain that they are labeled as safe or tested with laser printers.
Poor-quality materials can jam and damage your printer.
Me’n'zio'r*p is another important specification for EP printers. Because laser print-
ers assemble images as full pages of individual bits, the more memory that is avail-
able, the larger and more complex an image can be printed. Typical laser printers
offer 512 K (kilobytes) to 1 l/lb (megabytes), but 2-8 Mb is required for full-page
graphic images that might be produced with software such as CorelDraw. l/lost laser
printers offer memory upgrade options.
Print ciiaracreristics
Print characteristics specify just how printer images will appear, how they will
be produced, or how characters from the printer will be interpreted. Fonts, software
emulation, and character sets are the three specifications that you should be most
familiar with.
Ajbrit is a style of type with certain visual characteristics that distinguish it from
other type styles. These characteristics might include differences in basic character
formation, accents, and decorative additions (that is, Courier versus Helvetica type).
Figure l-.2 is an example of several basic printer fonts. Early laser printers relied on
font cartridges that contained ROl/is (read-only memories) that held the image data
for each font. To change a font, you changed the cartridge. However, with the rise of
Microsoft Windows and improved memory systems, most current laser printers use

11. Features and spec/zflcdt1lov'2,s 5
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soft fonts, where the data for desired fonts is downloaded to the printer during the
actual printing process. Soft fonts also allow easy enhancements such as underlin-
ing, bold, italic, superscript, subscript, and so on.
All printers use their own built-in software “language” that is in the printer per-
manent memory. The language specifies such things as font formation (dot place-
ment) and size, h.ow to i'ecognize and respond to control codes or control panel
input, and more. This software language also tells the printer how to operate, coin-
nninicate, and respond to problems.
l/lost of these languages were originally developed by leading printer manufac-
turers such as Hewlett-Packard, IBM, and Epson. Other manufacturers that wish to
make their printers functionally compatible must use a software language that enm-
ldtes one or more of the existiiig language standai'ds. For example, most laser print-
ers will emulate the operatioir of a Hewlett-Packard Lasenlet lll. E~niitldt?lo'2r. in this
use means that even though a laser printer is physically and electronically different
from a LaserJet lll, it will respond as if it were Lasenlet lll when connected to a
host computer. Another typical printer language is Postscript.
Ordinarily when a character code is sent to a printer, it is processed and printed
as a fully formed alphanumeric character or other special symbol. However, because
a character code is not large enough to carry every possible type of text or special
symbol (for example, foreign-language characters or block graphics), characters are
grouped into Cf/ZCZ/)"CtCf€i" sets that the printer can switch between. Switching a char-
acter set is often accomplished through a series of computer codes or control panel
commands. A standard character set consists of 96 ASCII (American Standard Oode
for information Interchange) ch.aracters. The 96 characters include 26 uppercase
letters, 26 lowercase letters, l0 digits, punctuation, symbols, and some control
codes. Other character s can include 96 italic ASOll characters, international
characters (German, French, Spanish, etc.), and unique block graphics.
CD (D c-l~r
Reiiability/iife inierniarien
Reliability and life ea¢pectcmc;y information expresses the expected working life
of the laser printer or its components in pages or time. For example, a typical EP

12. 0 1'/re etecwop/ziotog/mp/tic pm/rite*i"
toner cartridge is rated for 200»-250 pages, and the image-formation “engine” is rated
for up to 300,000 pages. Many printers are rated at 5,000 pages per month (about
200 pages per workday). You might see this same information expressed as MTBF
(mean time between failures).
