Electrophotographic (or EP) printers are fundamentally different from traditional moving-carriage printers (such as ink jet or dot-matrix). Those conventional printers develop dots as a one-step process moving a discrete print head across a page surface. EP printers are not nearly as simple. EP images are formed by a complex and delicate interaction of light, static electricity, chemistry, pressure, and heat - all guided by a sophisticated electronic control unit (ECU). This chapter details the background of EP technology, and provides you with a series of image formation troubleshooting procedures.

Electrophotographic printing is accomplished through a "process" rather than a "print head". The collection of components that performs the EP printing process is called an Image Formation System (or IFS). An IFS is made up of eight distinctive areas; a photosensitive drum (#14), cleaning blade, erasure lamp (#3), primary corona (#4), writing mechanism (#5 & 6), toner, transfer corona (#13), and fusing rollers (#18 & 19). Each of these parts, as shown in Fig. 26-1, play an important role in the proper operation of an IFS.

A photosensitive drum is generally considered to be the heart of any IFS. An extruded aluminum cylinder is coated with a non-toxic organic compound that exhibits photo-conductive properties. That is, the coating will conduct electricity when exposed to light. The aluminum base cylinder is connected to ground of the high-voltage power supply. It is the drum which actually receives an image from a "writing mechanism", develops the image with toner, then transfers the developed image to paper. Although you may think that this constitutes a print head since it delivers an image to paper, the image is not yet permanent - other operations must be performed by the IFS. Complete image development is a six-step process that involves all eight IFS components; cleaning, charging, writing, developing, transfer, and fusing. To really understand the IFS, you should know each of these steps in detail.

NOTE: Later CX-type EP engines use superior EP drum coatings which are more resistant to nicks and scratches, and offers much longer working life.

Before a new printing cycle can begin, the photosensitive drum must be physically cleaned and electrically erased. Cleaning may sound like a rather unimportant step, but not even the best drum will transfer every microscopic granule of toner to a page every time. A rubber "cleaning blade" is applied across the entire length of the drum to gently scrape away any residual toner that may remain from a previous image. If residual toner were not cleaned, it could adhere to subsequent pages and appear as random black speckles. Toner that is removed from the drum is deposited into a debris cavity as illustrated in Fig. 26-2. Keep in mind that cleaning must be accomplished without scratching or nicking the drum. Any damage to the drum's photosensitive surface would become a permanent mark that appears on every subsequent page. Some EP printer designs actually return scrap toner back to the supply for re-use. This kind of recycling technique can extend the life of your electrophotographic (EP) cartridge and eliminate the need for a large debris cavity.

Images are written to a drum's surface as horizontal rows of electrical charges which correspond to the image being printed. A dot of light causes a relatively positive charge at that point. This corresponds to a visual dot in the completed image. Absence of light allows a relatively negative charge to remain and no dots are generated. The charges caused by light MUST be removed before any new images can be written - otherwise images would overwrite and superimpose on one another. A series of erase lamps are placed in close proximity to the drum's surface. Their light is filtered to allow only effective wavelengths to pass. Erase light bleeds away any charges along the drum. Charges are carried to ground through the aluminum cylinder as shown in Fig. 26-3. After erasure, the drum's surface is completely neutral - it contains no charges at all.

A neutral drum surface is no longer receptive to light from the writing mechanism. New images can not be written until the drum is charged again. In order to charge (or condition) the drum, a uniform electrical charge must be applied evenly across its entire surface. Surface charging is accomplished by applying a tremendous negative voltage (often more than -6,000 volts) to a solid wire called a primary corona located close to the drum. Since the drum and high-voltage power supply share the same ground, an electrical field is established between the corona wire and drum as in Fig. 26-4.

For low voltages, the air gap between a corona wire and drum would act as an insulator. With thousands of volts of potential, however, the insulating strength of air breaks down and an electric "corona" forms. A corona ionizes any air molecules surrounding the wire, so negative charges migrate to the drum's surface. The trouble with ionized gas is that it exhibits a very low resistance to current flow. Once a corona is established, there is essentially a short-circuit between the wire and drum. This is NOT good for a high-voltage power supply. A primary grid (part of the primary corona assembly) is added between the wire and drum. By applying a negative voltage to the grid, charging voltage and current to the drum can be carefully regulated. This "regulating grid voltage" (often -600 to -1,000 volts) sets the charge level actually applied to the drum which is typically equal to the regulating voltage (-600 to -1,000 volts). The drum is now ready to receive a new image.

NOTE: Later CX-type EP engines replace the primary corona with a charging roller. The charging roller handles erasure also, and allows charging at a much lower voltage than corona wires.

In order to form a latent image on a drum surface, the uniform charge that has conditioned the drum must be discharged in the precise points where images are to be produced. Images are written using light. Any points on the drum exposed to light will discharge to a very low level (about -100 volts), while any areas left unexposed retain their conditioning charge (-600 to -1,000 volts). The device which produces and directs light to the drum surface is called a writing mechanism. Since images are formed as a series of individual dots, a larger number of dots per area will allow finer resolution (and higher quality) of the image.

For example, suppose a writing mechanism can place 300 dots per inch along a single horizontal line on the drum, and the drum can rotate in increments of 1/300 of an inch. This means your printer can develop images with a resolution of 300 x 300 dots per inch (DPI). Current EP printers are reaching 1200x1200 DPI. Lasers have been traditionally used as writing mechanisms (thus the name "Laser Printer"), and are still used in many EP printer designs, but new printers are replacing lasers with bars of microscopic light-emitting diodes (LEDs) to direct light as needed. Once an image has been written to a drum, that image must be developed.

Images written to the drum by laser or LED are initially invisible - merely an array of electrostatic charges on the drum's surface. There are low charges where the light strikes, and high charges where the light skips. The latent image must be developed into a visible one before it can be transferred to paper. Toner is used for this purpose. Toner itself is an extremely fine powder of plastic resin and organic compounds bonded to iron particles. Individual granules can be seen under extreme magnification of a microscope.

Toner is applied using a toner cylinder (or developer roller) as shown in Fig. 26-5. A toner cylinder is basically a long metal sleeve containing a permanent magnet. It is mounted inside the toner supply trough. When the cylinder turns, iron in the toner attracts it to the cylinder. Once attracted, toner acquires a negative static charge provided by the high-voltage power supply. This static charge level falls between the photosensitive drum's exposed and unexposed charge levels (anywhere from -200 to -500 volts depending in the intensity control setting). A restricting blade limits toner on the cylinder to just a single layer.

Charged toner on the cylinder now rotates into close proximity with the exposed drum. Any points on the drum that are not exposed will have a strong negative charge. This repels toner which remains on the toner cylinder and is returned to the supply. Any points on the drum that are exposed now have a much lower charge than the toner particles. This attracts toner from the cylinder to corresponding points on the drum. Toner "fills-in" the latent image to form a visible (or developed) image.

Notice that an AC booster bias (more than 1,500 Vpp) is added in series to the DC intensity bias. AC causes strong but brief fluctuations in the toner's charge level. As the AC signal goes positive, the intensity level increases to help toner particles overcome attraction of the cylinder's permanent magnet. As the AC signal goes negative, intensity levels decrease to pull back any toner particles that may have falsely jumped to unexposed areas. This technique greatly improves print density and image contrast. The developed image can now be applied to paper.

At this point, the developed toner image on the drum must be transferred onto paper. Since toner is now attracted to the drum, it must be pried away by applying an even larger attractive charge to the page. A transfer corona wire charges the page as shown in Fig. 26-6. The theory behind the operation of a transfer corona is exactly the same as that for a primary corona, except that the potential is now positive. This places a powerful positive charge onto paper which attracts the negatively charged toner particles. Remember that this is not a perfect process - not all toner is transferred to paper. This is why a cleaning process is needed.

Caution is needed here. Since the negatively charged drum and positively charged paper tend to attract each other, it is possible that paper could wrap around the drum. Even though the small-diameter drum and natural stiffness of paper tend to prevent wrapping, a static charge eliminator (or "static eliminator comb") is included to counteract positive charges and remove the attractive force between paper and drum immediately after toner is transferred. Paper now has NO net charge. The drum may be cleaned and prepared for a new image.

NOTE: Later CX-type EP engines replace the transfer corona with a transfer roller. The transfer roller allows transfer to paper at lower voltages than corona wires.

Once the toner image has reached paper, it is only held to the page by gravity and weak electrostatic attraction - toner is still in its powder form. Toner must be fixed permanently (or fused) to the page before it can be handled. Fusing is accomplished with a heat and pressure assembly like the one shown in Fig. 26-7. A high-intensity quartz lamp heats a non-stick roller to about 180 deg C. Pressure is applied with a pliable rubber roller. When a developed page is passed between these two rollers, heat from the top roller melts the toner, and pressure from the bottom roller squeezes molten toner into the paper fibers where it cools and adheres permanently. The finished page is then fed to an output tray. Note that both rollers are referred to as "fusing rollers", even though only the heated top roller actually fuses. To prevent toner particles from sticking to a fusing roller, it is coated with a non-stick material such as Teflon. A cleaning pad is added to wipe away any toner which may yet adhere. The pad also applies a thin coating of silicon oil to prevent further sticking.

Fusing temperature must be carefully controlled. Often a thermistor is used to regulate current through the quartz lamp in order to maintain a constant temperature. A snap-action thermal switch is also included as a safety interlock in the event that lamp temperature should rise out of control. If temperature is not controlled carefully, a failure could result in printer damage, or even a fire hazard.

After charging, the photosensitive drum contains a uniform electrostatic charge across its surface. In order to form a latent image, the drum must be discharged at any points that comprise the image. Light is used to discharge the drum as needed. Such a "writing mechanism" is illustrated in Fig. 26-8. Images are scanned onto the drum one horizontal line at a time. A single pass across the drum is called a trace or scan line. Light is directed to any points along the scan line where dots are required. When a scan line is completed, the drum increments in preparation for another scan line. It is up to the printer's control circuits to break down an image into individual scan lines, then direct the writing mechanism accordingly.

Lasers have been around since the early 1960s, and since have developed to the point where they can be manufactured in a great variety of shapes, sizes, and power output. To understand why lasers make such a useful writing mechanism, you must understand the difference between laser light and ordinary "white" light. Ordinary white light is actually NOT white. The light you see is composed of many different wavelengths, each traveling in their own directions. When these various wavelengths combine, they do so virtually at random. This makes everyday light very difficult to direct and almost impossible to control as a fine beam. As an example, take a flashlight and direct it at a far wall. You will see just how much white light can scatter and disperse over a relatively small distance.

The nature of laser light, however, is much different. A laser beam contains only ONE major wavelength of light (it is monochromatic). Each ray travels in the same direction and combines in an additive fashion (known as coherence). These characteristics make laser light easy to direct at a target as a hair-thin beam, with almost no scatter (or divergence). Older EP printers used Helium-Neon (HeNe) gas lasers, but strong semiconductor laser diodes have essentially replaced gas lasers in just about all laser printing applications.

