Updated: Aug 21, 2019
Laser Printer Mechanical Systems
For definitions of terms in this article a glossary has been provide near the footer.
More is needed than just good electronics and a fancy case to make a first-class laser printer. A series of tightly integrated mechanical systems are needed to perform the verity of physical tasks that every laser printer must do. The most obvious mechanism is the paper transport system responsible for moving paper through the printer, but the image formation system also needs mechanical force to function.
Laser printers need a well-regulated scanner motor assembly that can direct a laser beam across a light-sensitive drum surface. In previous articles you we discuss the selection of important mechanical parts in but this article shows you how to deal with mechanical problems.
Troubleshooting Laser Printer Paper Problems
Before you jump right into paper detection and handling symptoms, you should have a solid understanding of how paper is handled in a typical laser printer. Paper in a paper Tray is loaded into the printer. A paper-detect sensor makes sure that paper is available in the tray, and the tray itself actuates a series of tray size micro switches.
Each tray size actuates a different sequence of switches that tell the logic assembly just which tray size is now installed, which allows the printer to automatically size the image according to the paper tray being used.
When a printing cycle begins, the main motor turns, which causes mechanical linkages to turn the toner drum, fusing rollers, and any feed rollers that carry the paper along. The only two mechanisms that do not turn are the paper pickup roller and the registration rollers. Their actions are regulated by solenoid-driven clutches that remain open.
When the printing cycle is ready to receive a page, the pickup roller clutch engages. The notched pickup roller grabs the top page and draws it into the printer about 7.5 centimeters (about 3 inches), which is about the circumference of the pickup roller. After one turn, the pickup-roller clutch disengages, and the page rests against the registration rollers. A rubber separation pad just below the pickup roller prevents more than one page at a time from entering the printer.
When the developed drum image is properly positioned in relation to the page, the registration roller clutch engages and starts the page into the image formation system. Once the registration rollers start, they will remain engaged until the paper has exited the printer.
Feed rollers guide the page while the latent image is transferred to it, then gently transfer the page to the fusing assembly where the toner image is fixed. As the paper emerges from the fusing rollers, a paper exit sensor is actuated. When the page has left the printer, the sensor assumes its original condition, and the printer logic assembly knows that the page has left the printer. The main motor can now stop (or a new page can enter the printer for another cycle).
You find a PAPER OUT message. When the printer generates a PAPER OUT message, either the paper tray is empty or the paper tray has been removed. When a paper tray is inserted, a series of metal or plastic tabs contact a set of microswitches. The presence or absence of tabs will form a Paper tray 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.
Table 8-1 shows a typical laser printer paper tray code. Tray type SW1 SW2 SW3 Executive 1 1 1
A4 1 1 0
Legal 0 0 1
Envelope 0 1 1
Letter 1 0 0 No tray 0 0 0 1 = on (engaged).
0 = off (disengaged).
The presence of paper is detected by a mechanical sensing level. 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, which shows that paper is exhausted, The paper-out sensor is usually mounted on an auxiliary PC board (known as the paper control board).
Begin your check by removing the paper tray. Be sure that there is paper in the tray, and that any ID (identification) tabs on the tray 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 tray 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 8-1 and actuate each switch in turn ate one switch at a time and observe the printer 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 multi meter to check continuity across the suspect switch as you actuate it. Replace any defective switch or switch assembly. 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: this logic right be reversed depending on the particular logic of the printer.
Actuate the paper-out lever by hand and see that it moves freely and completely. This check confirms that the paper sensing arm works properly. If you see the lever mechanism jammed or bent, repair or replace the mechanism. If the problem persists, replace the paper-out optoisolator (or replace the paper handling PC board).
You see a PAPER JAM message. The laser printer must detect and report a paper-jam condition. For most printers, a jam occurs when any one of the four following situations do not take place. First, a sheet of paper must reach the fusing assembly within some amount of time after the printing cycle starts (usually under 30 seconds). Paper reaching the fusing assembly actuates the paper exit sensor.