Envirenrnentai inferinatien
Environmental specifications indicate the physical operating ranges of your
printer. Stomge Z€’l%]3€7"CLf2L’l/'8 and opercutmg rempemtitre are the two most
common environmental conditions. A typical laser printer can be stored in tern-
peratures between -10 and 50°C, but can only be used from 10 to 82°C (on aver-
age). It is a good idea to let your printer stabilize at the ambient temperature and
humidity for several hours before operating it. Reldtipe /in/n/2.vjd'1lty can often be al-
lowed to range from 10 to 90% during storage, but must be limited to a range of
40 to 70% during operation. Keep in mind that humidity limits are given as non-
condensing values. NO7ZCO'7Z-(.i€’l”L5i’)'Z-Q means that you cannot allow water vapor to
condense into liquid form. Liquid water in the laser printer would certainly dam-
age its image-formation system.
Your printer also might specify physical shoclr or mLb'2"dttoi"2, limits to indicate
the amount of abuse the printer can sustain before damage can occur. Shock or vi-
bration is usually rated in units of Q-force. Keep in mind that laser printers are re-
markably delicate devices; any substantial shock or vibration might disturb the
optics that direct the laser beam. LED printers are a bit more rugged, but also use
optics that can be damaged or misaligned by rough handling.
Pliysicai inforinatien
Physical information about a printer includes such routine data as the printer
height, width, depth, and weight. ln some cases, an operating noise level specifica-
tion is included to indicate just how loud the printer will be during operation and
standby. Noise specifications are usually given in dBA (A-weighted decibels).
icalasseies
No matter how diverse or unique EP printers might appear from one model to
another, their differences are primarily cosmetic. lt is true that each printer might
use different individual components, but every laser printer must perform a very
similar set of actions. As a result, most laser printers can be broken down into a se-
ries of typical sections, or functional areas as shown in Fig. 1-3. Before you trou-
bleshoot a printer, you must understand the purpose of each area.
The ac pewer suppiy
An ac power supply is usually a simple electronic module that provides en-
ergy for the fusing assembly heaters and erase lamp assembly. There is typically
little that goes wrong with the ac supply unless a serious fault in the fuser or erase
assemblies damages the supply. You can learn about power supply operation and
repair in chapter 6.

13. Z fyptcctt ctsse'mb£'2;es 7
Power ; Fllgmg
gupply _ C “l assembly
if all r t rat: Erase
module (ac) * k , 0 mi
"- -’* Paper exit sensor lampi C . _, ».£ _
fir "‘ Fusing bulb a'§SQmmy
. ‘*‘ Thermistor
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- Main-niotoi diivei ._
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.v.
"‘ Registration
Writing solenoid clutch
11 ' Main lflgig ”‘ Fusing interlockA (dc €0n_n_OHeI_) Papei out scnsoi A
3 Manual feed
at
‘ sensor
W ).._-.1...-.-....».__<_-_-::i_-i-é-..>l-#»..__i..M_.. ___________________.._
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interface/formatter i.
motor
-am‘-‘~!~.';-Jr:-i,-‘,;i:t W:-'5(optronal)
Scanner
.-'i§_~:$i;~v.zm_:-
i y f . High-voltage El?
@0nn.O3_p£me1 pgvvgr cartridge
assembly impply agsemmy
i--3 Diagram of a typical laser printer.
The dc power supply
The dc (direct current) power supply converts main ac entering the printer into
one or more levels of dc that are used to power the printer electronics and electro-
mechanical devices (such as motors and clutches). Like the ac supply, a dc power
supply is a fairly rugged and reliable device unless a fault in some part of the printer
circuitry damages the supply. A cover interlock in the dc supply shuts down printer
operation if a protective cover is opened. You can learn about power supply opera-
tion and repair in chapter 6.
l-ligh-wvolrage power supply
The EP process relies on high-voltage (1,000 V or more) that is used to create
and dissipate the powerful static charges that move toner Within an EP printer. Even
though specialized components are used in high-voltage supplies, high voltages
place a great deal of stress on electronic parts, so high~voltage supplies tend to fail
more commonly than ordinary ac or dc supply modules. See chapter 6 for more in-
formation on power supplies.