Laser diodes appear very similar to ordinary light-emitting diodes. When the appropriate amount of voltage and current is applied to a laser diode, photons of light will be liberated that have the characteristics of laser light (coherent, monochromatic, and low divergence). A small lens window (or "laser aperture" allows light to escape, and helps to focus the beam. Laser diodes are not very efficient devices - a great deal of power is required to generate a much smaller amount of light power, but this trade-off is usually worthwhile for the small size, light weight, and high reliability of a semiconductor laser.

Generating a laser beam is only the beginning. The beam must be modulated (turned on and off) while being swept across the drum's surface. Beam modulation can be accomplished by turning the laser on and off as needed (usually done with semiconductor laser diodes) as shown in Fig. 26-9, or by interrupting a continuous beam with an electro-optical switch (typically used with gas lasers which are difficult to switch on and off rapidly). Mirrors are used to alter the direction of the laser beam, while lenses are used to focus the beam and maintain a low divergence at all points along the beam path. Fig. 26-9 is just one illustration of a laser writing mechanism, but it shows some of the complexity that is involved. The weight of glass lenses, mirrors, and their shock mountings have kept EP laser printers bulky and expensive.

Alignment has always been an unavoidable problem in complex optical systems such as Fig. 26-9. Consider what might happen to the beam if any optical component should become damaged or fall out of alignment - focus and direction problems could render a drum image unintelligible. Realignment of optical systems is virtually impossible without special alignment tools, and is beyond the scope of this book. Finally, printing speed is limited by the speed of moving parts, and the rate at which the laser beam can be modulated and moved.

Fortunately, a laser printer's photosensitive drum is receptive to light from many different sources. Even light from light-emitting diodes (LEDs) can expose the drum. By fabricating a series of microscopic LEDs into a single scan line, an LED can be provided for every possible dot in a scan line. For example, the ROHM JE3008SS02 is an LED print bar containing 2560 microscopic LEDs over 8.53 inches. This equates to 300 dots per inch. Each LED is just 50 x 65 micrometers (m m) and they are spaced 84.6m m apart. The operation of an LED print bar is remarkably straightforward. An entire series of data bits corresponding to each possible dot in a horizontal line is shifted into internal digital circuitry within the print bar. Dots that will be visible are represented by a logic "1", and dots that are not visible will remain at logic "0". For a device such as the JE3008SS02, 2560 bits must be entered for each scan line.

You can probably see the advantages of an LED print bar system over a laser approach. There are no moving parts involved in light delivery - no mirror motor to jam or wear out. The printer can operate at much higher speeds since it does not have to overcome the dynamic limitations of moving parts. There is only one focusing lens between the print bar and drum. This greatly simplifies the optics assembly, and removes substantial weight and bulk from the printer. An LED system overcomes almost all alignment problems, so a defective assembly can be replaced or aligned quickly and easily.

Electrostatic printers mandate the use of extremely tight manufacturing tolerances to ensure precise, consistent operation. A defect of even a few thousandths of an inch could cause unacceptable image formation. Even the effects of normal mechanical wear can have an adverse effect on print quality. Many key IFS components would have to be replaced every 5000 to 10000 pages to maintain acceptable performance. Clearly it would be undesirable to send your printer away for a complete (and time-consuming) overhaul every 10000 pages.

In order to ease manufacturing difficulties and provide fast, affordable maintenance to every EP printer user, critical components of the IFS, as well as a supply of toner, are assembled into a replaceable electrophotographic (or EP) cartridge. As Fig. 26-10 shows, a typical EP cartridge contains the toner roller, toner supply, debris cavity, primary corona (and primary grid), photosensitive drum, and cleaning blade assembly. All necessary electrical connectors and drive gears are included. By assembling sensitive components into a single replaceable cartridge, printer reliability is substantially improved by preventing problems before they ever become noticeable. The cost of an EP cartridge is low enough to consider it a disposable (or recyclable) part.

A typical EP cartridge is capable of producing 200 to 5000 printed pages. The exact number varies depending upon just how much toner is available, and which critical parts are placed in the cartridge - highly-integrated EP cartridges will last longer than simple toner cartridges. Since toner is comprised partially of organic materials, it has a limited useful life (often 6 months after the cartridge is removed from its sealed container). Later "high-volume" engines can support 7000-10000 pages.

As you might imagine, the precision components in an EP cartridge are sensitive and delicate. The photosensitive drum and toner supply are particularly sensitive to light and environmental conditions, so it is important to follow several handling and storage guidelines:

• Keep light away from the drum - the photosensitive drum is coated with an organic material that is EXTREMELY sensitive to light. Although a metal shroud covers the drum when the cartridge is exposed, light may still penetrate the shroud and cause exposure (also known as fogging). Deactivating the printer for a time will often eliminate mild fogging. Do NOT defeat the shroud in open light unless ABSOLUTELY NECESSARY, and then ONLY for SHORT periods. This will certainly fog the drum. A seriously fogged cartridge may have to be placed in a dark area for several days. NEVER expose the drum to direct sunlight - direct sunlight can permanently damage the drum's coating.
• Avoid extremes of temperature and humidity - temperatures exceeding 40 deg C can permanently damage an EP cartridge. Extreme humidity is just about as dangerous. Do not allow the cartridge to become exposed to ammonia vapors or other organic solvent vapors - they break down the drum's coating VERY quickly. Finally, keep a cartridge secure and level. Never allow it to be dropped or abused in any way.
• Redistribute toner regularly - as the toner supply diminishes, it may be necessary to redistribute remaining toner so that it reaches the toner roller. Since toner is available along the entire cartridge, it must be redistributed by rocking the cartridge back and forth along its long axis. If you tip a cartridge upright, remaining toner will fall to one end and cause uneven distribution.

Now that you have an understanding of EP technology, the following sections of the chapter will present a series of printer problems and solutions. Given the proliferation of printer manufacturers - each requiring drivers and support, Table 26-1 lists an index of printer manufacturers for your reference. Before you begin troubleshooting, however, two important cautions are in order:

NOTE: SHOCK WARNING - Be sure to unplug the printer and allow ample time for the power supply (or supplies) to discharge before attempting to open the enclosure. High-voltage supplies are especially dangerous, and can result in a nasty shock if not allowed to discharge.

NOTE: BURN WARNING - Fusing assemblies in EP printers also reach over 200° F during normal operation. Even when opening the EP printer for routine maintenance, allow ample time for the fuser to cool (at least 10 minutes) before reaching inside.

You’ll also get a great deal of troubleshooting information from the diagnostics built into your EP printer. Many messages and codes displayed to the printer’s LCD panel are simply status indicators or reminders, but some printers (such as Hewlett-Packard’s line of LaserJet printers) use the LCD panel extensively for diagnostics. Table 26-2 presents the operating and error codes for the HP LaserJet family.

Most EP printers use an ECU consisting of two parts; a main board and a mechanical controller. The main board provides the core logic for the printer - CPU, memory, an interface for the control panel, the communication circuits, and other processing elements. The mechanical controller provides an interface between the pure logic and the electromechanical components of the printer. For example, a mechanical controller holds the driver circuitry controlling the printer’s motors and solenoids. Some printers integrate these functions onto a single PC board, while other printer designs employ two separate boards. While controller circuitry is generally quite reliable, it does fail from time to time, so it is important that you recognize the signs of trouble.

Symptom 26-1. The printer’s LCD shows a "CPU Error". Some printer designs may show this error as a series of blinking LEDs, or a sequence of beeps. The CPU is the heart of your printer’s logical operation. When you first start the printer the CPU and its associated core logic is tested - much like the BIOS of a computer will execute a self test (you can see the CPU in Fig. 26-11). If the CPU fails to pass all of its test requirements, an error will be generated. Unlike the CISC-type (Complex Instruction Set Computing) CPUs used in PC motherboards, the CPUs employed in EP printers are typically RISC-based (Reduced Instruction Set Computing) for added printer performance.

As you might imagine, a CPU failure is catastrophic - that is, the printer simply will not work without it. Start by turning off and unplugging the printer, then examine each of the connectors on the main controller. Each connector should be installed properly and completely. If problems persist, you will have to replace the CPU. Replacing the CPU can be either cheap or expensive depending on how it is mounted. If the CPU is socket-mounted on the main controller, you can often just remove the old CPU and plug in a new one. However, if the CPU is soldered to the main controller board, you will have to desolder and resolder the CPU (if you have the proper surface-mount soldering tools), or replace the entire main controller board.

Symptom 26-2. The printer’s LCD shows a "ROM Checksum Error". Your particular printer may use an error number (i.e. ERROR 11) to represent the condition. As in a computer, all of the printer’s on-board instructions and programming are held in a ROM on the main controller board. It is the ROM which provides the internal instructions and data (the "firmware") needed by the CPU for processing. When the printer starts, a checksum test is run on the ROM to verify the integrity of its contents. If the resulting checksum does not match the checksum reference number stored in the ROM, an error is generated. You can see the "program/font ROM" in Fig. 26-11.

First, check to see that any supplemental font or option cartridges are installed properly - you may try removing the cartridge(s) to find if the problem disappears. If there are no option cartridges, check to see that the ROM IC is installed securely. If the problem persists, you must replace the ROM IC. In many cases, ROM ICs are socket-mounted devices since they must be programmed outside of the logic board’s assembly process. When this is the case, you may be able to replace the ROM IC directly. If the ROM IC is soldered to the main logic board (or a replacement ROM IC is simply not available), you will have to replace the entire main logic board.

Symptom 26-3. The printer’s LCD shows a "RAM R/W Error", a "Memory Error", or other memory defect. Your particular printer may use an error number (i.e. ERROR 12 or ERROR 30) to represent the condition. Dynamic RAM (or DRAM) serves as the workspace for an EP printer. Where moving-carriage printers typically offer buffers of 8KB or 16KB, the EP printer can easily offer 4MB or more - some high-end printers can accommodate 48MB or more. This volume of memory is necessary because the EP printer must be able to construct the data needed to form an entire page at a time. For an 8.5"x11" page at high resolutions, this can be a phenomenal volume of data. Unfortunately, trouble in any part of the DRAM can adversely effect the image - especially PostScript images. Memory is tested when the printer is first initialized. Like PCs, the more memory that is installed, the longer it takes the printer to initialize. A typical test involves writing a known byte to each address, then reading those bytes back. If the read byte matches the written byte, the address is considered good - otherwise, a RAM error is reported. You can see the default 1MB of DRAM in Fig. 26-11.

It is rare that a RAM error message will indicate the specific location of the error, but you can easily isolate the fault to a bad memory module or the standard (resident) memory. Turn off and unplug the printer, then remove any expansion memory modules that may be installed. You may have to set jumpers or DIP switches to tell the printer that memory has been removed. If the problem disappears, one or more of your expansion memory modules has failed. Try re-installing one module at a time until the problem reoccurs - the last module to be installed when the error surfaced is the faulty module. If the problem persists when memory modules are removed, you can be confident that the fault is in your resident memory. Although memory modules often take the form of SIMMs or other plug-in modules, resident RAM is typically hard-soldered to the main controller board. You may attempt to replace the RAM if you have the proper desoldering tools and replacement RAM ICs on hand. Otherwise, simply replace the main controller board.

