Dot-matrix printers (Fig. 16-1) represent some of the most well-established, reliable, and inexpensive printing technology available for PCs. By firing an array of solid wires against an inked ribbon, a myriad of text styles and graphics can be printed on a variety of paper weights and finishes (it even works for multi-part forms). The low cost and long working life provided by dot-matrix printers have earned them a place as workhorses in modern homes and offices everywhere. This chapter will show you the inner workings of a typical dot-matrix printer (or DMP) and show you a series of fixes for each major sub-assembly.

A typical DMP is composed of five main areas as illustrated in the block diagram of Fig. 16-2; the print head, the paper transport, the carriage transport, the ribbon transport, and the electronic control unit (ECU). This part of the chapter discusses the first four sub-assemblies in detail.

Each dot is generated by an individual metal print wire driven through a solenoid. When an electrical pulse reaches the solenoid, it energizes the coil and produces a brief, intense magnetic field. This field "shoots" its print wire against the page. After the pulse passes, the solenoid's magnetic field collapses. A return spring pulls the wire back to a rest position. In actual practice, DMP solenoids and print wires are very small assemblies. A typical print wire may only travel about 0.5mm. This distance is known as the wire stroke. Not all DMP heads hold their print wire directly within the solenoid's coil. While this approach may work just fine for smaller general-purpose heads, heavy-duty heads need larger coils than can be stacked vertically. Each solenoid is mounted offset from one another, then connected to their respective print wires using a mechanical linkage. Due to the additional mechanical components, heavy-duty print heads operate somewhat slower than direct-drive heads, and they usually do not last as long.

Electrically speaking, solenoids are not very efficient devices. Only 1% or 2% of the energy provided is actually converted to force. The remaining energy is wasted as heat. Heating can have severe effects that include print wire jamming, coil burn out, and even a potential burn hazard. Solenoids usually require anywhere from 12 to 24 volts DC at currents greater than 1.5 amps, so the "on-time" for a typical solenoid must be kept very short (often below 1mS) to prevent excessive heating. Metal heat sinks are often die-cast into the head housing to dissipate excess heat as quickly as possible. Short pulses also allow extremely fast firing cycles. Most current DMP print heads can fire at greater than 300 Hz. Some models can be fired as fast as 600 Hz.

NOTE: The heat sinks of most DMP print heads can become extremely hot - especially after long printing jobs. Do not touch a DMP print head until the printer has had at least 15 minutes to cool.

DMP heads use an array of 7, 9, or 24 print wires arranged in vertical columns as shown in Fig. 16-3. There are three major mechanical specifications that you should be familiar with. Wire diameter specifies the diameter of each print wire (normally expressed in millimeters). This tells you how large each dot will be. The distance between the center of each dot is known as wire pitch (also expressed in millimeters). Finally, the height of each fully-formed character is specified (in millimeters) as character height (or CH). Wire diameter and pitch are much smaller for 24 pin heads than for 9 or 7 pin heads.

Not all wires may be used to form every character. For example, a 9 pin print head with a 2.5mm CH may only use 7 wires to form most characters. Wires 8 and 9 could be used to form characters with "descenders". The concept of true descenders is illustrated in Fig. 16-4. When all 9 wires are used to form characters, there will be no room left for descenders, so characters can be printed with false descenders. Overall character size may appear larger when all 9 wires are used, but many people find false descenders awkward to read.

Early DMPs lacked the dot resolution and electronic sophistication to produce letter-quality print as typewriters could. Characters were printed upper-case in a 5x7 dot matrix. Such printing was clear and highly legible, but not visually pleasing. Advances in electronics and printer communication allowed both upper and lower-case characters to be printed, while improvements in materials and construction techniques resulted in smaller, more reliable print heads. This made dot-matrix printing easier to look at, but still could not approach letter-quality. With the introduction and wide-spread acceptance of 9 pin print heads, enhanced features became possible.

The additional dots offered by a 9 pin head improved the appearance of each character even more, and provided enough detail to make several different font styles practical, but individual dots were still visible. Near-letter-quality (NLQ) was finally accomplished through multi-pass printing of the same line. Paper position is shifted just a fraction, then a subsequent series of dots would "fill-in" some of the spaces left by a previous pass. Two, three, and four passes may be required to form a line of NLQ characters (depending on the particular printer). A single pass per line became known as draft mode.