Second, paper that reaches the fusing assembly, must clear the fusing assembly within some amount of time (also about 30 seconds, but it depends on the paper size). The actuated paper sensor returns to its original state when paper passes.
Third, paper that is present in the fusing assembly must be present when fusing temperature is above some minimum temperature (about 150°C). If any one of these three conditions is false, a paper jam will be registered.
The electrostatic paper transport system is much more sophisticated than those used in more conventional serial or line printers. As a result this additional complexity, main logic circuitry must detect whether or not paper enters and exits the paper path as expected. For this discussion, assume that a paper jam can occur in four general areas:
Begin by checking paper in the paper tray. If a jam condition is shown, but there is no paper, it shows that your paper sensing lever is not functioning properly. It might be broken, bent, or jammed. When there is ample paper available, take a moment to be sure that paper is the right size, texture, and weight for your printer.
Unusual or special paper might not be picked up reliably. If you are uncertain as to the correct paper type, remove it and insert a quantity of good-quality xerographic paper.
This type of paper usually has the weight and texture characteristics ideal for laser printing. If the error continues, look at potential jam locations-paper-feed area, registration/transfer area, and exit area-as described below.
Paper-feed area The paper-feed area consists of the paper tray (and paper), pickup roller mechanical assembly, and electro mechanical clutch as shown in Fig. 8-5. If paper is not reaching your registration rollers, the trouble is probably in this area.
Paper-feed area The paper-feed area consists of the paper tray (and paper), pickup roller mechanical assembly, and electro mechanical clutch as shown in Fig. 8-5.
Inspect your paper tray carefully. Although the tray might seem foolproof, it actually plays an important role in paper feed. If the plastic tray housing is cracked or damaged, replace it with a new tray and re-test the printer. Note the movable metal plate in the tray bottom. This lift mechanism keeps paper positioned against the pickup roller at all times.
Remove the tray and paper. Make sure that this plate can move freely-replace your paper tray if it does not. Observe this lift plate as you insert it into the printer. The printer should lift this plate up as the tray is inserted. If this does not happen, repair or replace the printer lift mechanism assembly. Add some fresh paper (50 sheets or so) and gently insert the paper tray. Be sure to insert the tray fully and squarely. If there is any obstruction (or the tray does not seat squarely), find and remove the obstruction.
Next, make sure that your main motor is functioning. Keep in mind that the main motor drives all rollers in the printer, as well as the photosensitive has failed, paper will not be drawn into the printer at all. You can observe the main motor and its gear-train assembly by turning the printer on, opening an access cover, defeating the associated interlock switch (if any), defeating laser cartridge sensitivity switches, and initiating a printer self-test.
If the fusing roller temperature is above its lower temperature limit, you should see motor operation immediately. If the main motor fails to operate, replace the main motor driver circuit or logic assembly as required. If the main motor continues to malfunction, replace the main motor assembly.
The main motor turns when a print signal is first generated in main logic. It continues turning as long as there is paper in the feed path. However, even though the main motor supplies the force to operate every roller, pickup and registration rollers are operated only briefly during each print cycle. An electromagnetic clutch is used to switch the pickup roller on and off at desired times.
Main logic generates a clutch control signal that is amplified by driver circuits before being fed to the electromagnet. When deactivated, the plunger disengages the pickup roller from the drive train.
When activated, the plunger engages the pickup roller, which causes the pickup roller assembly to turn and grab the next available piece of paper. A separation pad beneath the pickup roller prevents more than one sheet from being taken at any one time. Paper stops when it reaches the idle registration rollers.
Notice how paper will bow-this is a normal and harmless function in the paper path. If the main motor operates, but the pickup roller does not turn (you can see this during an open-cover test print), inspect the pickup roller clutch solenoid. Note there are probably two major solenoids-one for the pickup roller, and one for the registration rollers.