Symptom 26-4. Your printer’s LCD shows a "Memory Overflow" error. Your particular printer may use an error code (i.e. ERROR 20) to represent the condition. When data is sent from the computer to the printer, part of that data consists of "user information" such as soft-fonts and macro commands. If the amount of "user information" exceeds the amount of RAM set aside for it, a "Memory Overflow" (or similar error) will be generated. While this error is not directly related to the image size or complexity, complex images typically carry a larger overhead of "user information" - so you may find that "simplifying" the image can sometimes clear the problem even though the image itself is not really at fault. Generally speaking, you can eliminate this error by adding optional memory, or reducing the amount of data that must be downloaded to the printer (such as reducing the image’s resolution or size - these are often functions of the application doing the printing).

Symptom 26-5. Your printer’s LCD shows a "Print Overrun" error. Your particular printer may use an error code (i.e. ERROR 21) to represent the condition. Unlike the last error, "print overrun" problems almost always indicate that the page to be printed is too complicated for the printer - there is just not enough memory to hold all of the data required to form the image. To overcome this type of problem, try simplifying the image (i.e. use fewer fonts or try using solid shading instead of dithering). You may also try making the printed area smaller. For example, instead of printing an image at 8"x8", try printing it at 5"x5". The smaller image requires less raw data. The ideal way to correct this problem over the long term is to add memory to the printer.

Symptom 26-6. The printer reports an "I/O Protocol Error". Your particular printer may use an error code (i.e. ERROR 22) to represent the condition. This is a communication fault. The term protocol basically means "agreement" or "rules". So when a protocol error arises, it suggests that the computer and printer are not communicating "by the rules". The most blatant protocol error is connecting a serial port to a parallel printer, or a parallel port to a serial printer - but this is an extremely rare oversight, and one that usually only occurs when a printer is first installed. Protocol errors among parallel ports are also very rare since parallel port operation is very well defined with handshaking designed right into the signal layout. The most likely protocol problems can arise with serial communication - there are so many variables in the serial process which must be matched between the printer and computer, that even the slightest error can cause problems.

Start by checking the connections between the computer and printer. See that the communication link is parallel-to-parallel or serial-to-serial. Also try a new, high-quality cable (serial or parallel as appropriate) between the printer and computer. When parallel communication is being used, a protocol error suggests a failure in the communication interface IC (IC10 in Fig. 26-11). You can try replacing the communication IC, or replace the main controller board entirely. When serial communication is being used, you should examine any DIP switches or jumpers inside the printer. Check to see that the communication speed and framing bits are all set as expected, then see that the corresponding COM port in the PC is configured similarly (through the printing application or the Windows Printer Control Panel). If problems persist - even when the serial communication link is set properly - suspect trouble in the communication interface IC. You can try replacing the communication IC, or replace the main controller outright.

Symptom 26-7. The image is composed of "garbage" and disassociated symbols. Your printer may also generate a "Parity/Framing Error", or use an error code (i.e. ERROR 40) to represent the condition. This error indicates that there is a problem with serial data framing. Serial data must be "framed" with the proper number of start, data, parity, and stop bits. These bits must be set the same way at the printer and the computer’s COM port. If either end of the communication link is set improperly, data passed from the computer to the printer will be mis-interpreted (resulting in highly distorted printout). Check the printer first and note any DIP switch or jumper settings that effect the data frame. Next, check the COM port settings at the computer (under the printing application or the Windows Printer Control Panel). The COM port’s start, data, parity, and stop bit configuration should all match the printer’s settings. If not, adjust either the COM port parameters or the printer DIP switch settings so that both ends of the communication link are set the same way. If problems persist, there may be a fault in the printer’s communication IC (i.e. IC10 in Fig. 26-11). You may try replacing the IC, or you may replace the main controller entirely.

Symptom 26-8. The image appears "stitched". Stitching is an image distortion where points in the image appear to have been "pulled" in the horizontal direction (typically to the right). Figure 26-12 illustrates the "stitching" effect, along with some manifestations of other controller errors. Images are formed by scanning a laser beam repetitively across the drum. Pixels are formed by turning the laser beam on and off while scanning - a function performed by the mechanical controller board as shown in Fig. 26-13. If there is an intermittent fault in the mechanical controller logic, beam modulation may fail during one or more scanning passes - resulting in random "pulls" in the image.

Start your examination by checking all of the cables between the laser/scanner assembly and the mechanical controller. Loose wiring may result in intermittent laser fire. Turn off and unplug the printer, then try removing and reinstalling each of the connectors. Check any other wiring on the mechanical controller as well. If problems persist, chances are very good that the mechanical controller has failed. You may attempt to troubleshoot the mechanical controller, but it is often more efficient to simply replace the mechanical controller board outright. If a new mechanical controller fails to resolve the problem, replace the laser/scanner assembly.

Symptom 26-9. The image appears elongated and "stitched". This is a variation of Symptom 26-8. Not only is the laser beam misfiring intermittently, but the image is being "stretched" along the page. Under most circumstances, there has been a logic failure on the mechanical controller. Before you attempt work on the controller, however, try reseating each of the connectors on the mechanical controller. Be sure to turn off and unplug the printer before fiddling with any connectors. If problems persist, there is a serious fault in the printer’s ECU. You may attempt to troubleshoot the ECU, but this type of fault can be very difficult to track down. As a result, it is often better to try replacing the mechanical controller board first. If that fails to correct the problem, try a new main logic board. If your particular printer design integrates all of the logic and controlling circuitry on a single ECU board, replace that board outright.

Symptom 26-10. Portions of the image are disassociated like a "jigsaw puzzle". Of all the controller failures, this is perhaps one of the most perplexing. You may note that some elements of the printed image are just fine, but other (larger) areas of print seem jumbled around. To make matters worse, the problem is often intermittent, so some printed pages may appear just fine. Under most circumstances, there has been a logic failure on the main controller board. Before you attempt work on the main controller, however, try reseating each of the connectors on the main and mechanical controller boards. Be sure to turn off and unplug the printer before working with any connectors. If problems persist, there is a serious fault in the printer’s ECU. You may attempt to troubleshoot the ECU, but this type of "jigsaw puzzle" operation can be very difficult to track down. As a result, it is often better to try replacing the main controller board first. If that fails to correct the problem, try a new mechanical controller board. If your particular printer design integrates all of the logic and controlling circuitry on a single ECU board, replace that board outright.

Symptom 26-11. The image appears to be shifted down very significantly. You can see this type of problem illustrated in Fig. 26-12. At first glance, you might be tempted to think that this is a registration problem (and cannot be ruled out), but it is also possible that fault on the mechanical controller (probably the "feed control" circuit as in Fig. 26-13) is passing the page through far too soon before the developed image is aligned. Chances are that the pickup and registration mechanics are working correctly - otherwise, the page would likely loose its top margin or appear smudged. When the top margin is excessive, suspect a logic fault. Specifically, you should suspect that a logic error is firing the registration system too soon after a printing cycle starts. You should address this type of problem by troubleshooting or replacing the mechanical controller board.

Symptom 26-12. The image appears "rasterized" with no intelligible information. A "rasterized" image is a complete distortion - there is rarely any discernible information in the printed page. Instead, the image is composed of broken horizontal lines such as in Fig. 26-12. The trick with this type of fault is that it is not always easy to determine the problem origin. Turn off and unplug the printer. Open the printer and check each cable and wiring harness at the controller board(s) and laser/scanning assembly. Try reseating each of the connectors. If the problem persists, the fault is almost certainly in the main controller board. You may be able to troubleshoot the ECU, but this type of logical troubleshooting can be extremely challenging and time consuming. So it is often easier to just replace the main controller board and re-test the printer. If a new controller board fails to correct the problem, you should troubleshoot or replace the mechanical controller board.

Symptom 26-13. The image is blacked out with white horizontal lines. This type of problem creates a page which is blacked out except for a series of white horizontal bars, and will typically eradicate any discernible image on the page. As it turns out, connector problems can readily cause this type of problem, so start your examination there. Turn off and unplug the printer, then check the wiring harnesses and reseat each connector on the main and mechanical controller boards. Be extremely careful to replace each connector carefully, and avoid bending any of the connector pins.

If the problem continues, your fault is likely to be in the main controller board. You may be able to troubleshoot the ECU, but this type of logical troubleshooting can be extremely challenging and time consuming - especially under these symptoms. So it is often easier to just replace the main controller board and re-test the printer. If a new controller board fails to correct the problem, you should troubleshoot or replace the mechanical controller board.

Symptom 26-14. The image is incorrectly sized along the vertical axis. Ideally, an image should be sized according the size of whatever paper tray is installed. When the image size is significantly smaller than expected, you should first check to see that the proper paper tray is installed, and that the printing application is set to use the correct paper size (especially under Windows). If everything is configured properly, you should examine the paper tray sensors as described in Chapter 9. Replace any defective tray sensor microswitches. If problems persist, you should also inspect any wiring harnesses and connectors at the mechanical controller. Loose or defective wiring can cause erroneous page sizing. If the connectors check properly, you should suspect a logical problem in the mechanical controller board. You may attempt to troubleshoot the mechanical controller if you wish, or you may simply choose to replace the mechanical controller outright. If that should fail to resolve the problem, try a new main controller board.

The "registration" process involves picking up a sheet of paper and positioning it for use. As a result, any problems in the paper tray, pickup roller, separation pad, registration rollers, or the related drive train can result in any one of the following problems. Registration problems are really quite common - especially in older printers - where age and wear can effect the rollers, gears, and critical mechanical spacing. In very mild cases, you may be able to correct a registration problem with careful cleaning and a bit of re-adjustment. For most situations, however, you will need to replace a defective mechanical assembly, or a failing electromechanical device (such as a clutch).

Symptom 26-15. The print contains lines of print - usually in the lower half of the page - that appear smudged. You can see a simple example of the problem in Fig. 26-14. This symptom is almost always the result of a problem with your registration rollers. Uneven wear can allow the registration rollers to grip the page firmly at one point, then loosely at another point. When the grip tightens up, the page jerks forward just a fraction - but enough to smudge (or "blur") the print at that point. Turn off and unplug the printer, then expose the registration assembly and examine it closely. Look for any accumulations of debris or obstructions that may force the registration rollers apart at different points. Remove any obstructions and try cleaning the roller pair. Examine the registration drive train and look for any gears which may be damaged or obstructed. Clean the drive train and replace any damaged gears. If the problem persists, you should consider replacing the registration assembly.

