Monday, December 19, 2011

Sticky Solenoids

Pictured below is a pickup solenoid from a LaserJet 4100's Tray 2. It's in its normal, de-energized state.

And here it is stuck in its energized state, in the absence of any applied energy.

On the solenoid's frame, there's a tiny rectangle of foam rubber adhered with pressure-sensitive adhesive; it's there to quiet the solenoid's operation. The foam rubber deteriorates with age, and the adhesive bleeds through. The result is a sticky solenoid that will cause the clutch it's controlling to double-cycle. That can result in some quite mystifying symptoms. Here's a view of the ruined foam rubber.

HP doesn't break out this solenoid as a service replacement part, but it's not at all difficult to get a sticky solenoid working properly.

First, you have to remove all traces of the adhesive with lacquer thinner. (The adhesive is tenacious stuff, but it does come off. Be sure to get rid of all traces of it.) Wrap four thicknesses of electrical tape around the flapper and reassemble the solenoid.

The solenoid will work fine, but it won't be as quiet as it was. It will make an audible, though not objectionable, muffled 'click' every time it's energized.

This style of solenoid is widely used to control clutches, and they can all get sticky with age. The symptom this one produced in a 4100 was rather odd.

On a multi-page print run, every third page would be a blank sheet. No error would be reported, but the printer would stop printing, and just keep running doing nothing. Opening and closing the lid would cause printing to resume for two pages, then the same blank sheet incident would occur.

Looking in the back of Tray 2, I could see that the pickup rollers were double-cycling. 'Explained a lot.

The double-cycling didn't affect the first page's timing, of course, and it didn't affect the second page either. But it always caused the third sheet to enter the printer too early. Whenever a sheet is fed too early, a misprint (blank sheet) ensues. The curious thing is that there was no error displayed. Misprints are supposed to give '41' errors. It looks to me like the 4100 has a bug in its firmware, such that it can't interpret the particular condition I was seeing. It just gets bewildered, and goes into a 'do nothing useful' loop.

Any solenoid that sticks long enough to cause double-cycling of a clutch will cause trouble of some sort. Any time you have access to a solenoid of this type, take a minute to press its flapper closed and see that it doesn't stick.

When you encounter a sticky solenoid in a machine, and the machine has other, similar solenoids in it, examine them all. The odds are good that they'll be sticky as well. The printer this solenoid was in had a Tray 1 solenoid that was even worse for stickiness. I think there might be some environmental factor that contributes to the foam rubber's deterioration.

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5 comments:

  1. This is a common problem with all HP LaserJet printers. Most will fail with age (10 years or so) but exposure to Ammonia fumes will cause the problem mush sooner. John

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  2. This is a common problem with all HP LaserJet printers. Most will fail with age (10 years or so) but exposure to Ammonia fumes will cause the problem mush sooner. John

    ReplyDelete
  3. Made a video on this... with thanks & shoutout in the video to this page. URL: https://www.youtube.com/watch?v=ChxmYK23kg8

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  4. I have the MFP3310 which is the minimum printer with just a front paper tray. Where do I find this solenoid?

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  5. Just an FYI, any repair which completely removes the adhesive and foam leaving bare metal can result in sticky solenoid symptoms due to magnetizing of the solenoid arm ("magnetic sticky"). My permanent fix for this - since electrical tape breaks down over time and can be inadvertently stretched by inexperienced techs - has been to do a 4" strip of scotch or packing tape wrapped the same way around the arm. I've never had a callback.

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