Easing asseruhiy
images are developed on paper using a fine powder called toner. The toner must
be fixed to the paper (otherwise, the toner would smudge or simply blow off the
page). Heat and pressure are used to fuse toner to the paper. The fusing assembly

14. b '1 '/"L63 etectiop/totogiup/2;zc ];)’)“"’Z'7"Z~Z§€7"
uses a set of two rollers in compression, where the top roller is heated to melt the
toner. A paper‘ exit sensor detects the passage of paper through the printer, and the
thermistor sensor is used to regulate temperature in the heated fusing roller. Chap-
ters £5 and '7 present more detailed information about the fusing assembly.
Erase lamp assembly
The image that appears on a printed page has been transferred there as a latent
image written to a special photosensitive drum. Each time the drum rotates, the la-
tent image must be erased before a new image is written. The erase lamps clear the
drum thoroughly and allow the photosensitive surface to accept a new image. Erase
lamp failure is usually easy to spot as you can see in chapter 7.
lllain motor
EP printers rely on substantial mechanical activity. Paper must be drawn from a
supply tray, fed to the image formation system, fixed, then fed to the output tray. The
mechanical force needed to support all of these activities is provided by a single mo-
tor and mechanical drive assembly. Chapter 8 describes mechanical systems in detail.
Writing rneebariism
The data that makes up an image must be transferred (or “written”) to the pho-
tosensitive drum. As you can see in chapter 5, this transfer is achieved by directing
light across the drum surface. For a laser printer, writing is accomplished by scans
ning a laser beam across a drum. For an LED printer, the light generated by individ-
ual microscopic LEDs (one LED for every dot) transfers image data to the drum.
Writing is controlled by the main logic assembly (or ECP).
Scanner»-inotor assembly
When a laser is used as a writing mechanism, the beam must be scanned back
and forth across the drum surface. This scanning process uses a hexagonal mirror
that is rotated with a motor. Note that scanners are not needed for LED printers be~
cause there is no beam to scan across the drum. You can find more information on
scanner assemblies in chapter 8.
Papeneontrol assembly
Paper must be grabbed from the paper tray, registered with the latent image,
passed through the image foi‘niationl§ybten1, fused, and passed out of the printer. Al-
though the main motor turns constantly, not all portions of the paper handling sys-
tem can be in motion at all times. The paper control assembly provides the sensors
that detect the presence of paper in the paper tray, the presence of paper in the
manual feed slot, and the sensitivity of each EP cartridge for optimum printing. in
addition to sensors, the papencontrol assembly provides the paper pickup and reg»
istration roller clutches that grab and register the page during printing. Chapter 8
discusses the paper-handling assembly in detail.

15. Typ/Zeal assemblies 9
Main logic assembly
The main logic assembly (or electronic control package—-ECP) is the heart and
soul of your EP printer. The main logic assembly has most of the circuitry that oper-
ates the printer, including electronics that communicates with the computer and
control panel. l/lain logic is also responsible for checking and responding to input
provided by a variety of sensors. Problems that occur in main logic can range from
subtle problems to major malfunctions. Chapter 9 covers the main logic circuits
found in EP printers.
EP cartridge assembly
The Electrophotographic (EP) cartridge of a printer is a remarkable piece of en-
gineering that combines the toner supply and much of the printer image-formation
system into a single, replaceable cartridge. By replacing the EP cartridge, you also
replace delicate, wear-prone parts such as the primary corona, EP drum, and devel-
oper roller. The modularity of the cartridge simplifies the maintenance of your
printer and improves its overall reliability. The image-formation system is discussed
in chapter '7.
Qontrol-panel assembly
Users must be able to interface with the printer to select various options or op-
erating modes. llot only do current control panels provide multifunction buttons,
but most also provide an LCD display for printer status and menu prompts. The con-
trol panel is covered with main logic in chapter 9.

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To troubleshoot any electromechanical system, you must be familiar with the indi-
vidual electronic and mechanical components that you find inside. This chapter in-
troduces you to a cross-section of components found in most typical laser printers.
