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)