When printing starts, one of the two solenoids (the pickup solenoid) should engage immediately. If no solenoid engages, there is an electronic problem. If the pickup solenoid engages, but your pickup roller does not turn (or does not turn properly), repair or replace the pickup mechanical assembly. There might be a faulty clutch or other mechanical defect.
When the pickup solenoid fails to actuate, use your multi meter to measure voltage across the solenoid. You should see voltage toggle switched. If voltage changes, but the solenoid does not function, replace the solenoid. If voltage remains at zero (or does not switch from some other voltage), there is probably a fault in the solenoid driver circuit.
Troubleshoot the solenoid signal back into main logic. You also might simply replace the pickup clutch PC board. Finally, you can check for feed roller wear by measuring the distance between the trailing edge of the paper and the end of the paper tray just as the sheet stops.
This time occurs between the point where the pickup roller stops, and the registration rollers start. Normally, this trailing edge should advance about 3 inches or more. on and off as the solenoid is If it does not advance this far, your pickup roller is probably worn out. Replace the pickup roller assembly and separation pad.
Registration/transfer area Registration rollers hold on to the page until its leading edge is aligned with the drum image. Force is supplied by the main motor, but another electromagnetic clutch switches the registration rollers on and off at the appropriate time. Once paper and the drum image are properly aligned, main logic sends a clutch control signal that is amplified by driver circuits to operate the registration clutch solenoid.
After the clutch is engaged, registration rollers will carry the page forward to receive the developed toner image. The registration/transfer assembly usually consists of registration rollers, the drive train, a registration clutch solenoid, a transfer guide, and the transfer corona assembly as shown in Fig. 8-6. If paper enters the printer but does not reach the fusing rollers, your fault is probably in this area.
You can see registration roller operation by opening a housing, defeating any corresponding interlocks, and defeating any laser cartridge sensitivity switches, then initiating a self-test, Use extreme caution to prevent injury from high-voltage optical radiation from the writing mechanism-especially from lasers.
Watch the paper path and drive train very carefully. If you see an obstruction in the paper path or in the drive train, unplug the printer and allow 10 minutes for the fusing assembly to cool before attempting to clear the obstruction! Replace any gears or bushings that appear damaged or worn. Pay close attention to any tension equalizing springs (called torsion springs) attached to the registration rollers. Re seat or replace any torsion springs that might be damaged or out of position.
If your main motor and drive train operate, but registration rollers do not tum (or turn properly), inspect the registration solenoid clutch. It will usually be adjacent to the pickup solenoid clutch.
The solenoid should engage moments after the pickup roller disengages. If the registration solenoid does not engage, there is an electrical problem. If the solenoid does engage, but registration rollers do not turn your mechanical clutch or registration rollers are probably worn out. Replace the mechanical registration assembly.
When your registration solenoid fails to actuate, measure the signal driving the solenoid. You should see the signal toggle on and off with the solenoid. If the signal changes, but the solenoid does not fire, replace the jammed or defective solenoid, If voltage does not change, there is probably a fault in the solenoid driver circuit or main logic. Troubleshoot the solenoid signal back into your main logic. You might replace the registration clutch PC board.
Exit area At the exit area, a page has been completely developed with a toner powder image. The page must now be compressed between a set of fusing rollers one provides heat, and the other applies pressure. Heat melts the toner powder, and roller pressure forces molten toner permanently into the paper fibers to fix the image. As a fixed page leaves the rollers, it might stick to the fusion roller.
A set of evenly spaced separation pawls pry away the finished page, which is delivered to the output tray. This step completes the paper feed process as shown in Fig. 8-7. Main motor force is delivered to the fusion rollers by a geared drive train.
There are no clutches involved in exit area operations, so the drive train moves throughout the entire printing cycle. It is important that the printer detect when paper enters and leaves the exit area. Based on paper size and fusion roller speed, a page has only a set amount of
Fig. 8-7 Simplified diagram of fuser exit or output area
time to enter and leave the exit area before a PAPER JAM is initiated. To detect the flow of paper, an optoisolator is usually actuated by a weighted plastic lever. An example lever assembly is shown in Fig. 8-7 Normally, a paper-flag lever protrudes down through a slot in the empty paper path, which leaves the optoisolator clear. Its resulting logic output indicates no paper.