Symptom 26-16. There is no apparent top margin. The image may run off the top of the page as in Fig. 26-14. In just about every case. There is a fault in the pickup assembly. As a consequence, the page is not being passed to the registration assembly in time to be aligned with the leading edge of the drum image - so the image appears cut off at the page top. Start by examining your paper tray. Make sure that the "lift mechanism" is not jammed or otherwise interfering with paper leaving the tray. If you’re not sure, try a different paper tray. Also consider your paper itself. Unusually light or specially-coated papers may simply not be picked up properly. Try a standard 20lb. bond xerography-grade paper. If problems persist at this point, chances are that your pickup assembly is failing. Turn off and unplug the printer, then examine your pickup system closely. Check for any accumulations of debris or obstructions that may interfere with the pickup sequence. Remove any obstructions, then clean the pickup roller and separation pad. Also examine the pickup drive train and clutch. Any jammed or damaged gears should be replaced. If the solenoid clutch is sticking or failing, you should replace the solenoid clutch or clutch PC board. If that fails to resolve the problem, you will have little alternative but to replace the pickup assembly and separation pad.

Symptom 26-17. There is pronounced smudging at the top of the image (generally near the top margin). This symptom suggests that the registration assembly is failing - or has not been installed correctly. Turn off and unplug the printer, then expose the registration system. Carefully inspect the system to see that the rollers and drive drain are installed properly. Try re-installing the registration assembly. If problems persist, try a new registration assembly.

Symptom 26-18. There is too much margin space on top of the image. When there is excessive margin space at the top of the image, it generally indicates that the paper has been allowed into the IFS too soon - paper is traveling through the printer before the drum image was ready. This fault is usually related to the registration roller clutch. You see, the registration rollers are supposed to hold the page until the drum image is aligned properly. This means that the registrations rollers must be engaged or disengaged as required - typically through a clutch mechanism. If the clutch is jammed in the engaged position, the registration rollers will always run (passing each new page through immediately). Turn off and unplug the printer, then examine your registration clutch closely. If the clutch is jammed, try to free it and clean surrounding mechanics to remove any accumulations of debris. If the clutch fails to re-engage or remains jammed again, replace the registration clutch entirely, or replace the clutch solenoid PC board.

Symptom 26-19. The image is "skewed" (not square with the page). Skew occurs when the page is passed through the printer at an angle (rather than straight). Typically, paper must enter the printer straight because of the paper tray, so the page must shift due to a mechanical problem. In actual practice, however, a loose or bent paper guide tab can often shift the paper as it enters the printer. Start your examination by checking the paper tray - specifically the paper cassette guide tab. If the tab is loose or bent, replace it or try a new paper tray. If the paper tray is in tact, consider the paper itself. Unusually light or specially coated papers can skew in the pickup and registration mechanics. If you are using an unusual paper, try some standard 20lb. xerography-grade paper. If the problem should continue, turn off and unplug the printer, then examine the pickup and registration mechanics. Check for obstructions, or any accumulations of foreign matter that might interfere with the paper path and cause the page to skew. If there is nothing conclusive, try replacing the pickup assembly and separation pad, then the registration assembly (in that order).

A laser beam must be modulated (turned on and off corresponding to the presence or absence of a dot) and scanned across the conditioned drum. Both modulation and scanning must take place at a fairly high rate in order to form an image - up to 24 pages per minute and more in some high-end models. However, the process of writing with a laser beam is not so simple a task as you see in Fig. 26-15. Variations in laser output power (often due to age), variations in polygon motor speed (also due to age and wear), and the accumulation of dust and debris on the polygon mirror and other optical components will all have an adverse impact on the final image. Faults can even creep into the laser sensor and effect beam detection and alignment. EP printer designers responded to the problems associated with such a delicate assembly by placing all of the laser, control, and scanning components into a single "laser/scanner" assembly. Today, the laser/scanner is an easily replaceable module - and that is how you should treat it.

Symptom 26-20. Right-hand text appears missing or distorted. Figure 26-16 illustrates a typical example. In many cases, this is simply a manifestation of low toner in your EP/toner cartridge. If any area of the development roller receives insufficient toner, it will result in very light or missing image areas. Turn off and unplug the printer, remove the EP/toner cartridge, and re-distribute the toner. Follow your manufacturer's recommendations for toner redistribution. If you see an improvement in image quality (at least temporarily), replace the EP/toner cartridge.

Examine the shock mountings that support your laser/scanner assembly. If the laser/scanner assembly is loose or not mounted correctly, scan lines may not be delivered to the proper drum locations. Try re-mounting the laser/scanner assembly. If the problem persists, replace the writing mechanism entirely. If you are using a laser writing mechanism, pay special attention to the installation and alignment of the laser beam sensor.

Symptom 26-21. You encounter horizontal black lines spaced randomly through the print. Remember that black areas are the result of light striking the drum. If your printer uses a laser/scanner assembly, a defective or improperly seated beam detector could send false scan timing signals to the main logic. The laser would then make its scan line while main logic waits to send its data. At the beginning of each scan cycle, the laser beam strikes a detector. The detector carries laser light through an optical fiber to a circuit which converts light into an electronic logic signal that is compatible with the mechanical controller’s logic. Circuitry interprets this "Beam Detect Signal" and knows the polygon mirror is properly aligned to begin a new scan. The mechanical controller then modulates the laser beam on and off corresponding to the presence or absence of dots in the scan line.

Positioning and alignment are critical here. If the beam detector is misaligned or loose, the printer's motor vibrations may cause the detector to occasionally miss the beam. Printer circuitry responds to this by activating the laser full-duty in an effort to synchronize itself again. Re-seat the laser/scanner assembly, or try re-seating the beam detector and optical fiber. If the problem persists, try replacing the beam detector and cable, or replace the laser/scanner unit outright.

Symptom 26-22. Your printer’s LCD reports a "polygon motor synchronization error". The printer may also display an error code (i.e. ERROR 31) to represent the condition. The polygon mirror is the heart of the laser scanning system. The motor’s speed must be absolutely steady. If the motor fails to rotate, or fails to synchronize at a constant rate within a few seconds of power-up, scanning will fail. In early EP printers, the polygon motor and mirror were implemented as discrete devices. In today’s EP printers, however, the laser, scanner motor, and polygon mirror are all integrated into a replaceable laser/scanner assembly. When a scanner error is reported, you should first shut down the entire printer, let it rest for several minutes, then turn it back on to see if the error clears. If the fault persists, your best course is simply to replace the laser/scanner assembly entirely.

Symptom 26-23. There are one or more vertical white bands in the image. At first glance, this symptom may appear to be a problem with the transfer corona. However, you will notice that the white band(s) appearing here are thick and well-defined (and cleaning the transfer corona will have no effect). A hard white band such as this suggests that the laser beam (or LED light) is being blocked. This is not as uncommon as you might imagine. Dust, foreign matter, and debris can accumulate on the focusing lens and obstruct the light path. It is also possible that there is a chip or scratch in the lens.

Turn off and unplug the printer. Start your examination by checking and cleaning the transfer corona - the trouble is probably not here, but perform a quick check just to eliminate that possibility. If the transfer corona should prove dirty, certainly retest the printer. If the problem persists, expose the "beam-to-drum" mirror and focusing lens, and examine both closely. Look for dust, dirt, toner, paper fragments, or any other foreign matter that may have accumulated on the optics. If you find foreign matter, you should not just blow it out with compressed air - it will make a mess, and the dust will eventually resettle somewhere else. Take the nozzle of a vacuum cleaner and hold it in proximity of the optical area, then blow the optics clean with a canister of compressed air. This way, the foreign matter loosened by the compressed air will be vacuumed away rather than re-settle in the printer. The key idea to remember here is; DO NOT TOUCH THE OPTICS.

For stains or stubborn debris, clean the afflicted optics GENTLY with a high-quality lens cleaner fluid and wipes from any photography store. Be very careful not to dislodge the "beam-to-drum" mirror or lens from its mounting. NEVER blow on a lens or mirror yourself - breath vapor and particles can condense and dry on a lens to cause even more problems in the future. Allow any cleaner residue to dry completely before re-assembling and re-testing the printer.

If the problem should persist, suspect a problem with the laser/scanner assembly. There may be some foreign matter on the laser aperture which blocks the scanned beam as it leaves the scanner. Check the laser/scanner’s beam aperture and clean away any foreign matter. If the material is inside the laser/scanner assembly, it should be replaced.

Symptom 26-24. There is a white jagged band in the image. This symptom is similar in nature to the previous symptom - foreign matter is interfering with the laser beam path. The major difference is that instead of a solid white band, you see a random jagged white band. A major difference, however, is that the obstruction is random (drifting in and out of the laser path unpredictably). This suggests that you are dealing with a loose obstruction such as a paper fragment which is able to move freely. Turn off and unplug the printer, then check for obstructions around the transfer corona. While the transfer corona itself is probably not fouled, a paper fragment stuck on the monofilament line can flutter back and forth resulting in the same jagged appearance.

Next, check the optical path for any loose material which can obstruct the laser beam. Be particularly concerned with paper fragments or peeling labels. Fortunately, such obstructions are relatively easy to spot and remove. When removing an obstruction, be careful to avoid scratching or moving any of the optical components. If the problem persists, you may have an obstruction inside of the laser/scanner assembly, so it should be replaced.

Symptom 26-25. There are repetitive waves in the image. You can see a simple example of this fault in Fig. 26-16. All of the image elements are printed, but there is a regular "wave" in the image. This kind of distortion is typically referred to as scanner modulation - where scanner speed oscillates up and down just a bit during the scanning process. In virtually all cases, the fault lies in your laser/scanner assembly. Turn off and unplug the printer, then try re-seating the cables and wiring harnesses connected to the laser/scanner unit. Try the printer again. If problems persist, replace the laser/scanner assembly.

Symptom 26-26. There are worsening waves in the image. This type of problem is a variation of the "scanner modulation" fault shown in the previous symptom. In this case, however, the modulation is relatively mild on the left side of the page, and gradually increases in magnitude toward the right side. These "worsening waves" can take several forms as shown in Fig. 26-16; typical manifestations can be heavy or light. Regardless of the modulation intensity, all of these symptoms can often be traced to a connector problem at the laser/scanner assembly. Turn off and unplug the printer, then carefully re-seat each connector and wiring harness between the laser/scanner unit and the mechanical controller board. If problems persist, replace the laser/scanner assembly.

Symptom 26-27. The image appears washed out - there is little or no intelligible information in the image. Typically, you will see random dots appearing over the page, but there are not enough dots for form a coherent image. Now, you may recall that light images may suggest a problem with the high-voltage power supply or the transfer system, but in many such circumstances, some hint of an image is visible. You may also suspect the toner supply, but toner which is too low to form an image will register a "low toner" error. Still, a quick check is always advisable. Remove the EP cartridge and try redistributing the toner, then try darkening the print density wheel setting. If the image improves, check the EP/toner cartridge and suspect the HVPS. Otherwise, you should suspect a failure at the laser diode itself. Although solid-state lasers do tend to run for long periods with little real degradation in power, an aging laser diode may produce enough energy to satisfy the laser sensor, but not nearly enough to discharge the EP drum. Try replacing the laser/scanner assembly.