NLQ printing is limited by the printer's speed. The more passes needed to form a line of print, the more time is required. For example, suppose a single pass (draft) takes 1 second. If two passes are required for an NLQ line, print time would be twice as long (2 seconds). If four passes are required, print time would be four times as long (4 seconds). For long documents, this additional time can really add up. As a general rule, you can save a great deal of time (and your ribbon) by using the draft mode until you are ready to print the final version of a document.

It was not until the introduction of a 24 pin DMP print head that NLQ print could be accomplished in only one pass. Two vertical columns of 12 wires are offset from one another as in Fig. 16-3. As a 24 pin head moves across the page, the leading column places a first "pass" of dots. This forms a basic character image which is already superior to the 9 pin equivalent. Immediately after the leading column fires, the lagging column places a second "pass" of dots to fill in each character. In this way, two passes are made effectively at the same time. Today, letter quality printing is relatively easy to achieve with DMPs. Since today's DMPs typically offer a selection of built-in high-quality fonts, letters appear much more "complete" and visually pleasing then older models. With graphics-mode drivers and operating systems such as Windows 95, virtually any TrueType font can be printed in any size or enhancement on an ordinary DMP.

To this day, DMP technology remains a cornerstone of commercial printing technology. They are flexible devices - capable of a wide variety of fonts and enhancements, as well as draft or NLQ performance and bit-mapped graphics. DMPs are reasonably fast, so they can achieve speeds easily exceeding 160 CPS. They are reliable devices. Heavy-duty print heads can last through more than 30 million characters. Smaller, general-purpose heads can last for more than 100 million characters. Impact printing is mandatory for printing multi-copy forms. Finally, they require little maintenance - except for periodic routine cleaning.

However, impact printing is very noisy. The continuous drone of print wires striking paper can become quite annoying. Although DMP printers are now made with plastic coverings that baffle much of its noise, they do not quiet the printer completely. Limited dot resolution is another concern. You may not notice this for NLQ text, but you can see individual dots in draft or bit graphics modes. You can only achieve just so many dots per inch.

Another problem with dot-matrix printing is the eventual buildup of heat. The substantial current needed to fire a solenoid is mostly given up as heat. Under average use, the metal housing will dissipate heat quickly enough to prevent problems. Heavy use, however, can cause heat to build faster than it dissipates. This happens most often when printing bit-image graphics where many print wires can fire continuously. Heating can cause unusual friction and wear in print wires. In extreme cases, uneven thermal expansion of hot pins within the housing may cause them to jam or bend.

Although individual paper handling mechanisms will vary slightly from manufacturer to manufacturer, dot-matrix printers use two distinct types; friction feed and tractor feed. Each of these systems operates in a different manner, and suffers from its own unique set of problems.

As its name suggests, a friction feed paper transport uses friction to push paper through the printer as shown in Fig. 16-5. Paper is threaded into the printer along a metal feed guide. The guide ensures that paper is maneuvered properly between the platen and pressure roller(s), then up in front of the print head assembly. A set of small, free-rolling bail rollers press gently against the paper to help keep it flat around the platen. In order to allow the free passage of paper during threading, a lever is often included to separate pressure rollers from the platen (not shown in Fig. 16-5). After paper is positioned as desired, the lever can be released to re-apply pressure. From then on, paper can only be moved by hand-turning the platen, or the printer's actual operation.

A typical drive system for a friction feed paper transport is shown in Fig. 16-6. High-energy square wave pulses provided by the ECU's motor drive circuits feed a stepping motor. Depending on the quantity of pulses and their sequence, the motor can be made to step clockwise or counterclockwise by any amount. The stepping motor provides force to a drive train of gears. The drive gear (or primary gear) operates a secondary gear attached to the platen. Gear assemblies are usually used, but pulley systems can sometimes be found. In some designs, a stepping motor is used to operate the platen directly (known as direct drive). The drive assembly offers several useful features. First, the use of gears provides a reliable drive train - gears do not stretch or tear with age, and they will not jam or slip as long as they are kept clean and aligned properly. Second, the use of a smaller primary gear provides greater positioning accuracy for the platen.

