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HP LaserJet Error #50

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HP LaserJet 4L gear

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Replacing the HP LaserJet 4L drive gear

My HP LaserJet 4L
This HP LaserJet 4L was bought in 1993, 6 years after the appearance of the LaserJet Series II on the market. Where the LaserJet Series II appears like a professional instrument, the tiny 4L targeted the home user. Its design objective might have been to remove everything not absolutely necessary without compromising quality. I underestimated this printer back in 93, with its unassuming look and its single push button, and stand corrected ever since the maintenance operation described here. The design is extremely well thought, no space is wasted while still maintaining easy serviceability. The 4L uses the PX engine, capable of 600dpi with HP's resolution enhancement technique RET.

The grinding noise
After 17 years of loyal service, my printer's attempt to feed a sheet of paper failed, and was accompanied by a loud grinding noise. A quick search on the net revealed that this failure is well known, and its origin is a small vinyl gear, which is readily available at various parts suppliers. I got mine from Fuser-Zentrum, where it has the part number RG5-0765-gear. Another supplier in Germany I already dealt with is Compucare; they also offer to carry out the repair for you. Note that I am not affiliated with either of these.

Replacing the gear

After removing the toner cartridge and the paper cassette, I remove 4 screws accessible in the toner cartridge compartment. The printer plastic case can then be removed after unlocking 6 catches. Start with the two front catches, then the two on the sides, and finally the two hooks at the back, and you see the printer naked as in the left picture. Turn the printer sideways to access the printed circuit board under a protective cover, as seen in the picture to the right.

The cover plate can be removed after removing 4 screws. The remaining two screws hold the PCB itself. After removal of the screws, it can be simply pulled out. On the back of the board is a plug, which connects to a socket underneath.

After removal of the PCB, we have to remove the gear panel. It is attached with 2 small screws holding the ground clips, 6 screws on the panel, and two screws on the front attached to the motor. I took the opportunity to clean it from toner residue, and to verify the lubrification.

One remaining screw must be removed before the motor can be pulled out, as seen in the picture. The white plastic gear attached to the axe looks intact. Only a closer inspection revealed a small crack. The picture to the right shows the motor with the new gear, which must be mounted flush with the axe. It simply snaps on.

Reassembly is simply done in reverse order. The entire operation took me less than an hour, including a thorough cleaning of the case.

I received feedback from several readers who successfully carried out the repair. Two difficulties were reported:

The plastic cover is a bit tricky to remove. The two front catches are easy to unlock, but the others and especially the hooks on the side may require some extra effort. Patience and perseverance are in order.

One reader reported that one of the little springs at the back's straight paper path switch unsnapped during the operation, preventing the path switch to register completely, and impeding the paper from being turned 180° towards the output tray. The spring apparently unsnapped while removing the printer's cover, and was easy to put back in place.

LaserJet 4L paper jam

The sticky solenoid problem
After about 20 years of loyal service, my HP LaserJet 4L developed a strange paper jam problem. It would pull in the first sheet of paper and print correctly, but fail to pull in the second. After clearing the jam, the same pattern would repeat.

My initial diagnosis was that it was time to replace the D-shaped pickup roller. That was wrong - the LaserJet 4L has a particularly well built pickup roller (compared to my LaserJet II), and the problem turned out to be a sticky solenoid. HP put some foam on the solenoid, to reduce noise I would guess, and after about 20 years this foam disintegrates into some adhesive goo which causes the solenoid to stick and to release with a delay. This delay messes up the timing for the paper feed through the printer and causes a paper jam error.

I did not come to this conclusion myself - I found a helpful video on youtube which explains the steps necessary to disassemble the printer to get access to the solenoids and to clean them. The pictures below show the two solenoids, the one on the main board being disassembled to facilitate access. The video on youtube is very explicit, it only lacks one step - the fuser should be removed before removal of the main board, and be reinserted after the main board is reinstalled. Otherwise the two white connectors might get damaged.

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