Your troubleshooting efforts are simplified if you can identify important components
on sight, understand their purpose, and spot any obvious defects. Keep in mind that
this chapter is by no means a complete review of every possible type of component,
but it will give you a good idea of what to expect.
1.. 1*» .Cv W I.-i yr ‘-r. iii
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For the purpose of this discussion, mechanical parts basically serve a single pur-
pose--to transfer force from one point to another. For example, a laser printer uses
a single motor to operate the printer paper transport and image-formation systems.
The physical force of a motor must be transferred to the paper, as well as to the var-
ious rollers and mechanisms that make the printer work. This transfer is accom-
plished through a series of gears, pulleys, rollers, and belts. Whenever mechanical
parts are in contact with one another, they produce friction that causes wear. Lubri-
cants, bushings, and bearings work to minimize the damaging effects of friction.
Chapter 8 presents troubleshooting procedures for mechanical systems.
Qears
Gears perform several important tasks. Their most common application is to
transfer mechanical force from one rotating shaft to another. The simplest arrange-
ment uses two gears in tandem as in Fig. 2-1. When two gears are used, the direction
of secoi'ic£cz.ry rotation is opposite that of-the p'rimcm"y shaft. If secondary direction
must be the same, a third gear can be added as shown in Fig. 2-2. The orientation of
applied force can be changed by using angled (or beoeled) gears shown in Fig. 2-8.
By varying the angles of both gears, force,,can be directed almost anywhere. Several
secondary gears can be run from a single drive gear to distribute force to multiple lo-
cations simultaneously, which is a critical feature for laser printer operation.
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Not only can gears transfer force, they also can alter speed and amount of force
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parts that rotate, the force in the rotating parts is known as torqrue). The effect is
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slower, but with more force. Finally, an equal ratio causes a primary and secondary
gear to turn at the same speed and force.
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2-4 An example of gear ratios.
Fuiieys
Pulley assemblies are common in many laser printers. Like gears, they are used
to transfer force from one point to another. Instead of direct contact, however, pul-
leys are joined by a CIZ'7""Z'/U6 Z/miactge, which is usually a belt, wire, or chain. The action
is much the same as the fan belt in your automobile or the drive chain on your bicy-
cle. A basic pulley set is shown in Fig. 2-5. A motor turns a drive pulley that is con-
Drive pulley
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2-5 Top view of a basic pulley set.

19. M8CflCZ7’?/ZCCLZ pciits 13
nected to a secondary pulley through a drive belt that is under tension. As the drive
pulley turns, force is transferred to the secondary through the linkage, so the sec-
ondary pulley also turns. Notice that both pulleys turn in the same direction. A pul-
ley/belt configuration is sometimes used as a conveyor belt to carry paper evenly to
the fusing rollers.
Pulleys and drive linkages will vary depending on their particular application.
Low-force applications can use narrow pulleys (little more than a wheel with a
groove in it) connected with a wire linkage. Wire is not very rugged, and its contact
surface area with both pulleys is small. Therefore, wire can slide when it stretches
under tension or if load becomes excessive. Belts and their pulleys are wider, so
there is much more surface contact around each pulley. Belts are usually stronger
than wire, so there is less tendency to stretch under tension. Greater strength makes
belt-driven pulleys better suited for heavier loads.
You can replace pulleys with sprocket wheels and a chain linkage. Because each
chain link meshes with the sprocket wheels, any chance of slipping is eliminated.
Chains are almost immune to stretching under tension, so chain drives are used to
handle the highest loads.
Boilers
Rollers are really a focal point of laser printer operation. Rollers not only serve
to grab a sheet of paper from the paper tray, but rollers position (or register) the pa-
per before printing. The image formation system uses several rollers to distribute
toner and transfer the image to paper. By passing the paper through a set of heated
rollers, the transferred image is fused to paper. Damaged, old, or dirty rollers might
have an adverse effect not only on paper handling, but on overall image quality.