Tractor feed does not rely on friction to transport paper. Instead, a set of sprocket wheels are mechanically linked to the platen drive train. Pegs on each sprocket wheel mesh perfectly with specially made paper. This type of paper (also called continuous-feed or fan-fold paper) has holes perforated along both sides. Paper is threaded into the printer along a metal feed guide. There is very little resistance from its contact rollers, so paper can easily be fed through and secured into its sprocket wheels. Most sprocket wheels can slide left or right to accommodate a selection of paper widths or tractor feed label products. Bail rollers are included to help keep paper flat against the platen. Once paper is threaded as shown in Fig. 16-7, it can only be advanced by hand-turning the platen knob, or in actual printer operation.

A typical drive system for a tractor feed paper transport is shown in Fig. 16-8. As with a friction feed system, electrical pulses from the ECU's motor driver circuits operate a stepping motor. Depending on the quantity of pulses and their sequence, a motor can be made to rotate clockwise or counterclockwise by any amount. Force developed by the motor is used to operate the drive train. The platen may also be driven directly. For the system shown, a drive gear operates a secondary gear attached to the platen roller. The ratio of primary to secondary gears offers the same advantages as it does for friction feed systems. An additional drive train of gears or pulleys links the platen and sprocket wheel assemblies.

Impact printers use serial print heads - fully-formed text and graphics are formed by passing a print head left and right across a page surface. As the head moves, it places a series of vertical dots that creates the image. In this way, a complete line of text can be generated in a single pass (letter quality text or graphics may require additional passes). As you might imagine, the process of moving a serial print head becomes a serious concern. It must move at the proper time, at the proper speed, and over the proper distance to within several thousandths of an inch on every pass.

The task of moving a serial print head is handled by the carriage transport. A print head is mounted to a platform or holder (the carriage) as shown in Fig. 16-9. Most serial printers use some type of belt drive. A stepping motor (often in conjunction with a gear train) drives a primary pulley which is connected to a secondary pulley by a drive linkage. A drive linkage may be a wire, belt, or chain, depending on the weight of the print head and its desired left-right speed (or slew). At one point, the drive linkage is connected at the carriage, which rides along one or more low-friction rails. Rails keep the carriage rigidly parallel to the platen at all points. When the stepping motor turns counter-clockwise, the carriage assembly slides left, and vice versa.

Sometimes, a pulley system is replaced by a lead-screw type of assembly such as the one in Fig. 16-10. The print head and stepping motor remain unchanged, but the carriage and drive train are modified. A stepping motor now drives a gear train which operates a long, coarsely-threaded lead screw. This lead screw is threaded into the carriage to become one of its "rails". The second rail remains a simple, low-friction shaft which provides stability and support for the carriage (and print head). When the stepping motor turns, it rotates the lead screw (often through a gear train). Clockwise rotation of the lead screw pushes the carriage left, while counter- clockwise rotation pulls the carriage right.

All forms of printing require some sort of media. It is the media which becomes a permanent page image. There are various media for different technologies - impact printing requires ink from a fabric ribbon, thermal printing requires heat-sensitive chemicals already in the paper, ink jet printing uses a reservoir of liquid ink, and EP printing takes a supply of toner powder. Media is consumed by the printer during normal operation, so it must be fresh and available at all times. Ribbons must be advanced during the printing process in order to keep fresh media available to the print head. This often complex task is handled by a ribbon transport mechanism. Transports for packed ribbon cartridges are often uni-directional - ribbon is advanced in one direction only until it wears out. A typical fabric ribbon will survive several complete passes before wearing out.

Most ribbon transports are driven from carriage motion - either directly from the carriage motor, or from a gear on the carriage itself. An intricate "transmission" of contact rollers and gears serve to keep the ribbon advancing in the same direction regardless of carriage direction. Ribbon transport mechanisms vary greatly between printers, but you can easily recognize a ribbon transport mechanism from the long spindle that inserts into a ribbon cartridge. In some printer designs, the ribbon cartridge is a small container that snaps into place on the carriage itself. In other systems, a long ribbon cartridge is typically driven from a mechanism on either side of the printer.

