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I have a problem with our old HP Laserjet 4L.
The paper jam indicator flashes after the printer fed the 2nd sheet of paper a few centimetres into the machine. The first page after powering on the printer after some time always gets printed, but it gets stuck at all following pages. However, when I press the button, the printer is correctly transporting the sheet and throwing it out of the correct slot, but without printing on it.

After studying several service manuals and long researching on the web I found out that there is a photo sensor (TLP833, I think) that checks the state of an arm that gets moved when the sheet of paper is passing through the machine. The paper stops about where the second position it detects is: Paper is taken from the box or manual feed. Normally, this position commands the paper transport motor to stop and wait until the fixing unit is hot and the page is loaded into memory etc so it would be ready to print.
But this does not happen, as the paper jam indicator immediately flashes and the printing process stops.

I assumed first that the photo sensor is defect because it seemed like it does not detect that the paper arrived and therefore assumes that it got stuck anywhere = paper jam before entering the printing module. But I read that those photo sensors almost never die and it does not fit to the fact that every first page after a long period without AC connection was printing successfully.

Now my second guess is that there must be a capacitor that charges at some time during the first printing process and does not get properly discharged, but has to be in order to allow a new sheet of paper to be detected. No idea whether this is true, just a guess. I unfortunately was not able to see any suspicious changes like a blown up capacitor, a defect soldering joint or anything that looked burned on the circuit board.

My question now is whether somebody had a similar problem and how it could be solved or at least if you have any ideas on where to get a circuit diagram, how to check which components or any other advice that could help me to get that good old box back to work.
Thank you in advance!

(PS: They don't want that question on Electronics.SE because it is a repair question and told me to try it here)

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    No direct help here, but I just retied a HP LJ4 that was built in 1995. It owed us nothing and was approx. the same cost to replace with similar newer HP LJ than to order a maintenance kit for the LJ4. How much is your time worth? ;) – Ƭᴇcʜιᴇ007 Feb 20 '15 at 20:23
  • If you haven't already, you may want to peruse the service manual: fixyourlaserjet.files.wordpress.com/2010/06/… – Ƭᴇcʜιᴇ007 Feb 20 '15 at 20:26
  • @Ƭᴇcʜιᴇ007 I already spent some nights over the service manual, but it is another one than that one you linked to. It is a 4L, not 4/4plus/4M or whatever... And it is not my printer, but a relative's. She wants to keep it or replace with an even older and much bigger LJ 4 we still have somewhere, that does not even fit under the desk properly... – Byte Commander Feb 20 '15 at 20:30
  • RE "found out that there is a photo sensor (TLP833, I think) that checks the state of an arm that gets moved when the sheet of paper is passing through the machine." Correct. The sensor can 'close the circuit' so to speak, by hanging a piece of paper with scotch tape. The paper emulates the "arm that gets moved when the sheet of paper is passing through the machine." – user504080 Oct 1 '15 at 11:59
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I'm not so sure I'd jump to blaming the electronics so quickly, especially since it sounds like the first page always feeds and prints correctly. I worked in an HP-certified repair shop for 5 years and repaired dozens, if not hundreds, of HP printers exhibiting the exact same problem as the 4L you're currently troubleshooting. This included the LaserJet 4L as well as every model from every series ranging from the original LaserJet all the way up to the LaserJet 5 and 4000 series, from personal laser printers to workgroup and high-volume departmental printers. The sensors themselves rarely failed (though the sensor flags sometimes broke), and I don't recall ever tracing a paper jam to a defective logic board. Although an extra 10 years may have taken their toll on the electronics, I'd still suspect the mechanical components first. If the printer had a defective logic board, I'd expect a more catastrophic failure, though your theory may not be completely outside the realm of possibility.

I think it's much more likely that the second page is not being picked up soon enough and as a result it is not fed far enough into the machine to register correctly. Check the pickup roller. If it's smooth and shiny, replace it.

If you cannot source replacement parts or they're prohibitively expensive, you can also try using some "rubber rejuvenator" on the roller as a short-term fix. It often did the trick on HP DeskJet printers which did not have replaceable rollers, but this fix won't last as long as replacing the roller. The stuff we used when I worked in the HP-certified shop was made by Techspray, but other brands may also work fine.

Another possibility is a bad solenoid. Or perhaps the printer has a defective sensor flag, but I would expect that to also prevent the first page from printing correctly.

You didn't mention that the printer is grabbing more than one sheet at a time or that the first page is not fed all the way through, so I'm assuming the separator pad and the output rollers, respectively, are fine. The separator pad is a small rectangular piece of cork or rubber just below the pickup roller. On some printer models we always replaced the pickup roller and separator pad at the same time. I suppose a bad separator pad could also cause the second page to be partially fed prematurely, causing a paper jam. If you've observed any of these symptoms, be sure to check the separator pad and/or output rollers.

Lastly, humidity and poor-quality paper can cause paper jams. If nothing else, try running some newly-purchased premium laser printer paper through the printer.

Service manual

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  • Thanks for your long and detailed answer! But to clarify, I can print one single page after connecting the printer to AC successfully. But if the printer now stays connected and I print a second page minutes later, after checking the paper box etc..., it fails. I am not talking about print jobs of two pages, but two single jobs of one page. Are you referring to that D-shaped, maybe 4cm big roll as pickup roller? I did clean that and it is neither sticky nor shiny. – Byte Commander Mar 4 '15 at 13:07
  • I also cleaned the gum stuff away from the solenoid responsible for unlocking that D-shaped roller because it got a bit adhesive. After that, every first page of a power cycle went through, before that, no page worked. – Byte Commander Mar 4 '15 at 13:08
  • Thanks for clarifying that it jams on the second single-page job. Hmm, so just power cycling the printer without doing anything else fixes the paper jam, but removing and re-inserting the paper tray between jobs doesn't help? Yes, I'm talking about the D-shaped roller which is the first roller that pulls the paper out of the tray. Does it also jam on the second page of a multi-page job? I had to look it up again, but if you press and hold the front panel button for 10 seconds the lights should start blinking and it will print 5 test/configuration pages. – rob Mar 4 '15 at 19:02
  • The first page in every power cycle prints, all others (2nd page of multi-page job or 1st page of 1nd job and all following pages) fail with the sheet of paper fed in about 4cm and flashing paper jam LED. It does not depend on whether I hold the button to print the internal test page or whether I send one from the PC, page is page. I can do with the paper tray etc what I want, the only thing that allows me to print the next page is to disconnect it from AC and plug the power back in after maybe 15 min or so. – Byte Commander Mar 5 '15 at 16:11
  • If you have any idea left, please come fast, because the bounty period is over and I have only 45 minutes left to award it! – Byte Commander Mar 7 '15 at 16:01

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