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HP LaserJet 1000-series continuously feeds paper.

2024-03-23 22:00:12,  In: DIY, Electronics

This happened to my old LaserJet 1005, but will happen to any printer of this line, as this (not to confuse with types starting with P or M, these are totally different printers!) is one of the last decent HP laser printers. The first symptom is that the printer, instead of feeding the paper once, does it twice. It means that you cannot use auto-feeder and have to feed every page separately, but it still works.
Then, it starts to repeat the paper feed cycle three times. Quickly three becomes four, and when it becomes around five, then the printer stops with error. If you try to feed the paper then, it may even work, depending on time passed - you then have to make it try to feed the paper by quickly putting paper back and forth, and when it resets to the "ready to feed" state, you put the paper and it prints.
Or, in extreme cases, it will still repeat the paper feed cycle and, when paper is fed, will pass it thru without printing. But if you wait, you hear a metalic cling from the right side of the printer and then you can feed the paper, and it will happily return to the typical operation, going back thru the paper feed cycle.

Trying to shorten the time to become usable does not work. You can try to bang it, move it or force it back and forth - no acceleration seems to make it faster.

The source of the problem is in the right side of the printer, in a small electromagnet. This electromagnet (solenoid) moves a small curved anchor which, when the solenoid is activated, will move back to the solenoid, away from the clutch it sets into, and it will put the printer to the "paper feed" state. By default, a short pulse makes the anchor move for a short moment, so the clutch is activated only once. In my printer I could see that the anchor, although has a small spring on the other end, does not pop back from the electromagnet.
The photo showing the problem is below:

The problem became simple: the electromagnet (red arrow) will not reliably work with a closed frame, e.g. when the anchor closes the loop around the coil. So engineers from HP installed a small bar of sponge between the anchor (green arrow) and the rest of electromagnet. This sponge gets sticky and the anchor doesn't want to go back even if the spring (blue arrow) forces it back.

The solution is following:
0. Disconnect the printer from AC mains, remove toner, wait a quarter.

1. Open the rear part and loosen all screws on the left (right if you look at the front of the printer).

2. Open the toner door, by losening screws you may lift the top part a little, unlatching the right wall of the casing. It should be also kept by the latch on the rear, latch on the bottom (easy to break!) and a latch on the front. Sometimes it may be needed to partially remove rear panel to unlatch and remove it.

3. There is a fan, below is white, round clutch and to the left of it, an electomagnet with a shiny (usually blue) solenoid and two wires going from it. The anchor (marked with a green arrow in the picture) is hinged near the coil, and there is a spring (marked with blue arroa) near the hinge, one side of it is locked on a small protruding piece of metal in the solenoid's frame, while the other is on the latch. Carefully remove it and don't loose it!

4. Now you can remove the anchor, it's a L-shaped piece of metal with a obtuse angle of it's arms. It always goes with its shorter arm into the clutch.

5. Clean it REALLY well. Nothing sticky must be on it. Instead, put a very small, rectangular piece of electrician's (or even better - silver) tape in the place away from the spring mounting, in the place in which it touches similar part in the electromagnet. While having a sticky sponge there is not a good thing, having a closed magnetic frame, without any gap, will cause it to quickly magnetize.

6. Now the most difficult thing, as removing the electromagnet is troublesome - using a small knife, a flat screwdriver, a cloth with rubbing alcohol, clean the part in which the old sponge is glued to (solenoid's casing to which the anchor sticks) VERY well. There must not be any pollution there, and it's quite difficult. As I had a pretty limited tools, I had to use a cloth with IPA alcohol, a flat-bladed screwdriver and a small paper-cutting knife - all worked perfectly. Finally, apply a layer of insulating tape there.

7. Put the anchor back, do not install the spring yet. Does it fit well? Doesn't it stick?

8. CAREFULLY (the spring likes to fly) install the spring. Check one more time doesn't it stick.

9. If you're already in this place, why not clean the fan and the casing? OK, now put the casing back and test.

If it still sticks, you may try to demagnetize solenoid's guide by moving a transformer-based soldering iron around for a few minutes (only turn it on and off far away from the solenoid), or lock the spring a bit (maximum 2 turns) tighter.

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