The technique of dot-matrix printing is straightforward, but the actual formation of each letter, number, or symbol is a bit more involved. Data sent from a host computer is interpreted by the printer's main logic and converted to a series of vertical dot patterns. Motor commands start the carriage (and print head) moving across the platen. Simultaneously, printer circuits will send each dot pattern to the print head in series. Each dot pattern fires the corresponding print wires through an inked ribbon to leave a permanent mark on the page. This is also called "serial" or moving-head operation. This part of the chapter covers symptoms and solutions for each main area of the DMP.

Symptom 16-1. The print quality is poor (dots appear faded or indistinct). All other operations appear normal. Begin by carefully examining the ribbon. It should be reasonably fresh and it should advance normally while the carriage moves back and forth. A ribbon that is not advancing properly (if at all) may be caught or jammed internally, so install a fresh ribbon and retest the printer. A fresh ribbon may improve image quality, but that will fade again quickly if the ribbon does not advance. If the ribbon still does not advance properly, the ribbon transport (part of the carriage transport) may be defective. You can inspect the transport assembly as discussed later in this chapter, or replace the transport assembly outright.

Examine the print head spacing next. Most printers are designed with a small mechanical lever adjustment that can alter the distance between a print head and platen by several thousandths of an inch. This adjustment allows print intensity to be optimized for various paper thicknesses. If the print head is too far away from the platen for your current paper thickness, the resulting print may appear light or faded. If spacing is already close or non-adjustable, turn your attention to the print head itself.

Check each print wire in the head assembly. They should all be free to move, sliding in and out without restriction -- except for mechanical tension from the return spring. Keep in mind that you will probably have to remove the print head from its carriage assembly. WARNING: Hot print heads are a burn hazard - allow plenty of time for the print head to cool before removing it. Over an extended period of time, paper dust and ink forms a sticky glue that can work its way into each print wire. As this gunk dries, it can easily restrict a wire's movement or jam it all together. If you find a tremendous buildup of foreign matter, wipe off each wire as gently as possible. Use a stiff cotton swab dipped lightly in alcohol or light-duty household oil. Do NOT use harsh chemical solvents ! Finally, wipe down the front face of the print head. Once all wires are moving freely again, replace the print head and retest the printer.

Remember that it is the print head driver circuitry that supplies energy necessary for print wire operation. There could be a loss of solenoid driving voltage or some other defect in your drivers. If the problem persists, try replacing the ECU assembly.

Symptom 16-2. Print has one or more missing dots that resemble horizontal "white" lines. This also takes place during a self test. Assuming that all other operations of the printer are correct, a loss of one or more dot rows suggests that the corresponding print wire(s) will not fire. Unplug the printer and remove the print head from its carriage assembly. WARNING: Hot print heads are a burn hazard - allow plenty of time for the print head to cool before removing it. While you have the print head free, check each print wire to be sure that they are free to move without restriction - except for normal return spring tension. Over extended periods of use, a print wire may bend and jam within the head housing. Print wires can also jam from an accumulation of dust and oils that build up during normal printing. Foreign matter can often be wiped away gently with a stiff cotton swap dipped lightly in alcohol or light-duty household oil. You should quickly notice an improvement in the wire's freedom after cleaning. If you encounter bent or broken print wires, your best course is to simply replace the print head. You will find several printer parts suppliers at the end of this chapter.

When a new print head fails to correct the problem, your next step should be to check continuity across each wire of the print head cable. An open or intermittent cable wire can render any firing solenoid inoperative. Turn off and unplug the printer before checking continuity. WARNING: Hot print heads are a burn hazard - allow plenty of time for the print head to cool before removing it. You may have to disconnect the cable at one end to prevent false readings. Wiggle the cable to stimulate any intermittent connections. Replace any print head cable that appears to be defective.

If everything checks out up to this point, your problem is probably located in a faulty wire driver circuit or a data error from the ECU. Try replacing the ECU board.

Symptom 16-3. Printer does not print under computer control. Operation appears correct in self-test mode. Before you attempt to disassemble the printer, take a moment to check its "on-line" status. There is almost always an indicator on the control panel which is lit when the printer is selected. If the printer is not selected (on-line), then it will not receive information from the computer - even if everything is working correctly.

