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HP Laser Printer Mechanical Assemblies - Exploring And Troubleshooting The Printer Path Path.

Updated: Jun 10, 2019

Understanding the basic mechanics of a HP laser printers is the very first step towards successful troubleshooting. The following is designed to give a basic understanding of each printer assembly and its primary functions – to save you a lot of time and money troubleshooting and ordering HP printer parts.

HP Laser Printer Mechanical Assemblies - Exploring And Troubleshooting The Printer Path Path.

Remember: while not every printer is the same, each model has the same basic assemblies in some way, shape, or form. The Cassette Pickup Assembly Depending on the model of the machine, “the pickup assembly” could just be a pickup roller with a metal shaft and clutch, or a series of clutches, gears, and rollers. Regardless of how different they look, they all perform the same basic function.


Typical HP printer picks paper up from the paper tray and transports it to the registration assembly through a series of clutches, gears, and rollers. Printers have been getting much faster and more complex throughout the years.


The HPLJ II/III series pickup assembly consisted of a clutch, shaft and gear. The HPLJ 4000 series has a pickup assembly and a feeder assembly, and the HP LJ 5si and HPLJ 8000 models have the pickup and feeder assemblies integrated into the Paper Input Unit, or PIU.


HP Laser Paper Input Units PIU

Some paper input units have their own motor within the unit to drive the assembly. The rest are driven off the printer’s main drive assembly. The “pickup assembly” is responsible for two things: its first function is to pick up paper. Its second function is to prevent multi-feeding, through the use of a separation pad, separation roller or wear strips.


The goal of this assembly is to get one sheet of paper to the feeder assembly. In most printers, this assembly has a sensor to let the machine know that the tray is out of paper. Some models do not have this sensor, and the machine will give a paper jam error when the tray is empty.


This is because it assumes there is always paper in the tray, and the first sensor does not get tripped. The feeder assembly has one function. It assists in getting the paper to the registration assembly from the cassettes. On some of the smaller machines, the pickup roller also acts as the feed assembly. This is due to the more compact design of the machine.


This assembly is also used in machines that duplex. The duplexer feeds the paper from its output to the input of the feeder assembly to print the second side of the paper. Many machines with paper feed assemblies have a jam sensor in the unit.



HP Laser Printer Manual Pickup Tray / Multi Purpose Tray Pickup Assembly

The Manual Pickup Tray The multi-purpose tray pickup assembly uses a slightly different paper path, but its function is simple: get the paper from the tray to the registration assembly. This tray works very well for labels or media that need a more direct paper path (such as thicker media), along with odd-sized paper. Most of the time, media in the tray is recommended to exit out the face-up bin on one side of the machine, depending on the model.


This assembly has a sensor to inform the DC Controller that there is paper in the tray. If you forget to plug in this sensor, the machine will do one of 2 things. Either it will always think there is paper in the tray, or it will never think there is paper in the tray.


If your machine has a control panel and keeps prompting for paper in the MP tray, check what paper size and types are in the cassettes. This can be done by printing a configuration page. If none of the cassettes match what media the print job is asking for, or the print job is requesting to be printed from the manual feed tray, the unit will prompt the user to put the selected media in the MP tray.


This assembly also has either a clutch or solenoid to engage the assembly. When the controller sends a signal to pick paper from the MP tray, it engages the solenoid or clutch. This causes the pickup roller to turn the separation pad to lift and paper to be picked up from the MP tray. From there, the paper is fed into the registration assembly.

The Registration Assembly The registration assembly has one function.


This is to ensure that the paper is going through the image formation process straight so the image is not skewed. This assembly can be as simple as a few offset rollers guiding the paper to a straight-edge on the side of the paper path, to a set of rollers with a metal guard operated by a solenoid or clutch. Once the paper passes this assembly, the top of page sensor is tripped.


This sensor is located right after the registration assembly. When this sensor is tripped the image formation process begins. If the registration is not functioning properly, paper will either jam in front of it or the image on the page will not be centered properly. If the top of page sensor is missing or not tripped, you will get a blank page, followed by a paper jam in the fuser.


This is because the fuser jam sensor tripped, yet the controller is still waiting for the top of page sensor to be tripped. Image Formation After going through the registration assembly, the paper is sent to the imaging section of the printer.


