HP Laserjet 4l paper jam - probably electronics behind phototransistor defect - how to check?

Question

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)}

Answer 1

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.

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