Reducing fricrion
As with all mechanical systems, parts that are in contact with one another will
wear while the system operates due to unavoidable friction that occurs between parts.
Therefore, reducing friction will extend the working life of your printer. Lubrication,
bushings, and bearings are three commonly accepted methods of reducing friction.
Use of oils or grease is one way to reduce friction (and might prove effective in
small doses), but this Zabr'"iccttvLo'iv. must be replaced regularly for it to remain effec-
tive. Otherwise, it can wear away, dry out, or harden into thick sludge. Lubricants
also are notorious for collecting dust and debris from the environment, which even-
tually defeats any benefits that the lubricant can provide.
BU-S/’Z/Z71-Q8 are usually “throw-away” wear surfaces as shown in Fig. 2-6. A bush-
ing is made of softer materials than the parts it is separating, so any friction gener-
ated by moving parts will wear out the bushing before the other parts touch each
other. When a bushing wears out, simply replace it with a new one. Bushings are
much less expensive and easier to replace than major mechanical parts such as
slides or frames. Today, bushing materials are reliable, and can last throughout the
working life of the printer.
Probably the most effective devices for reducing friction between parts are
bearings. Bearings consist of a hard metal case with steel balls or rollers packed in-
side as shown in Fig. 2-7. Because each steel ball contacts a load-bearing surface at

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Ball bearings
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only one point, friction (and Wear) is substantially lower than for bushings. Unfortu-
nately, bearing assemblies are often much more expensive than bushings, so bear-
ings are used only to handle heavy loads, or in places that Would be too difficult to
change bushings. Most laser printers avoid the expense of bearings in favor of inex-
pensive bushings.
lectrecaicalcets
Electromechcm"iccil co'mpo’2'tents are a particular class of devices that convert
electrical energy into mechanical force or rotation. Relays, solenoids, and motors are
three common electromechanical components that you should understand. Each of
these important devices relies on the principles of electromagnetism.
Electromagnetisrn
Whenever electrical current passes through a conductor, a magnetic field is gen-
erated around the circumference of that conductor as shown in Fig. 2-8. Such a mag-
netic field can exert a physical force O11p67"’l’l’L8CLbl8 materials (any materials that can
be magnetized). The strength of a magnetic field around a conductor is proportional
Magnetic
force
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Metal Direction of
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to the amount of current flowing through it. Higher amounts of current result in
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Unfortunately, it is virtually impossible to pass enough currentthrough atypical
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magnetic field must somehow be concenitiated, usually by cro'tlrmg the wire as shown
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permanent magnet. Notice how the direction of magnetic itlux always points to the
‘rt-O'7‘tlt pole of the coil. if the direction of current flow were reversed, the magnetic
poles of the coil also would be reversed.
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21-=9 The concentra.tion of magnetic force in a coil.
To concentrate magnetic forces even furtheif, a permeable core material can be
inserted into the coil center as in Fig. 2-10. Ty_3ica.lfly, iron, steel., and cobalt are con-
sidered the classical core materials, but iron-ceranuc blends are used as well. Coils
of wire such as these form the foundation of all electromechanical cleviceU2
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Reiays
A relay is simply a mechanical switch that is actuated with the electromagnetic
force generated by an energized coil. A diagram of typical relay is shown in Fig.
2-ll. The switch (or coittctct set) can be normally open (NO) or normally closed
(NC) while the coil is de~energi:z.ed. When activated, the magnetic field of the coil
causes normally open contacts to close, or normally closed contacts to open. Con-

22. l@* C';§' T;y,p'tcrz.l conrtpo~iz.ei2.ts
Contacts
l l & 2-ll
_ . _ __ A schematic view of relay
*~ Co1.l *- 1- . -configurations.
Normally Normally
Open (NC) Closed (NC)
tacts are held in their actuated positions as long as the coil is energized. lf the coil is
turned off, contacts will return to their normally open or closed states. Keep in mind
that a coil might drive more than one set of contacts.