Symptom 26-28. The print appears "jeweled". You can see this kind or print in Fig. 26-16. This is caused when the laser beam in totally unable to synchronize with the printer - the laser sensor is failing to detect the beam. In many cases, the fiber optic cable carrying the laser signal has been detached or broken. When the optical cable is a stand-alone component, it is a relatively easy matter to replace the cable and sensor. If the cable and sensor are integrated into the laser/scanner assembly, your best course is to re-seat the cables and wiring harnesses between the laser/scanner and the mechanical controller board. If problems persist, replace the laser/scanner assembly outright.

Symptom 26-29. You see regular "smudging" in the print. When dirt, dust, and other foreign matter accumulate on the "beam-to-drum" mirror or compensating lens, they tend to block laser light at those points - resulting in vertical white bars or lines down the image. However, mild accumulations of dust or debris which may not be heavy enough to block laser light may be enough to "scatter" some of the light. This "scattered light" spreads like shrapnel resulting in unwanted exposures. Since each point of exposure becomes dark, this often manifests itself as a "dirty" or "smudged" appearance in the print. Your best course is clean the printer’s optical deck. Turn off and unplug the printer, the expose the optical area. Place the nozzle from your vacuum cleaner in the immediate area, and blow away any dust and debris with a can of photography-grade compressed air. Do not attempt to vacuum inside of the printer! Just let it remove any airborne contaminants dislodged by the compressed air.

Symptom 26-30. The print is blacked out with white horizontal lines. In order to modulate the laser beam to form dots, the data must be synchronized with the position of the laser beam. This synchronization is accomplished by the beam detector which is typically located in the contemporary laser/scanner assembly. If the detector fails to detect the laser beam, it will fire full-duty in an attempt to re-establish synchronization. When the beam fires, it will produce a black line across the page. Multiple subsequent black lines will effectively black-out the image. The white gaps occur if the beam is sensed, or if a time-out/retry period has elapsed. In most cases, the beam sensor in the laser/scanner assembly has failed or become intermittent. Turn off and unplug the printer, then try re-seating each of the cables from the laser/scanner. If the problem persists, your best course is simply to replace the laser/scanner assembly entirely.

Symptom 26-31. The image forms correctly except for random white gaps that appear horizontally across the page. This is another manifestation of trouble in the laser beam detection process. A kink in the fiber optic cable can result in intermittent losses of laser power. In older printers with a discrete fiber optic cable, it was a simple matter to replace the cable outright. Now that beam detection is accomplished in the laser/scanner assembly itself, your best course is simply to replace the unit outright.

With so much emphasis placed on the key electronic and mechanical sub-assemblies of an EP printer, it can be easy to forget that each of those mechanical assemblies are coupled together with a comprehensive drive train of motors, gears, and (sometimes) pulleys. A failure - even an intermittent one - at any point in the drive or transmission will have some serious consequences in the printed image. Figure 26-17 outlines the printer’s mechanical system.

The mechanical controller (sometimes referred to as the "DC Controller") starts the main motor. Once the main motor starts, a gear train will operate the EP drum, the transfer (or "feed") roller(s), and the fusing rollers. In some designs, the main motor will also operate a set of exit rollers which direct the page to an output tray. Of course, there must also be a provision to pickup and register each page, but those assemblies cannot run "full-duty" - instead, they must be switched on and off at the proper time. To accomplish this timing, a solenoid-driven clutch (marked "Solenoid") is added to the pickup roller and registration roller assemblies. For the system in Fig. 26-17, a separate motor (marked "Magnetic motor") is used to drive the development system.

Symptom 26-32. There are gaps and overlaps in the print. Figure 26-18 shows you an example of this symptom. The problem here is a slipping gear or failing drive motor. Unfortunately, this is not so simple a problem to spot - gear assemblies are generally quite fine, and an intermittent gear movement can easily go unnoticed. Start with a careful inspection of the gear train. Make sure that all gears are attached and meshed securely. It is not uncommon for older gear assemblies to loosen with wear. Also check that the main motor is mounted securely and meshed properly with other gears. Be especially careful to check for obstructions or foreign matter that may be lodged in the gear train. Finally, you will need to check each gear for broken teeth - a time consuming and tedious process to be sure, but preferable to dismantling the entire drive train. A high-intensity pen light will help to highlight broken gear teeth. Replace any gears that may be damaged. If the problem persists and the drive train is flawless, try replacing the main motor assembly.

Symptom 26-33. The print has a "roller-coaster" appearance. This type of roller coaster distortion is typically the result of a fault in the gear train. Start with a careful inspection of the gear train. Make sure that all gears are attached and meshed securely. It is not uncommon for older gear assemblies to loosen with wear. Also check that the main motor is mounted securely and meshed properly with other gears. Be especially careful to check for obstructions or foreign matter that may be lodged in the gear train. Finally, you will need to check each gear for broken teeth - a time consuming and tedious process to be sure, but preferable to dismantling the entire drive train. A high-intensity pen light will help to highlight broken gear teeth. Replace any gears that may be damaged.

Symptom 26-34. The image is highly compressed in the vertical axis. A highly compressed image can indicate a failing main motor - especially when the amount of "compression" varies randomly from page to page. Since the main motor is responsible for driving the entire system, a fault can interrupt the page transport. Check the main motor to see that it is mounted securely to the frame and meshed properly with other gears. Also check the connector and wiring harness at the main motor and mechanical controller board to be sure that everything is attached properly. If problems persist, try replacing the main motor. If that fails to correct the problem, replace the mechanical controller board.

High-voltage is the key to the electrophotographic process. Huge electrical charges must be established in order to condition the EP drum, develop a latent image, and transfer that image to a page. An HVPS for the classical "SX-type" engine develops -6000, +6000, and -600 volts. The newer "CX-type" engine requires far less voltage (-1000, +1000, and -400 volts). Still, high voltages impose some important demands on the power supply and its associated wiring. First, high-voltage supplies require precise component values that are rated for high-voltage operation. While ordinary circuits might easily tolerate a "close" component value, HV supplies demand direct replacements. Installing a "close" value (or a part with a loose tolerance) in an HVPS can throw the output(s) way off. The other factor to consider is the wiring. Most commercial wire is only insulated to 600 volts or so - higher voltages can jump the inexpensive commercial insulation and arc or short-circuit - even electrocute you. So HVPS wiring harnesses and connectors are specially designed to operate safely at high voltages.

As a technician, these factors present some special problems. Replacement components are expensive and often difficult to find. Installing those components can be tedious and time-consuming. And even when things are working perfectly, you can not measure the outputs directly without specialized test leads and equipment. When all of this is taken into account, it is almost always preferable to replace a suspect HVPS outright rather than attempt to troubleshoot it.

Symptom 26-35. Your printer’s LCD displays a "high-voltage error". The printer may also use an error code (i.e. ERROR 35) to represent the condition. This indicates that one or more outputs from the HVPS are low or absent. The preferred technique is to replace the HVPS outright. Before replacing an HVPS, turn off and unplug the printer, and allow at least 15 minutes for charges in the HVPS to dissipate. When replacing the HVPS, be very careful to route any wiring away from logic circuitry, and pay close attention when installing new connectors. It is also important that you bolt the new HVPS securely into place - this ensures proper grounding.

Symptom 26-36. The image is visible, but the printout is darkened. You can see this type of symptom in Fig. 26-19. In order for an image to be developed, the EP drum must be discharged. This can also happen if the primary corona fails to place a conditioning charge on the drum. At first, you might suspect that the -6000 volt source is low, but in actual practice, this type of symptom is typically the result of bad HVPS grounding. Turn off and unplug the printer, then allow at least 15 minutes for the HVPS to discharge. Open the printer and inspect the mounting bolts holding the HVPS in place. Chances are that you’ll find one or more grounding screws loose. Gently tighten each of the mounting/grounding hardware (you don’t want to strip any of the mounting holes). Secure the printer and re-test.

Symptom 26-37. There are random black splotches in the image. Generally, the image will appear, but it will contain a series of small black marks spaced randomly throughout the page. This type of image problem suggests that the HVPS is arcing internally (and is probably close to failure). Turn off and unplug the printer, then allow at least 15 minutes for charges in the HVPS to dissipate. Check the high-voltage connectors and high-voltage wiring harness. Try re-seating the connectors to check for failing contacts. If problems persist, replace the HVPS.

Symptom 26-38. There is "graping" in the image. The "graping" effect places small, dark, oval-shaped marks on the page - usually along one side of the page. Graping is often due to a short-circuit in the primary corona HV connector - HV is arcing out. Turn off and unplug the printer, then allow at least 15 minutes for any charges in the HVPS to dissipate. Inspect the primary corona wiring, and check for any shorts along the corona, or along the HV lead from the HVPS. Try re-seating the HV connectors and wiring. If problems persist, replace the primary corona HV lead (if possible) - otherwise, replace the HVPS.

Symptom 26-39. The image appears, but it contains heavy black bands. You can see an example of this in Fig. 26-19. Although this symptom may look quite different from Symptom 2, it is really quite similar. If the HVPS ground is loose, the image can be darkened, but if the HVPS ground is simply intermittent, portions of the image may be exposed just fine. As the grounding cuts out, however, primary voltage fails, and the lack of conditioning voltage causes a black band to form. When the ground kicks in again, the image formation resumes - and so on. Turn off and unplug the printer, and allow at least 15 minutes for the HVPS to discharge. Gently tighten or re-seat each of the grounding screws holding the HVPS in place (careful not to strip the threaded holes). Also check the HV wiring harness to see that it is not crimped or shorted by other assemblies. If problems persist, replace the HVPS.

Symptom 26-40. The image appears fuzzy - letters and graphics appear "smudged" or "out of focus". This type of problem suggests a fault in the AC bias voltage. You see, toner is heavily attracted to the exposed drum. Then toner jumps to the drum, some toner lands in non-exposed areas near the exposed points. By using an AC developer bias, the developer voltage varies up and down. As developer voltage increases, more toner is passed to the exposed drum areas. As developer voltage decreases, toner is pulled back from the non-exposed drum areas. This action increases image contrast while cleaning up any "collateral" toner that may have landed improperly. If the AC component of your developer voltage fails, that contrast-enhancing feature will go away - resulting in fuzzy print. Since developer voltage is generated in the HVPS, try replacing the HVPS outright.

Symptom 26-41. There are weakly-developed areas in the image. The image appears, but various areas of the image are unusually light. This can be attributed to moisture in the paper. Try a supply of fresh, dry paper in the paper cassette. Also make sure that the paper does not have a specialized coating. If the problem persists, you should suspect that the HVPS is weak and nearing failure. For a symptom such as this, your best course is usually just to replace the HVPS outright. Be sure to turn off and unplug the printer, and allow at least 15 minutes for the HVPS to discharge before attempting replacement.