When paper reaches the lever, it is pushed up to the paper level. This action, in turn, moves the flag into the optoisolator slot, causing a logic change that shows paper is present-a timer is started in main logic.
When everything works properly, paper moves through the fusion roller assembly. As paper passes, the lever falls again, returning the optoisolator to its original logic state. If the optoisolator returns to its initial value before the timer expires, it means that paper has moved through the exit area properly. If paper remains, a PAPER JAM is indicated.
A long-term timer was started at the beginning of the printing cycle. If this long term timer expires before paper reaches the paper flag lever, a PAPER JAM also is generated. As you might suspect, there are a variety of problems that can cause a jam error.
Begin by checking the paper path for any obstructions. Unplug the printer and wait 10 minutes for the assembly to cool before exposing the fusing assembly. It might be necessary to remove secondary safety guards covering the heater roller. Remove any obstructions or debris that you find to be interfering with the paper.
Make sure that your plastic separation pawls are correctly attached. Clean the pawls if they appear dirty. Inspect the paper flag lever carefully to be sure that it moves freely. Replace the flag lever assembly if it appears damaged or wor-out. Also check all interconnecting cables and wiring to see that the paper lever optoisolator is attached.
The drive gears that run your fusion rollers are often attached to a door housing In this way, fusing rollers are disengaged whenever that access door is opened. This set of gears is sometimes called the delivery coupling assembly.
If these gears are not engaging properly because of wear or damage, the fusion rollers will not operate (or operate only intermittently). Repair or replace any faulty delivery coupling components. If the mechanics of your exit area appear to be operating correctly, you should examine the operation of the paper flag optoisolator.
Replace any safety guards for the fusing assembly. Turn on the printer and use your multimeter to measure voltage across the optoisolator output. Note that you might have to defeat any open cover interlocks to ensure proper voltage in the printer. Use extreme caution when measuring, and stay well clear of the high-voltage coronas. Move the paper lever to actuate the optoisolator.
You should see the output voltage toggle on and off as the optoisolator is actuated. If output voltage does not change, replace the faulty optoisolator. If voltage changes as expected, but paper jams are still indicated, troubleshoot your sensor signal back into the main logic circuit.
The printed image appears with a smudged band and over print. This type of symptom is usually the result of a worn or damaged paper-pickup assembly. A worn pickup assembly can allow the pickup roller to rotate past its idle position such that it is pressing slightly on the page.
If this happens while the registration rollers try to transfer the page into the printer, friction can delay paper movement for a moment. This delay results in a dark band that appears rather like an overprinted smudge. The solution to this problem is to replace the entire pickup assembly You also might wish to replace the pickup roller clutch. Check the associated gear train to make sure there are no obstructions or damaged components.
Laser Printer Sensor and interlock problems
Sensors play a major role in laser printer operation. With so many variables in the outside world influencing the ultimate print quality that you see in a finished page, it is vital that the laser printer detect the physical conditions within the printer. For the purposes of this book, the primary interest is in monitoring temperature, as well as tie state of physical contacts. There are three types of sensors used for these purposes: resistive sensors, mechanical sensors, and optical sensors.
Laser Printer Resistive sensors
Electrostatic printers use a fusion-roller assembly to apply heat and pressure to fix a toner image on paper. Fusion temperature must be carefully maintained at about 180°C to achieve an optimum toner melt. To control temperature, it is necessary to detect temperature, which is the job of a thermistor.
Thermistors (thermal resistors) are resistors whose values change in proportion to their temperature. Depending on their formulation, thermistors can be constructed to increase or decrease with temperature. For the purpose of this book, a thermistor is assumed to increase its value with temperature.