Relays are not always easy to recognize on sight. l/lost relays used in electronic
circuits are housed in small rectangular containers of metal or plastic. Low-power re-
lays are available in oversized lC (integrated circuit) style packages and soldered
right into a PC (printed circuit) board just like any other integrated circuit. Unless
the relay internal diagram is printed on its outer case, you will need a printer
schematic or mamifacturer’s data for the relay to determine the proper input and
output functions of each relay pin.
Ruienuiris
SOZ-8"l‘2.-O'Z(Tt-S convert electromagnetic force directly into motion as shown in Fig.
2-l2. Unlike ordinary electromagnets whose cores remain fixed within a coil, a sole-
noid core is allowed to float back and forth without restriction. When energized, the
magnetic field generated by a coil exerts a force on its core (called a ptim.ge"r')that
pushes it out f'i'om its rest position.
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lf left unrestrained, a plunger would simply shoot out if its coil and fall away.
Flungers usually retained by a spring or some other sort of mechanical return as-
sembly. Being retained, a plunger extends only to some known distance when the
coil is fired; then it automatically returns to its rest position when the coil is off.
Solenoids are commonly used as clutches in laser printer paper-handling systems.
Remember that the motor providing mechanical torque in the printer is constantly
running while a. page is being printed. However, the paper-grab and registration rollers
need only turn briefly during the printing cycle. Solenoid “clutches” are used to en-
gage and disengage the motor torque as needed.

23. Electrom.ec/trim/tcctl componems i7
Meters
./l/[otors are an essential part of every laser printer manufactured today. Motors
operate the entire mechanical transport system. Chapter 8 presents a detailed dis-
cussion of mechanical systems and service. For now, concentrate on the motor itself.
All motors convert electrical energy into rotating mechanical force (torque). ln
turn, that force can be distributed with mechanical parts to turn a roller or move a
belt. An "t"7’tCZ"L(.iCZ’Z-O71 motor provides torque through a series of powerful electromag-
nets (coils) around a permanent magnet core as shown in Fig. 2-13. The core
(known as a rotor) is little more than a shaft that is free to rotate as its poles en-
counter electromagnetic forces. Each coil (also called a phase or p/tctse 2.u“mct2Trtg) is
built into the motor stationary frame (or stcttor).
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2»-i3 Simplified view of a typical induction motor.
By powering each phase in its proper order, the rotor can be made to turn with
some amount of force. The amount of angular force generated by a motor is known
as torque. lnduction motors generally require two ac signals separated by a 90 de-
gree phase difference. These sinusoidal driving signals vary the strength of each
phase evenly to create smooth rotation. lnduction motors are rarely used in today’s
commercial printers because they do not lend themselves to the precise positioning
requirements of most printers. lnstead, a close cousin of the induction motor is used,
and the motor is called a ctc servo motor.
The dc servos are powered by bipolar dc signals rather than by sinusoidal ac.
Like the induction motor, a dc servo is turned by bipolar driver signals in rapid suc-
cession. Although dc servos are not very precise for positioning, they can hold a con-
stant speed very accurately. This characteristic makes dc servos ideal for driving the
scanner mirrors in laser printers that must rotate at a precise rate.
A popular variation of the dc servo motor is called a SZ§€].’)j_lJ’Z7’tQ motor. Physically,
a stepping motor (or stepper) is very similar to a dc servo, but the rotor/stator
arrangement is much more intricate. Like dc servos, stepper motors are driven by a
series of square wave pulses separated by a phase difference. The sudden shift in
drive signals coupled with the intricate mechanical arrangement causes the rotor to
jump (or step) in certain angular increments, not a smooth, continuous rotation.
Once the rotor has reached its next step, it will hold its position as long as driver sig-

24. i% Typiccil covrzpo'rze'mis
nals maintain their conditions. If driver? signals hold steady, the motor could remain
stationary indelimitely. A typical stepping motor can achieve 1.8 degrees per step,
which means a motor must make 200 (360 degrees divided by 1.8 degrees) individ-
ual steps to complete a single rotation. lilovvever, gear ratios can break down motor
movement into much finer divisions.