A printer can be off-line for several reasons. Paper may have run out, in which case you will often have to re-select the printer explicitly after paper is replenished. Even the simplest printers offer a variety of options that are selectable through the keyboard (i.e. font style, character pitch, line width, etc.). However, you must often go off-line in order to manipulate those functions, then re-select the printer when done. You may have selected a function incorrectly, or forgotten to re-select the printer after changing modes. Also consider software compatibility. If you are using a "canned" software package, make sure that its printer driver settings are configured properly for your particular printer. If you are working through Windows, make sure that the proper Windows driver is selected.

Check your communication interface cable next. It may have become loose or unattached at either the printer or computer end. If this is a new or untested cable, make sure that it is wired correctly for your particular interface (i.e. serial or parallel). An interface cable that is prone to bending or flexing may have developed a faulty connection, so disconnect the cable at both ends and use your multimeter to check cable continuity. If this is a new, home-made cable assembly, double check its construction against your printer and computer interface diagrams. Try a different cable.

Double-check the printer's dip switch settings or set-up configuration. Dip switches are often included in the printer to select certain optional functions such as serial communication format, character sets, default character pitch, or automatic line feed. If you are installing a new printer, or you have changed the switches to alter an operating mode, it may be a faulty or invalid condition. Dip switches also tend to become unreliable after many switch cycles. If you suspect an intermittent dip switch, rock it back and forth several times, then retest the printer. If everything else checks out up to this point, try replacing the ECU board.

Symptom 16-4. Print head moves back and forth but does not print - or prints only intermittently. This also takes place during a self-test. Check your ribbon first. Make sure that it is installed and seated properly between the platen and print head. If the ribbon has dislodged from the head path or is totally exhausted, no ink will be deposited on paper. If the ribbon is in place, make sure that it advances properly as the carriage moves. A ribbon that does not advance properly may be caught or jammed internally, so install a fresh ribbon and retest the printer.

Intermittent connections in the print head or print head cable can lead to highly erratic head operation. A complete cable break can shut down the print head entirely, especially if the break occurs in a common (ground) conductor. Turn off and unplug the printer, then use your multimeter to check continuity across each cable conductor. WARNING: Hot print heads are a burn hazard - allow plenty of time for the print head to cool before removing it. You may have to disconnect the cable at one end to prevent false readings. Replace any print head cable that appears defective. There could also be an open lead in the print head itself. Try a new print head assembly.

There may be a problem with the print head driver supply voltage(s). Use your multimeter to check each output from the power supply. If you find that one or more power supply voltage(s) are low or erratic, you can attempt to troubleshoot the supply, or replace the supply outright. If the power supply is integrated on the ECU, try replacing the ECU outright.

Symptom 16-5. The paper advance does not function, or functions only intermittently. All other functions check properly. When a paper advance fails to work at all, begin by observing the paper feed drive train assembly. Check any pulleys or gears to ensure that all parts are meshed evenly and are able to move freely. You can watch this by turning the platen knob located outside of the printer. Remove any foreign objects or obstructions that may be jamming the drive train. NEVER try to force a drive train that does not turn freely! Realign any parts that appear to be slipping or misaligned. Replace any damaged mechanical parts or assemblies.

Turn off all printer power and examine the electrical connections for your paper advance motor. Make sure that all connectors are installed and seated properly. If you suspect a faulty wiring connection, turn off printer power and use your multimeter to measure continuity across any suspicious wires. It may be necessary to disconnect the cable from at least one end to prevent false continuity readings. Replace any faulty wiring.

If everything checks properly up to this point, either the motor or ECU has failed. With printer power still off and the motor disconnected from the ECU, use your multimeter to measure the continuity of each motor winding. This is not difficult, but you will need to check the schematic of your particular printer for specific pin numbers and resistance measurements. A working motor winding exhibits between 4 to 40 ohms. If you read a short circuit (about 0 ohms), or an open circuit (substantial or infinite resistance), the paper advance motor is probably defective and should be replaced. If you can not determine the location of each motor winding, try replacing the paper advance motor.