Symptom 26-42. The image appears washed out. This type of symptom can often be the result of several causes. Before proceeding, check the print density wheel and try increasing the density setting. If problems continue, check your paper supply. Specially or chemically-coated papers may not transfer very well. If the problem continues, you should suspect that the transfer voltage is weak or absent, or the primary grid voltage may be failing. In either case, you should replace the HVPS. Be sure to turn off and unplug the printer, and allow at least 15 minutes for the HVPS to discharge before attempting replacement.

Symptom 26-43. The page is blacked out. This symptom suggests that the primary corona voltage has failed. Without a conditioning charge, the EP drum will remain completely discharged - this will attract full toner which will result in a black page. Before attempting to replace the HVPS, check the primary corona to see that it is still intact, and check the wiring between the primary corona and the HVPS. If the primary corona is damaged, replace the EP/toner cartridge. Otherwise, replace the HVPS. Be sure to turn off and unplug the printer, and allow at least 15 minutes for the HVPS to discharge before attempting replacement.

The fusing assembly is another focal point for many printer problems. In order to fix toner to the page surface, a combination of heat and pressure is applied with a set of fusing rollers. The upper roller provides heat while the lower roller provides pressure. In order for the fusing assembly to work properly, there are several factors that must be in place. First, the heating roller must reach and maintain a constant temperature - that temperature must be consistent across the roller’s surface. Second, pressure must be constant all the way across the two rollers, so the two rollers must be aligned properly. Third, not all melted toner will stick to the page - some will adhere to the heating roller. So there must be some provision for cleaning the heating roller. Finally, there must be a reliable method for protecting the printer from overheating.

The fusing unit design shown in Fig. 26-20 addresses these concerns. Heat is generated by a bar heater or a long quartz lamp mounted inside of the upper fusing roller. Power to operate the heater is provided from the DC power supply (typically 24 volts). A separate thermistor in the roller changes resistance versus temperature, so it acts as a temperature detector. The thermistor’s resistance is measured by a circuit on the mechanical controller board which, in turn, modulates the power feeding the heater. This process "closes the loop" to achieve a stable operating temperature. If a failure should occur which allows the heater to run continuously, a thermal fuse will open and cut off voltage to the heater above a given limit. Although Fig. 26-20 does not show it, the upper and lower fusing rollers are held together with torsion springs - the springs keep both rollers together with the right amount of compression, and can adjust for slight variations in paper weight and system wear. Toner that sticks to the upper fusing roller can transfer off the roller elsewhere on the page - resulting in a "speckled" appearance, so the upper roller is coated with Teflon to reduce sticking, and a cleaning pad rubs any toner off the roller. You will find that temperature, alignment, and cleaning problems are some of the most frequent fusing troubles.

Symptom 26-44. The printer’s LCD indicates a "Heater Error" or other type of fusing temperature malfunction. Your particular printer may use an error number (i.e. ERROR 32) to represent the condition. Fusing is integral to the successful operation of any EP printer. Toner that is not fused successfully remains a powder or crust that can flake or rub off onto your hands or other pages. Mechanical controller logic interprets the temperature signal developed by the thermistor and modulates power to the quartz lamp. Three conditions will generate a fusing malfunction error; fusing roller temperature falls below about 140 deg C, fusing roller temperature climbs above 230 deg C, or fusing roller temperature does not reach 165 deg C in 90 seconds after the printer is powered up. Your particular printer may utilize slightly different temperature and timing parameters. Also note that a fusing error will often remain with a printer for 10 minutes or so after it is powered down, so be sure to allow plenty of time for the system to cool before examining the fusing system.

Begin by examining the installation of your fusing assembly. Check to see that all wiring and connectors are tight and seated properly. The quartz heater power supply is often equipped with a fuse or circuit breaker that protects the printer (this is NOT the thermal switch shown in Fig. 26-20). If this fuse or circuit breaker is open, replace your fuse or reset your circuit breaker, then retest the printer. Remember to clear the error, or allow enough time for the error to clear by itself. If the fuse or breaker trips again during retest, you have a serious short circuit in your fusing assembly or power supply. You can attempt to isolate the short circuit, or simply replace your suspected assemblies - fusing assembly first, then the DC power supply.

Turn off and unplug the printer, allow it to cool, and check your temperature sensing thermistor by measuring its resistance with a multimeter. At room temperature, the thermistor should read about 1 Kohm (depending on the particular thermistor). If the printer has been at running temperature, thermistor resistance may be much lower. If the thermistor appears open or shorted, replace it with an EXACT replacement part and retest the printer.

A thermal switch (sometimes called a "thermoprotector") is added in series with the fusing lamp. If a thermistor or main logic failure should allow temperature to climb out of control, the thermal switch will open and break the circuit once it senses temperatures over its preset threshold. This protects the printer from severe damage - and possibly a fire hazard. Unplug the printer, disconnect the thermal switch from the fusing lamp circuit, and measure its continuity with a multimeter. The switch should normally be closed. If you find an open switch, it should be replaced. Check the quartz lamp next by measuring continuity across the bulb itself. If you read an open circuit, replace the quartz lamp (or the entire fusing assembly). Be sure to secure any disconnected wires. If the printer still does not reach its desired temperature, or continuously opens the thermal switch, troubleshoot your thermistor signal conditioning circuit and the fusing lamp control signal from the mechanical controller, or replace the mechanical controller board entirely.

Symptom 26-45. Print appears smeared or fused improperly. Temperature and pressure are two key variables of the EP printing process. Toner must be melted and bonded to a page in order to fix an image permanently. If fusing temperature or roller pressure is too low during the fusing operation, toner may remain in its powder form. Resulting images can be smeared or smudged with a touch. You can run the PRINTERS fusing test to check fusing quality by running a series of continuous prints. Place the first and last printout on a firm surface and rub both surfaces with your fingertips. No smearing should occur. If your fusing level varies between pages (one page may smear while another may not), clean the thermistor temperature sensor and repeat this test. Remember to wait 10 minutes or so before working on the fusing assembly. If fusing performance does not improve, replace the thermistor and troubleshoot its signal conditioning circuit at the mechanical controller. If smearing persists, replace the fusing assembly and cleaning pad.

Static teeth just beyond your transfer corona are used to discharge the paper once toner has been attracted away from the EP drum. This helps paper to clear the drum without being attracted to it. An even charge is needed to discharge paper evenly, otherwise, some portions of the page may retain a local charge. As paper moves toward the fusing assembly, remaining charge forces may shift some toner resulting in an image that does not smear to the touch, but has a smeared or pulled appearance. Examine the static discharge comb once the printer is unplugged and discharged. If any of its teeth are bent or missing, replace the comb.

A cleaning pad rubs against the fusing roller to wipe away any accumulations of toner particles or dust. If this cleaning pad is worn out or missing, contamination on the fusing roller can be transferred to the page, resulting in smeared print. Check your cleaning pad in the fusing assembly. Worn out or missing pads should be replaced immediately.

Inspect your drive train for any gears that show signs of damage or excessive wear. Slipping gears could allow the EP drum and paper to move at different speeds. This can easily cause portions of an image to appear smudged - such areas would appear bolder or darker than other portions of the image. Replace any gears that you find to be defective. If you do not find any defective drive train components, try replacing the EP cartridge. Finally, a foreign object in the paper path can rub against a toner powder image and smudge it before fusing. Check the paper path and remove any debris or paper fragments that may be interfering with the image.

Symptom 26-46. There are narrow horizontal bands of smudged print. Figure 26-21 shows you an example of this problem. While smudging usually suggests a fusing problem, its occurrence in relatively narrow bands actually points to a problem with the paper feed - the registration or transfer rollers are not moving evenly, so they are jerking the paper. When the paper jerks, the toner immediately being transferred from the EP drum becomes smudged.

Check your paper stock first. Unusually light or specially-coated papers may slip periodically - resulting in a slight jerking motion. Try a standard 20lb. xerography-grade paper. If the paper is appropriate, there are three causes for this kind of "paper jerk" - either your rollers are worn (allowing loose contact at some point in their rotation), the rollers are obstructed (effectively jamming the paper at some point in their rotation), or there is a fault in the drive train. Unfortunately, observing the paper path while the printer is running will rarely reveal subtle mechanical defects, so turn off and unplug the printer, then inspect your registration rollers for signs of wear or accumulations of foreign matter. If the registration rollers appear damaged or worn, replace the registration assembly. If there is a buildup of foreign matter, carefully clean the registration rollers.

If the problem persists, inspect the drive train carefully. Check each gear to see that they are meshed properly, and see if there are any broken gear teeth. A small, high-intensity pen light will make this inspection easier. Replace any gears that are worn or damaged. If there are obstructions in the gear train, clean them away carefully with a cotton swab lightly dampened in isopropyl alcohol. If this still fails to correct the problem, the fault is probably in the EP engine mechanics. Try replacing the EP/toner cartridge (the "engine").

Symptom 26-47. There are wide horizontal areas of smudged print. You can see this type of symptom in Fig. 26-21. Although this problem may sound quite similar to the previous symptom, the fault is almost always in the fusing assembly. Excessive pressure from the lower fusing roller squeezes the page so tightly that the print is smudged - typically across a wide area. Turn off and unplug the printer, and allow 15 minutes or so for the fusing assembly to cool. Inspect the fusing assembly carefully. If there are torsion springs holding the upper and lower fusing rollers together, you can probably reduce the tension to relieve some of the pressure. You may have to work in small increments to get the best results. Also check the lower fusing roller itself - if the roller is worn or damaged, it should be replaced. As an alternative, you can replace the entire fusing assembly outright.

Symptom 26-48. There are dark creases in the print. These are visible creases (also referred to as "pencil lines") in the page itself - not just in the printed image. In virtually all cases, pencil lines are the result of a bloated lower fusing roller. The way in which it applies pressure on the page causes a crease in the page. First, check your paper supply. Light bond or specially coated papers may be especially susceptible to this kind of problem. Try a standard 20lb. xerography-grade paper. If the problem persists, you will need to inspect the fusing assembly. Turn off and unplug the printer, and allow 15 minutes for the fusing system to cool before opening the printer. Check the lower fusing roller for signs of bloating, excessive wear, or other damage. Try replacing the lower fusing roller. Otherwise, you should replace the entire fusing assembly.

Symptom 26-49. There is little or no fusing on one side of the image. However, the other half is fused properly. This problem occurs when there is a gap in the fusing rollers. Even if the upper fusing roller is producing the correct amount of heat, it will not fuse toner without pressure from the lower fusing roller. Gaps are often caused from physical damage to the fusing assembly, or an accumulation of foreign matter that forces the rollers apart. Turn off and unplug the printer, then wait about 15 minutes for the fusing assembly to cool. Inspect the rollers carefully. You can expect to find your problem on the side which does not fuse. For example, if the right side of the page is not fusing, the problem is likely on the right side of the fusing rollers. Check for mechanical alignment of the rollers. You may be able to restore operation by adjusting torsion spring tension. If problems continue, you should replace the entire fusing assembly.