This consists of the toner cartridge/ transfer roller sections, the feed belts (if applicable), and the fuser. When paper passes between the toner cartridge and the transfer roller, the toner is transferred from the cartridge to the paper, and pushed along.


If the transfer roller is not installed properly or the springs are missing, you will get an uneven print density on one side of the page. But we can explain this in more detail later. Depending on the machine, the registration assembly is doing the advancement of the paper during this process until it reaches the fuser. Smaller media uses feed belts to transport paper between the image transfer and fusing section of the printer in some machines.


When the paper enters the fuser, it is squished between two rollers to fuse the toner to the paper. The paper then exits the fuser rollers, where it goes out the delivery rollers. Between the fuser rollers and the delivery rollers is a jam sensor. This sensor is there to inform the controller that paper is in the fuser. If this flag is tripped when it shouldn’t be, it tells the printer's DC Controller to not turn on the fuser.


This prevents the fuser from melting transparencies or starting a fire. When the paper exits the fuser, it goes through the fuser’s delivery roller. From here, the paper can go to many different places from the face-up bin, the face- down bin, an optional accessory or duplexer. The face-up bin would be the door on the side of the machine (some are in front, others in back, and others on the side of the machine.) In most cases the face-up bin bypasses the output assembly.



HP Laser Printer Paper Output

Output Assembly After the paper exits the fuser to go to the face-up bin (the bin on top of the machine) it enters the output assembly. This assembly usually contains a few sets of rollers to assist in getting the paper to the top output bin. In some models, there is a jam sensor in the output assembly to inform the controller of a problem.


In the more compact models without a removable duplexer assembly, the output assembly is also used as a reversing assembly for the duplex functions. After the last set of rollers in the paper path for the top output bin is usually a sensor flag to let the printer's DC Controller know that the output bin is full. This stops the print engine to prevent a large mess of paper in the output assembly or all over the floor.


Troubleshooting the Mechanicals Throughout the print process, the printer's DC Controller is expecting the paper to be at certain flags at certain times. The print process is all based on timing. If the page slows down across the top of page sensor (due to a roller slipping) you will get a 41.3 unexpected paper size error.


This error is due to the paper size sensor telling the printer's DC Controller that there should be one size, for example letter, yet the top of page sensor registers the paper as another size, such as A4.


Many different tests can be performed to check for faulty assemblies. Halfway tests can be used to test for imaging problems and some jamming issues such as corners of the paper being folded over and wrinkling of the page. Another great test to use for troubleshooting mechanical assemblies is to do the process of elimination.


If the printer has four paper trays, try all four trays with the same media. Also try a different type of media in all the trays since the problem may be with the media. If the printer has a print defect from the cassette, try the MP tray. If there is no problem from the MP tray, the problem is most likely before the registration assembly.


If the HP printer has multiple paper output assemblies, try a different output tray. Remember that HP service manuals are very handy for a repair person to have when on a call. In most HP manuals there is a section for theory of operation.


This section will explain how the print process works. The laser printer service manual also has error codes and troubleshooting advice for that particular printer and error code. This should help the technician troubleshoot a printer relatively quick and accurately.


Check All HP Laser Printer Mechanical Connections

If the old printer assembly works, but your new assembly doesn’t, check to make sure everything is fully plugged in. Connectivity can be the cause of many problems. If a sensor, clutch or cable is not fully plugged in or off by a pin you can have some serious problems. If you are off by a pin, you can destroy components due to voltages being in places they shouldn’t be in.


If the connector is not fully plugged in the printer's DC Controller will not be getting the correct signals, and it will throw up an error due to this. Just because the printer says “13.2 Jam in fuser” does not mean the fuser is bad. It could be a bad toner cartridge preventing the paper from getting to the sensor at the fuser in time. Accordion jams would equate to an assembly that is pushing the paper, but it’s not being pulled by the next step.


This could be caused by something blocking the laser printer paper path, to a gear not lined up or a bad clutch. Usually the suspect assembly is located right where the accordion jam occurs. Remember, HP lasers printers are based on timing. If paper gets there early or late, paper jams will occur.


Other Helpful HP Printer Support Resources

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