Stepping motors are ideal for precise positioning. Because the motor moves in
knovvn angular steps, it can be rotated to any position simply by applying the appro-
priate series of driver pulses. For example, suppose your motor had to rotate 180 de-
grees. ll’ each step equals l.8 degrees, you need only send a series ol’ lOO (180
degrees divided by 1.8 degrees) pulses to turn the rotor exactly that amount. Logic
circuits in the printer generate each pulse, then driver circuits amplify those pulses
into the high-power signals that actually operate the motor. Chapter 9 explains the
operation and repair of printer electronics.
h=_ _:,4-' - , ~,_ ._, ‘ »_‘ _, . ,‘»¢3,‘ . -V.
it .>.§;~_-.< =->. .~ ll is tr it 5::as-., it w ‘ii e 15='_ g; .4‘ rt,
li you intend to do any subassembly or component-level troubleshooting of laser
printer systems, you must have an unde1;'stan<;ling of the various electronic components
that are available. l/lost circuits contain both a.c:t"Z’ve and ,pcr».s.s/we components working
together. Passive components include resistors, capacitors, and inductors. They are
called pcissiue because their only purpose is to store or dissipate circuit energy.
Active components make up broader group of semiconductor-based parts such
as diodes, transistors, and all types of integrated circuits. They are referred to as ac-
tive because each component uses circuit energy to perform a specific set of func-
tions—they all do something. A component might be as simple as a recti.i’ier or as
complex as a microprocessor, but active parts are the key elements in modern elec-
tronic circuits. This part of the chapter shows you each general type of component,
how they Work, hovv to read their markiiigs, and how they fail.
Resistors
All resistors ever made serve a single purpose--to dissipate povver in a con-
trolled fashion. Resistors appear in most circuits, but they are usually used for such
things as voltage division, current limiting, etc. Resistoiis dissipate power by pre-
senting a resistance to the flovv of current. Wasted energy is then shed by the resis-
tor as heat. ln printer logic circuits, so little energy is Wasted by resistors that
virtually no temperature increase is detectable. ln high-energy circuits such as
power supplies, resistors can shed substantial amounts of heat. The basic unit of re-
sistance is the ohm. The symbol for resistance is the Greek symbol omega (Q). You
will see resistance also presented as kilohins (lrtl or thousands of ohms) or megohms
(Mil or nullions of ohms).
CCL7*"bO'7'L—f’ll7?'Z resistors, as shown in Fig. 2-14, have largely replaced carbon-com-
position resistors in most circuits requiring through-hole resistors. instead of carbon
filling, a very precise layer of carbon film is applied to a thin ceramic tube. The thick-
ness of this coating al’l’ects the amount ol’ resistance--thicker coatings yield lovver
levels of resistance, and vice versa. l/letal leads are attached by caps at both ends,

25. 1 i-ooo1,~c=n: L/L)!/L1./U‘Ibt7'l(/bib 17
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andlluienurefnushedaueennflyischppedinegxuqrorceranuc.Carbonefihnresumors
are generally more accurate than carbon composition resistors because a film can be
deposited very precisely during manufacture.
A swij‘crce-'n2..oin/2r2._t resistor is shown in Fig. 2-15. As with carbon-film resistors,
surface-mount resistors are formed by depositing a layer of carbon film onto a thin
ceramic substrate. Metal tabs are attached at both ends of the Wafer. Surface-mount
resistors a.re soldered directly on the top or bottom sides of a printed circuit board
instead of using leads to penetijate the PC board. Surface-mount resistors are in-
credibly small devices (only a few square millimeters in area), yet they offer very
tight tolerances. Surfa.ce-mount resistors are used extensively in computers and
printers.
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Highly magnit'ied view of a
surface-mount resistor.
Ceramic substrate
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Metal tabs