On 09/11/2017 08:42 PM, Jeff Liebermann wrote:

I moved the armatures until they touched the foam pads, and there was
not the slightest sign of sticking.

http://www.learnbydestroying.com/jeffl/hp4200/Sticky-Relay.jpg
http://www.learnbydestroying.com/jeffl/hp4200/New-Felt-Pad.jpg
You have to smash the armature hard against the solenoid, simulating
what the energized core would do, in order to check for sticky
problems. I've found that even the slightest residue on the either
the armature or core will cause problems.

No sign of permanent magnetism in any of the solenoids.

This is for a different model HP printer, but with the same sticky
solenoid problems:
http://www.learnbydestroying.com/jeffl/hp4200/hp4200.html
It still paper jammed on double sided printing even after replacing
the pads. Here's a video of the residual magnetism problem with the
same solenoids:
http://www.learnbydestroying.com/jeffl/crud/hp2200-solenoid.wmv
(600K)
and a rather marginal test with a compass needle:
http://www.learnbydestroying.com/jeffl/crud/hp2200-compass.wmv
If you decide to use a tape head demagnetizer on the solenoid, be sure
to unplug the solenoid from the circuitry or you may fry something on
the motor controller board.

http://www.ebay.com/120894286540

Those parts were replaced a few months ago. That solved the problem that
it kept trying to pick up several sheets at once.

Ok, so much for that theory. Am I correct that it ONLY jams when
trying to print double sided? Did you have the paper jam problem
immediately after you replaced those parts, or did it take "several
months" for the problem to appear? I'm thinking that something might
have gummed up the feed roller or coated them with something stick,
such as printing sheets of mailing labels with some of the labels
removed. That will transfer rubber glue to the rollers.

I wouldn't really call it a ratcheting sound. It's much softer and
"gentler" than that. More like parts rubbing together.

I'll stand by my guess(tm) that it was reassembled incorrectly.

The only solenoid I removed was the first one, the one that did have the
sticky goo problem. I was very careful to position it correctly when I
reinstalled it.

So much for easy and obvious solutions. I don't know what might be
misassembled so the only way to find out is to take it apart again and
look for problems. It's a bit tricky to get it to print while
disassembled, but not impossible.

Good luck 2.0.

Even though I could detect no sign of stickiness on the other two
solenoids, and no residual magnetism on any of them, I gave all of them
the recommended treatment, and now the printer is printed double-sided
without problems.

Thank you.

Perce