There are two high-voltage charge areas in the EP printer; the primary area and the transfer area. Classical "SX-type" engines use corona wires, so the primary area will use a primary corona, and the transfer area will use a transfer corona. The newer "CX-type" engines replace the corona wires with charge rollers, so the primary area will use a primary charge roller, and the transfer area will use a transfer charge roller. Although these areas very rarely fail, there are a suite of problems that plague the coronas. This part of the chapter shows you some of the more pervasive faults.

Symptom 26-50. Pages are completely blacked out, and may appear blotched with an undefined border. You can see an example of this problem in Fig. 26-22. Turn off and unplug the printer, remove the EP cartridge, and examine its primary corona wire. Remember that a primary corona applies an even charge across a drum surface. This charge readily repels toner - except at those points exposed to light by the writing mechanism which discharge those points and attract toner. A failure in the primary corona will prevent charge development on the drum. As a result, the entire drum surface will tend to attract toner (even if your writing mechanism works perfectly). This creates a totally black image. If you find a broken or fouled corona wire, clean the wire or replace the EP cartridge.

If your blacked out page shows print with sharp, clearly defined borders, your writing mechanism may be running out of control. LEDs in a solid state print bar or laser beam may be shorted in an ON condition, or receiving erroneous data bits from its control circuitry (all logic 1s). In this case, the primary corona is working just fine, but a writing mechanism that is always on will effectively expose the entire drum and discharge whatever charge was applied by the primary corona. The net result of attracting toner would be the same, but whatever image is formed would probably appear crisper - more deliberate.

Use your oscilloscope to measure the data signals reaching your writing mechanism during a print cycle. You should find a semi-random square wave representing the 1s and 0s composing the image. If you find only one logic state, troubleshoot your main logic and driving circuits handling the data, or replace the mechanical controller board. If data entering the writing mechanism appears normal, replace your writing mechanism (LED bar or laser/scanner assembly). You may wish to cross-reference this symptom with an HVPS problem earlier in this chapter.

Symptom 26-51. Print is very faint. Turn off and unplug the printer, remove the EP cartridge, and try re-distributing toner in the cartridge. Your user's manual probably offers preferred instructions for re-distributing toner. Keep in mind that toner is largely organic - as such, it has only a limited shelf and useful life. If re-distribution temporarily or partially improves the image, or if the EP cartridge has been in service for more than six months, replace the EP cartridge. If you are using a paper with a moisture content, finish, or conductivity that is not acceptable, image formation may not take place properly. Try a standard 20lb xerography-grade paper.

Check your transfer corona or transfer charge roller. It is the transfer corona that applies a charge to paper which pulls toner off the drum. A weak transfer corona or charge roller may not apply enough charge to attract all the toner in a drum image. This can result in very faint images. Turn off and unplug the printer, allow ample time for the high-voltage power supply to discharge completely, then inspect all wiring and connections at the transfer corona. If the monofilament line encircling the transfer corona is damaged, replace the transfer corona assembly, or attempt to re-thread the monofilament line. If faint images persist, repair or replace the high-voltage power supply assembly.

Finally, check the drum ground contacts to be sure that they are secure. Dirty or damaged ground contacts will not readily allow exposed drum areas to discharge. As a result, very little toner will be attracted and only faint images will result. If the problem persists, replace the EP engine.

Symptom 26-52. Print is just slightly faint. Print that is only slightly faint does not necessarily suggest a serious problem. There are a series of fairly simple checks that can narrow down the problem. Check the print density control dial. Turn the dial to a lower setting to increase contrast (or whatever darker setting there is for your particular printer). Check your paper supply next. Unusual or specially coated paper may cause fused toner images to appear faint. If you are unsure about the paper currently in the printer, insert a good-quality, standard-weight xerographic paper and test the printer again.

Over time, natural dust particles in the air will be attracted to the transfer corona and accumulate there. This eventually causes a layer of debris to form on the wire. This type of accumulation cuts down on transfer corona effectiveness, which places less of a charge on paper. Less toner is pulled from the drum, so the resulting image appears fainter. Turn off and unplug the printer, allow ample time for the high-voltage power supply to discharge, then gently clean the transfer corona with a clean cotton swab or corona cleaning tool. Be very careful NOT to break the monofilament line wrapped about the transfer corona assembly. If this line does break, the transfer corona assembly will have to be re-wrapped or replaced.

Check your toner level. Unplug the printer, remove the EP cartridge, and re-distribute toner. Follow all manufacturer's recommendations when it comes to re-distributing toner. The toner supply may just be slightly low at the developing roller. Unplug your printer and examine the EP cartridge sensitivity switch settings. These microswitches are actuated by molded tabs attached to your EP cartridge. This tab configuration represents the relative sensitivity of the drum. Main logic uses this code to set the power level of its writing mechanism to ensure optimum print quality. These switches also tell main logic whether an EP cartridge is installed at all. If one of these tabs are broken, or if a switch has failed, the drum may not be receiving enough light energy to achieve proper contrast. Check your sensitivity switches as outlined for a "NO EP CARTRIDGE" error shown later in this chapter. If the problem persists, your high-voltage power supply is probably failing. Replace your high-voltage power supply.

Symptom 26-53. There are one or more vertical black streaks in the print. Black streaks may range from narrow lines to wide bands depending on the severity of the problem. In most cases, this fault is due to foreign matter accumulating on the primary corona. Foreign matter will prevent charges from forming on the drum. In turn, this will invariably attract toner which creates black streaks. Typically, the edges of these streaks are fuzzy and ill-defined. Your best course is simply to clean the primary corona - most printers enclose a cleaning tool for just this purpose. The process takes no more than a minute. If the problem should persist (very unlikely), replace the EP engine.

Symptom 26-54. There are one or more vertical white streaks in the print. Begin by checking your toner level. Toner may be distributed unevenly along the cartridge's length. Turn off and unplug the printer, remove the EP cartridge, and re-distribute the toner. Follow your manufacturer's recommendations when handling the EP cartridge. If this improves your print quality (at least temporarily), replace the nearly-exhausted EP cartridge.

Next, examine your transfer corona for areas of blockage or extreme contamination. Such faults would prevent the transfer corona from generating an even charge along its length - corrosion acts as an insulator which reduces the corona's electric field. Uncharged page areas will not attract toner from the drum, so those page areas will remain white. Clean the transfer corona VERY carefully with a clean cotton swab. If your printer comes with a corona cleaning tool, use that instead. When cleaning, be sure to avoid the monofilament line wrapped around the transfer corona assembly. If the line breaks, it will have to be re-wrapped, or the entire transfer corona assembly will have to be replaced.

Check the optical assembly for any accumulation of dust or debris that could block out sections of light. Since EP drums are only scanned as fine horizontal lines, it would take little more than a fragment of debris to block light through a focusing lens. Gently blow off any dust or debris with a can of high-quality, optical-grade compressed air available from any photography store. For stains or stubborn debris, clean the afflicted lens GENTLY with a high-quality lens cleaner and wipes from any photography store. Be very careful not to dislodge the lens from its mounting. NEVER blow on a lens or mirror yourself - breath vapor and particles can condense and dry on a lens to cause even more problems in the future.

Symptom 26-55. The print appears "scalloped". You can see an example of "scalloping" in Fig. 26-22. The scalloping effect has a unique and unmistakable appearance, and almost always indicates that the primary corona has broken. The image that forms is then expressly the result of random discharge from the erase lamps. In many cases, the failure of a primary corona will simply blacken the page. In some circumstances, however, the erase lamps will leave a latent image which be developed into the scalloped pattern. You should immediately suspect a failure in the primary corona. Your best course is simply to replace the primary corona by exchanging the EP/toner cartridge.

Symptom 26-56. The print contains columns of horizontal "tic" marks. An image appears as expected, but it is marked vertical swatches of small horizontal tics. Experience has demonstrated that this type of symptom is frequently due to a short-circuited transfer corona. Turn off and unplug the printer, then allow at least 15 minutes for the printer to cool and discharge. Inspect the transfer corona carefully, as well as any wiring at the corona. Gently clear away any foreign material - especially conductive material - from the transfer area, and try the printer again. If the problem persists, try replacing the transfer corona assembly.

Symptom 26-57. Print appears speckled. In almost all cases, speckled print is the result of a fault in your primary corona grid. A grid is essentially a fine wire mesh between the primary corona and drum surface. A constant voltage applied across the grid serves to regulate the charge applied to the drum to establish a more consistent charge distribution. Grid failure will allow much higher charge levels to be applied unevenly. A higher conditioning charge may not be discharged sufficiently by the writing mechanism - toner may not be attracted to the drum even though the writing is working as expected. This results in a very light image (almost absent except for some light speckles across the page). Since the primary grid assembly is part of the EP cartridge, replace the EP/toner cartridge and retest the printer. If speckled print persists, you should suspect a fault in the HVPS.

Symptom 26-58. There are light/white splotches in the image. When you see a symptom such as this, your first suspicion should be moisture in the paper supply - a common occurrence in humid summer months. When the paper becomes damp (even just from the air’s humidity), charges to not distribute properly across the page. As a result, paper will not charge in the damp areas, so toner is not attracted from the drum. Damp areas then remain very light or white. Paper that is unusually coated can have similar problems. In virtually all cases, a supply of fresh, dry, 20lb, xerography-grade paper should correct the problem. To correct the problem over the long term, consider adding a de-humidifier or air-conditioner in the work area to keep paper dry.

Symptom 26-59. There are light/white zones spread through the image. At first glance, you may think that this symptom is similar to the previous one. In practice, however, random white zones in the printed page are much larger and more distinct than simple light splotches - in effect, the white areas have just disappeared. This symptom is indigenous to the CX-type EP engine which uses charge rollers rather than coronas. In most cases, you will find that the transfer charge roller has failed or is missing. Even without a working transfer corona, the CX engine can transfer portions of the latent image to the page, but you can see from Fig. 26-22 that the transfer is very unstable. Check and replace the transfer charge roller - or replace the EP engine outright.

This chapter has focused on problems that plague key areas of the EP printer. However, there are some symptoms that cannot easily be associated with any particular area of the printer. As a consequence, these problems can be difficult to track down and correct. This part of the chapter will illustrate some of the printer’s miscellaneous problems.

Symptom 26-60. Your printer never leaves its warm-up mode. There is a continuous "WARMING UP" status code. EP printers must perform two important tasks during initialization. First, a self-test is performed to check the printer's logic circuits and electromechanical components. This usually takes no more than 10 seconds from the time power is first applied. Second, its fusing rollers must warm up to a working temperature. Fusing temperature is typically acceptable within 90 seconds from a cold start. At that point, the printer will establish communication with the host computer and stand by to accept data, so its "WARMING UP" code should change to an "ON-LINE" or "READY" code.

When the printer fails to go on-line, it may be the result of a faulty communication interface, or a control panel problem. Turn the printer off, disconnect its communication cable, and restore power. If the printer finally becomes ready without its communication cable, check the cable itself and its connection at the computer. You may have plugged a parallel printer into the computer's serial port, or vice versa. There may be a faulty communication interface in your host computer.