If the motor checks out (or a new motor fails to resolve the problem), a fault has likely developed in the ECU. One or more of the motor drive circuits has failed, or the motor voltage output from the power supply has failed. Replace the ECU outright.

Symptom 16-6. Paper slips or "walks" around the platen in friction-feed operation. Friction feed paper transports are only designed to work with certain types of paper - brands within a certain range of thickness and weight. Very fine (light bond) paper or very heavy (i.e. card stock) paper will probably not advance properly. Check the specifications for your particular printer to find its optimum paper type. If you find that you are using an unusual type of paper, try the printer using standard 20lb bond paper such as photocopier paper.

Keep in mind that friction-feed was intended for single-sheet operation. Some small amount of walking is natural, but feeding extremely long lengths of paper may result in noticeable walking. It is possible to use continuous-feed paper in a friction-feed transport, but you should expect to see a certain amount of walking eventually - try to stay with single sheets.

If the problem persists, take careful note of each roller condition. An even, consistent paper feed depends on firm roller pressure applied evenly across its entire length. Rollers that are very dirty, or old and dry, may no longer be applying force evenly. Clean and rejuvenate your rollers with a good-quality rubber cleaning compound such as "Kleen-a-Platen" (TM) available from almost any comprehensive office supply store. CAUTION: rubber cleaning compounds can be dangerous, and may not be compatible with all types of synthetic roller materials - read instructions on the chemical container carefully, and follow ALL safety and ventilation instructions.

Old rollers may also be out of alignment. Mechanical wear on shafts and bushings (or bearings) can allow some rollers to "float" around in the printer. Carefully examine the condition of each roller shaft. Use the paper loading lever to separate pressure rollers from the platen, then wiggle each shaft by hand. Ideally, each shaft should be fixed firmly within its assembly, so you should feel little or no "slack". If you feel or see a roller move within its assembly, replace its bushings, bearing, or shaft. Newer mechanical assemblies make it easier to simply replace the entire transport outright. Some pressure roller assemblies can be adjusted slightly to alter their contact force. If your particular printer uses non-adjustable pressure rollers, there is little more to be done (other than replace the mechanical assembly). If you can adjust roller force (using spring tension or a screw adjustment), do so ONLY as a last resort -- and only then in small increments. Careless adjustment can easily worsen the problem.

Check your paper path for any debris or obstructions that may be catching part of the paper. A crumpled corner of paper jammed in the paper path or caught in the feed guide can easily interfere with subsequent sheets. Adhesive label fragments are even more troublesome. Remove all obstructions being careful not to mark any of the rollers. A straightened paper clip can often get into spaces that your fingers and tools will not. Use your needlenose pliers to put a small hook in the wire's end for grabbing and pulling the obstruction. Do not disassemble the rollers unless absolutely necessary.

Symptom 16-7. Paper wrinkles or tears through the printer in tractor-feed operation. Tractor feed paper transport systems are remarkably reliable. As long as the paper advance drive is working, it is very rare to encounter wrinkling problems using the tractor mechanism. Many printer families (especially DMP's) offer a selection of paper feed paths (i.e. friction-feed, tractor-feed pull, and tractor-feed push). A mechanical lever switches between tractor and friction feed modes. If paper suddenly seems to wrinkle or tear along its perforations during printing, the first thing to check should be the paper feed selector lever.

If your printer's paper feed mode is set correctly, check the paper path for any debris or obstructions that may be catching the paper. Fragments of torn paper or adhesive labels caught in the feed guide can easily jam the paper path. Carefully remove all obstructions that you may find. Use extreme caution to prevent damage to your rollers or feed guide. Do not disassemble the paper transport unless it is absolutely necessary.

Symptom 16-8. The carriage advance does not function - or functions only intermittently. All other functions check properly. During printer initialization, the carriage is taken to its "home" position. Since the printer has no way of knowing where its carriage is when it is first turned on, finding the carriage "home" gives the ECU physical evidence of an actual carriage position. Under normal circumstances, the ECU will initialize the carriage to the left side of the printer. A mechanical or optical "home" sensor will inform the ECU when home is reached. At that point, the exact carriage position is known for certain. As the carriage moves, pulses generated from an optical position encoder confirm that the carriage is in motion.

