Magnetics experts (RG) ? Theory and mystery to solve...

[nuke]

I'm trying to solve a bit of a mystery in dealing with magnetic components of Schaller humbucker pickups during repair.

There's nothing special about Schaller humbuckers, other than the dimensions being metric instead of inch and the screws are usually north instead of south like Gibson. Otherwise, basically same deal, Alnico V bar magnet, screws/slugs plastic bobbins, nickel silver and brass baseplates, plastic spacers. They usually put a little smear of solvent cement on the bobbins under the magnet during assembly. Just a plain older vanilla humbucker.

These are 30-40 year old pickups, mostly from the "pointy guitar" era.

Now the mystery:

Prior to dis-assembly: Measure magnetic flux at the slugs at about 45mT. (with digital gauss meter at the centers of the slugs).

After reassembly: Measure magnetic flux at the slugs at about 32mT.

Why?

And it doesn't take significant disassembly. Simply moving the magnet and breaking the physical connection to the keeper bar and slugs and putting it right exactly where it came from, the flux at the slugs and poles is reduced.

This is consistent on every slug/pole and happens on every pickup I've worked on.

Under a microscope, the magnet's physical contact is good, no gaps, no debris, nice clean physical contact to the magnets.

The magnet seems just fine with a full charge as expected. I even recharged them to be sure they're fully saturated, consistent with alnico-V.

The pickups seem to work fine.

But why the significant drop in magnetic flux?

I've exhausted all my ideas, including some really dumb ones that AI generated up.

The only thing I can think of is the soft magnetic parts, the screws, slugs, keeper over a very long time, are internally aligning their domains more and more in the static field and simply conducting more of the flux. Once breaking that long connection with the magnet, they go back to their normal state.

Beats me. I wish I understood it.

[martin manning]

I even recharged them to be sure they're fully saturated, consistent with alnico-V.

Were you able to confirm that the magnets themselves had the same field strength after you recharged them?

[Helmholtz]

As you're speaking of the slugs only, did you have the pole screws inserted when measuring at the slugs?
Is the battery of your meter fresh?

[didit]

Do you find any measurement variability across the dozen pole points across each of the pickups?

..

[nuke]

Yes, the magnet is fully saturated upon removal. Even putting it in the magnetizer and hitting again show no increase. So, it's fine.

The flux reduction is at both the screws and slugs, and evenly reduced across every one of them, and every single pickup worked on shows exactly the same effect, across every pole and slug.

The meter is working correctly. So the indications are real.

I've done everything I can think of, particularly in cleanliness of assembly in particular. If it were that, I'd expect more variation pole to pole and more variation among the population of pickups.

I even tried completely degaussing the pickup (with the magnet removed of course).

ChatGPT came up with some crazy ideas that some kind of magnetic buckling was going on during the assembly process, suggested putting the pickup face down on a keeper plate. (no effect) Then it suggested putting the pickup on top of magnets to complete the magnetic circuit as the bar magnet is re-assembled. (no effect) and then the kooky hallucination of heating the entire pickup to 130F, "to relax the crystalline structure in the steel" while in the said magnetic fixture. (no effect). Everything, same-old, same-old. No change whatsoever. Not better, not worse, just exactly the same.

The only thing I can come up with is that the "soft" magnetic parts (the non-permanent magnetic components) simply becoming more aligned somehow after 30-40 years in contact with the alnico 5 magnet.

But if I ever learned how that worked in university, I've sure as heck forgotten it long ago. But the aligning of the ferromagnetic structure over time makes more sense to me than any other explanation I've come up with.

[R.G.]

So much for my magnetics expertise. I can't come up with anything rational other than the other metal parts doing ... something ... or the assembly process offending them.

That is - I don't know.

[Helmholtz]

The only thing I can come up with is that the "soft" magnetic parts (the non-permanent magnetic components) simply becoming more aligned somehow after 30-40 years in contact with the alnico 5 magnet.

Didn't you re-use the original soft magnetic parts?
Did you use the same Teslameter before and after?

I just measured an unaltered 15 years old A5 humbucker and all slugs showed 32mT.
None of my 5 books on magnetic materials mentions something like permeability of soft magnetic materials increasing over time when exposed to a field.
Generally magnetization is an instantaneous effect.

[nuke]

So much for my magnetics expertise. I can't come up with anything rational other than the other metal parts doing ... something ... or the assembly process offending them.

That is - I don't know.

Well, at least my self-esteem is intact, 'cause you're a lot smarter at this than me!

The only thing I can come up with is that the "soft" magnetic parts (the non-permanent magnetic components) simply becoming more aligned somehow after 30-40 years in contact with the alnico 5 magnet.

Didn't you re-use the original soft magnetic parts?
Did you use the same Teslameter before and after?

I just measured an unaltered 15 years old A5 humbucker and all slugs showed 32mT.
None of my 5 books on magnetic materials mentions something like permeability of soft magnetic materials increasing over time when exposed to a field.
Generally magnetization is an instantaneous effect.

Yeah, I'm working on Schaller pickups. They're built like other humbuckers, except everything is metric. That makes the bar magnet 59mm x 12mm x 3mm instead of 2.33" (or 2.5") x 0.5" x 0.125".

I have to re-use the original parts, since no off the shelf parts will fit. Even Chinese humbuckers today use inch-sized components, except for the threads on the mounting screws.

Some of these are very unique pickups, hard to find and that I'd like to keep working.

The gauss meter is working fine. It's an inexpensive digital one with a hall-effect probe, but it works correctly. I've worked on plenty of pickups, never encountered this before, but not sure I was ever looking for it.

I had one in the junk drawer that's not likely repairable, as the bobbin has a slight crack across all the screw holes. I hadn't messed with the magnet on it at all until the other night. This mystery has been bugging me a while, so I took the screws out far enough to unthread from the baseplate, no change in strength. I took the base plate completely off. Flux unchanged. Lifted the magnet off and set it right back where it was - flux dropped. No amount of dinking with it has restored it.

I'm puzzled.

ChatGPT is suggesting that I construct a keeper for the bar magnet, charge it in the magnetizer with the keeper on the magnet, then slide the alnico V magnet out of the keeper into the pickup so it never is out of "circuit" and in an airgap. I guess ChatGPT doesn't understand the pickup itself is an air-gap...

I've watched vids of factory assembly of humbuckers. They don't do anything like that. Magnets come uncharged, they pop them in the Magnetool M24-C one at time, and rest them in a block of wood with a bunch holes so they don't touch, take them to the assembly line. Wind the bobbins, press the slugs in, put them face down in a wooden holding tray with dozens of others. Just set the magnet, keeper bar and spacers down, then the back plate, screw it down, flip it over and drive in the pole screws. No special handling.

The only difference is Schaller puts a little schmear of solvent cement on the plastic bobbins under the magnet and under the plastic spacer (vs. maple on PAF copies).

They're mass produced.

But I'll be darned if all the unmolested samples I've tested didn't have really good flux readings at the poles. But just interrupt the magnet and bang, it drops. Not just at the face of the slugs, the whole field reads lower, everywhere.

The magnet itself reads totally fine. My magnetizer is a stack of neo magnets on either side of an iron vise. It seems capable of evenly zapping even uncharged magnets.

Beats me. I'm mostly out of guesses.

Maybe the steel soft-magnetics part was fresh out of annealing when it was assembled, and as time has gone by, the annealing reverted?

[Astronomicum]

Maybe over time and under use, the flux begins to climb. If you have records and have access to ones rebuilt, check and see. If not, run an experiment with rebuilt pickups, maybe one in use, one not, and see. Who knows. Maybe there is a “settling in period”. Be interesting to see.