If the printer still fails to become ready, unplug the printer and check that the control panel cables or interconnecting wiring is attached properly. Check the control panel to see that it is operating correctly. Also check the control panel interface circuit (sometimes called an "interface/formatter" circuit). Repair or replace your faulty control panel or interface/formatter circuit. Depending on the complexity of your particular printer, the interface/formatter may be a separate printed circuit plugged into the main logic board, or its functions may be incorporated right into the main logic board itself.

Symptom 26-61. You find a "PAPER OUT" message. When the printer generates a "PAPER OUT" message, it means that either paper is exhausted, or the paper tray has been removed. When a paper tray is inserted, a series of metal or plastic tabs make contact with a set of microswitches as shown in Fig. 26-23. The presence or absence of tabs will form a code that is unique to that particular paper size. Microswitches are activated by the presence or tabs. Main logic interprets this paper type code, and knows automatically what kind of media (paper, envelopes, etc.) that it is working with. This allows the printer to automatically scale the image according to paper size. Table 26-3 shows a typical paper code table.

The presence of paper is detected by a mechanical sensing lever as shown in Fig. 26-24. When paper is available, a lever rests on the paper. A metal or plastic shaft links this lever to a thin plastic flag. While paper is available, this flag is clear of the paper out sensor. If the tray becomes empty, this lever falls through a slot in the tray which rotates its flag into the paper out sensor. This indicates that paper is exhausted. The paper out sensor is usually mounted on an auxiliary PC board (known as the "paper control" board), and its signal is typically interpreted by the mechanical controller board. Begin your check by removing the paper tray. Be sure that there is paper in the tray, and that any ID tabs are intact - especially if you have just recently dropped the tray. Re-insert the filled paper tray carefully and completely. If the "PAPER OUT" message continues, then there is either a problem with your paper ID microswitches, paper sensing lever, or the paper out optoisolator.

You can check the paper ID microswitches by removing the paper tray and actuating the paper sensing lever by hand (so the printer thinks that paper is available). Refer to Table 26-3 and actuate each switch in turn using the eraser of pencil. Actuate one switch at a time and observe the printer's display. The "PAPER OUT" error should go away whenever at least one microswitch is pressed. If the error remains when a switch is pressed, that switch is probably defective. Unplug the printer and use your multimeter to check continuity across the suspect switch as you actuate it. Replace any defective switch. If the switches work electrically, but the printer does not register them, troubleshoot or replace the main logic board. Inspect the paper out lever and optoisolator next.

When paper is available, the paper out lever should move its plastic flag clear of the optoisolator. When paper is empty, the lever should place its flag into the optoisolator slot. Note that this logic may be reversed depending on the particular logic of the printer. This check confirms that the paper sensing arm works properly. If you see the lever mechanism jammed or bent, repair or replace the mechanism. Check the paper out optoisolator, and replace the optoisolator if it appears defective. If the sensors appear operational, replace the mechanical controller board.

Symptom 26-62. You see a "PRINTER OPEN" message. Printers can be opened in order to perform routine cleaning and EP cartridge replacement. The cover(s) that can be opened to access your printer are usually interlocked with the writing mechanism and high-voltage power supply to prevent possible injury from laser light or high-voltages while the printer is opened. The top cover (or some other cover assembly) uses a pushrod to actuate a simple electrical switch. When the top cover is opened, the interlock switch opens, and the printer's driver voltage (+24 Vdc is shown) is cutoff from all other circuits. This effectively disables the printer's operation. When the top cover is closed again, the interlock switch is reactivated, and printer operation is restored.

Make sure that your cover(s) are all shut securely (try opening and re-closing each cover). Inspect any actuating levers or pushrods carefully. Replace any bent, broken, or missing mechanical levers. Unplug the printer and observe how each interlock is actuated (it may be necessary to disassemble other covers to observe interlock operation). Adjust the pushrods or switch positions if necessary to ensure firm contact.

Turn off and unplug the printer, then use your multimeter to measure continuity across any questionable interlock switches. It may be necessary to remove at least one wire from the switch to prevent false readings. Actuate the switch by hand to be sure that it works properly. Replace any defective interlock switch, re-attach all connectors and interconnecting wiring, and retest the printer. If the switch itself works correctly, check the signals feeding the switch. Check the DC voltage at the switch. If the voltage is low or absent, trace the voltage back to the power supply or other signal source. If signals are behaving as expected but a "PRINTER OPEN" message remains, trace the interlock signal into the mechanical controller board and troubleshoot your electronics, or replace the mechanical controller outright.

Symptom 26-63. You see a "NO EP CARTRIDGE" message. An electrophotographic engine assembly uses several tabs (known as sensitivity tabs) to register its presence, as well as inform the printer about the drum's relative sensitivity level. The ECU regulates the output power of its writing mechanism based on these tab arrangements (i.e. high-power, medium-power, low-power, or no-power - NO Cartridge). Sensitivity tabs are used to actuate microswitches located on a secondary PC board. The sequence of switch contacts forms a "sensitivity" code that is interpreted by the mechanical controller.

Begin by checking the installation of your current EP engine. Make sure that it is in place and seated properly. Check to be sure that at least one sensitivity tab is actuating a sensor switch. If there are no tabs on the EP engine, replace it with a new or correct-model EP engine having at least one tab. Retest the printer. If your "NO EP CARTRIDGE" error persists, check all sensitivity switches in the printer. Turn off and unplug the printer, then use your multimeter to measure continuity across each sensitivity switch. It may be necessary to remove at least one wire from each switch to prevent false continuity readings. Actuate each switch by hand and see that each one works properly. Replace any microswitch that appears defective or intermittent. Replace any connectors or interconnecting wiring, and retest the printer. If the sensitivity switches are working properly, troubleshoot or replace the mechanical controller board or replace it outright.

Symptom 26-64. You see a "TONER LOW" message constantly, or the error never appears. A toner sensor is located within the EP/toner cartridge itself. Functionally, the sensor is little more than an antenna receiving a signal from the high-voltage AC developer bias as shown in Fig. 26-25. When toner is plentiful, much of the electromagnetic field generated by the presence of high-voltage AC is blocked. As a result, the toner sensor only generates a small voltage. This weak signal is often conditioned in the mechanical controller by an amplifier using some type of operational amplifier circuit which compares sensed voltage to a preset reference voltage. For the sensor in Fig. 26-25, sensed voltage is normally below the reference voltage, its output is a logic 0. Main logic would interpret this signal as a satisfactory toner supply. As toner volume decreases, more high-voltage energy is picked up by the toner sensor, in turn developing a higher voltage signal. When toner is too low, sensed voltage will exceed the reference, and the comparator's output will switch to a logic 1. This is handled in main logic, and a "TONER LOW" warning is produced.

Unfortunately, there is no good way to test the toner sensor. High voltage is very dangerous to measure directly without the appropriate test probes, and the signal picked up at the receiving wire are too small to measure without a sensitive meter or oscilloscope. Turn off and unplug the printer and begin your check by shaking the toner to re-distribute the toner supply (or insert a fresh EP/toner cartridge). Refer to the user's manual for your particular printer to find the recommended procedure for re-distributing toner, then retest the printer. If the problem persists, there may be a fault in the mechanical controller board’s detection circuit. Troubleshoot the mechanical controller or replace it entirely.

Symptom 26-65. Your printer’s LCD displays a "fan motor error" or similar fault. The printer may also use an error code (i.e. ERROR 34) to represent the condition. The typical EP printer uses two fans; a high-voltage cooling fan, and an ozone venting fan. In most cases, the ozone venting fan runs off-line and is not detected by the printer. The power supply cooling fan, however, is vital for the supply’s reliability - if the cooling fan fails, the power supply will quickly overheat and fail. To prevent this from happening, the fan’s operation is often monitored. If the fan quits, an error will be produced. In most cases, the fan is simply defective and should be replaced. Check the voltage available at the fan. If fan voltage is available (but the fan does not spin), the fan is defective and should be replaced. If fan voltage is missing, you will need to work back into the printer to find where fan power was lost - check any loose wiring or connectors.

Symptom 26-66. There is ghosting in the image. The expected image prints normally, but upon inspection, you can see faint traces of previous image portions as shown in Fig. 26-26. This is a case of poor housekeeping - ordinarily, a cleaning blade should scrape away any residual toner remaining on the EP drum prior to erasing and conditioning. If the cleaning blade is worn out, or the scrap toner reservoir is full, cleaning may not take place as expected, and toner will remain on the EP drum for one or more subsequent rotations. If the residual toner comes off on another rotation, it will often appear as the "ghost" of a previous image. Unfortunately, the only way to really correct this problem is to replace the entire EP engine. Cleaning blades are hardly replaceable parts, and if the scrap toner bin is full, there is no way to recycle the toner back into the reservoir. Turn off and unplug the printer, replace the EP engine, and try the printer again.

Symptom 26-67. The print appears fogged or blurred. This may appear somewhat like smudging in previous symptoms, but where smudging was generally limited in other symptoms, it occurs throughout the page here. This is a situation where you should examine the paper transfer guide - the passage between the static discharge comb (the transfer area) and the fusing assembly. Most transfer guides are coated with Teflon or similar material to reduce static. Over time and use, the anti-static coating can wear off revealing the static-prone plastic below the coating. If the plastic of the transfer guide causes a static charge, it may be strong enough to "drag" the toner image just slightly - resulting in a blurred of fogged image. If you find wear in the transfer guide, replace the transfer guide assembly.

Symptom 26-68. Nothing happens when power is turned on. You should hear the printer respond as soon as power is turned on. You should see a power indicator on the control panel (alphanumeric displays will typically read "self-test"). You should also hear and feel the printer's cooling fan(s) in operation. If the printer remains dead, there is probably trouble with the AC power. Check the AC line cord for proper connection with the printer and wall outlet. Try the printer in a known-good AC outlet. Also check the printer's main AC fuse. When the AC and fuse check properly, there is probably a problem with the printer's power supply.

If the printer's fan(s) and power indicator operate, you can be confident that the printer is receiving power. If the control panel remains blank, there may be a problem with the DC power supply or ECU. You may troubleshoot or replace the DC power supply at your discretion. When the power supply checks properly, the trouble is likely somewhere in the ECU or control panel assembly itself. Remove power from the printer and check the control panel cable. If there are no indicators at all on the control panel, replace the control panel cable. If problems remain, try replacing the ECU. When only one or a few indicators appear on the control panel, try replacing the control panel cable. If problems remain, replace the control panel.

That’s all for Chapter 26. 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 laser/LED printer resources listed below (in addition to the printer manufacturers listed in Table 26-1):

Environmental Laser: http://www.toners.com/welcome.html

Laser Pros International: http://www.laserpros.com/

Laser Saver: http://rampages.onramp.net/~laser/

Laser Supplies: http://www.mind.net/laser/

Printer Drivers Page: http://www.primenet.com/~penguink/printers.html

The Printer Works: http://www.printerworks.com/index.html

USENET Newsgroups:

comp.periphs.printers (General Printer Newsgroup)

comp.periphs (General Peripheral Newsgroup)