When the carriage fails to move (or moves only intermittently), suspect a serious mechanical problem. Watch your carriage advance motor and drive train assembly while the printer initializes. A slipping belt or misaligned gear may have to be tightened or replaced. Make sure the primary pulley is attached securely to its drive shaft. Also examine the drive linkage to be sure that it is properly attached to the carriage. Replace any parts that are broken or excessively worn. Remove any obstructions or foreign objects that may interfere with the drive train.

If you see that the carriage advance motor does not turn, turn off and unplug the printer, then check all electrical connections to the motor. Make sure that all connectors are installed properly and firmly seated. Use your multimeter to check continuity across any suspicious wiring. When checking continuity, it may be necessary to disconnect the wire or cable at one end to prevent false readings. Replace any faulty wiring.

If everything checks properly up to this point, either the motor or ECU has failed. With printer power still off and the motor disconnected from the ECU, use your multimeter to measure the continuity of each motor winding. This is not difficult, but you will need to check the schematic of your particular printer for specific pin numbers and resistance measurements. A working motor winding exhibits between 4 to 40 ohms. If you read a short circuit (about 0 ohms), or an open circuit (substantial or infinite resistance), the paper advance motor is probably defective and should be replaced. If you can not determine the location of each motor winding, try replacing the paper advance motor.

If the motor checks out (or a new motor fails to resolve the problem), a fault has likely developed in the ECU. One or more of the motor drive circuits has failed, or the motor voltage output from the power supply has failed. Replace the ECU outright.

Symptom 16-9. The carriage operates, but it does not always position properly. Improper carriage positioning can take many forms - character spacing may be erratic, or the carriage may sometimes ram into the left or right side frames. Faulty mechanics are often at the heart of carriage problems. Turn off and unplug the printer, then inspect the carriage drive train very carefully. Pay particular attention to the drive linkage to be sure that it is reasonably tight. One or both pulleys may be loose. Inspect any drive gears between the motor and primary pulley for signs of slipping or broken gear teeth. Replace any parts that appear broken or excessively worn.

An optical encoder provides pulses to the ECU as a carriage moves. The pulses allow the printer to position its carriage. Erratic or inconsistent positioning may be the fault of the encoder. Try replacing the encoder. If the problem persists, there is probably a logic fault in the ECU. Replace the ECU outright.

Symptom 16-10. Print is light or non-existent. All other functions appear correct. Before you actually begin to troubleshoot a ribbon transport, examine the ribbon cartridge as the printer operates. If the ribbon advances, inspect the ribbon itself - it may simply be exhausted. Replace the ribbon cartridge and retest the printer. A ribbon cartridge that does not advance may be kinked or jammed within its cartridge. Install a fresh ribbon and retest. If normal operation returns, discard the defective ribbon cartridge.

If a fresh ribbon does not correct your problem, examine the ribbon transport mechanics. Turn off and unplug the printer, then remove the ribbon cartridge. WARNING: Hot print heads are a burn hazard - allow plenty of time for the print head to cool before removing it. You will observe a long sprocket gear that inserts into the ribbon cartridge. Grouped just below the sprocket gear, you will find a series of other small gears and friction rollers that make up the ribbon transport. The mechanism can be assembled on the carriage or on the printer frame.

Although it is never desirable to operate a printer without its ribbon, it is usually safe to do for limited periods of time as long as paper is available to absorb print wire impact. Refer to your owner's manual for any specific warnings or cautions. You may have to perform some minor disassembly to observe the entire ribbon transport. While the printer is running, watch the ribbon transport mechanism for any parts that may be loose, sticking, or jammed together. Dust and debris may have accumulated to jam the mechanism. Use a clean cotton swab to wipe away any foreign matter. If the mechanism is severely worn, it should be replaced entirely.

That concludes Chapter 16. Be sure to review the glossary and chapter questions on the accompanying CD. If you have access to the Internet, take a look at some of the dot-matrix printer manufacturers listed below:

Epson: http://www.epson.com

Mannesmann Tally: http://www.tally.com

OKI: http://www.oki.com

Panasonic: http://www.panasonic.com

Star Micronics: http://www.cygnet.co.uk/star