The adjustable resistor (R sets the alarm trip point, and the thermistor (T) forms a voltage divider with the fixed resistor (R). While temperature is below the alarm setting, V exceeds V the output is about +V as shown. When temperature rises above the alarm setting, V drops below V so the output drops to about ground (0 V).
Because a thermistor value is roughly proportional to temperature, it can be used in a proportioning circuit to regulate such things as fusion roller temperature. As temperature increases, the oscillator will produce shorter pulses, and vice versa. The output can be detected by a microprocessor or ASIC, or used to drive a heater control circuit directly.
Laser Printer Mechanical sensors
When position or presence must be detected, a simple mechanical switch can be used. A set of mechanical contacts might be normally open or closed, then actuated by the presence of paper, the paper tray, closed housings, etc.
The condition of each switch (whether used individually or in sets) is often detected and acted upon directly by a logic circuit. The switches used in a printer's control panel often are considered to be mechanical sensors because they are detecting your input from the outside world.
Laser Printer Optical Sensors
Mechanical sensors generally lack reliability over long-term use. Electrical contacts wear out through use and environmental corrosion. Contacts also are subject to electrical ring-an output that might vary on and off for several milliseconds before reaching a stable condition. Optical sensors are immune to these problems.
A basic optical sensor (called an optoisolator). An optoisolator consist of two parts: (1) a transmitter, and (2) a receiver. Both are separated by a physical gap. The transmitter is usually an IR (infrared) LED kept on at all times. The receiver is typically a photosensitive transistor that is most sensitive to light wavelengths generated by the LED.
When its gap is clear, light passes through unobstructed and saturates the phototransistor, in turn producing a logic 0 output signal. If an object (such as paper or the carriage) interrupts the light path, the phototransistor will turn off and allow a logic 1 output signal.
There are no moving parts in an optoisolator, so it can operate at high speeds, and it will never wear out mechanically. Your printer paper supply and paper exit sensors are typically optical.
Fig. 8-11 Simplified diagram of optical sensor.
Troubleshooting laser printer sensors and interlocks
Before performing sensor checks, make it a point to examine any connectors of interconnecting wiring that tie the sensor into its conditioning circuit. Be certain that all cabling is installed correctly and completely before proceeding. Also remember that most sensors are mounted on small PC board assemblies that might contain other sensors or electronics.
If you do not have the schematics or test instruments to check a sensor as discussed below, it is acceptable simply to replace that sensor PC board outright. If you do check the sensor and it checks properly (and its Conditioning circuit appears to be functional), there is likely a problem with the logic assembly.
You see a PAPER OUT message even though paper is available. Also, the error might not appear when paper is exhausted. If your paper sensor is a mechanical switch, place your multi-meter across its leads and try articulating it by hand. You should see the voltage reading shift between a logic 1 and logic 0 as you trigger the switch.
If you measure some voltage across the switch but it does not respond (or responds only intermittently) when actuated, replace the defective switch. If it responds as expected, check its contact with paper to be sure that it is actuated when paper is present. You might have to adjust the switch position or thread paper through again to achieve better contact.
Check an optical paper sensor by placing your multi meter across the photosensitive output; then, try to actuate the sensor by hand. You might need to place a piece of paper or cardboard in the gap between transmitter and receiver.
You should see the phototransistor output shift between logic 1 and logic 0 as you trigger the optoisolator. If it does not respond, check for the presence of dust or debris that might block the light path. If excitation voltage is present, but the phototransistor does not respond, it is probably defective. Replace the optoisolator or sensor PC board assembly When a sensor responds correctly, the trouble is probably in your logic assembly.
Fusing temperature control is ineffective. Temperature never climbs, or climbs out of control. Temperature fluctuations affect print quality or initialization for laser printers. Unplug the printer and allow at least 10 minutes for the printer to cool and discharge. Disconnect the thermistor at its connector. Use your multimeter to measure its resistance. A short or open circuit reading might indicate a faulty thermistor, so replace any suspect part.