How do I re-magnetize old Brandes headphones?

[W3RSW]

non-destructively, of course.
When a kid we used to magnetize nails by putting them in a wooden vise, pointing the nails, North South and hammering the sh'' out of them.  Borrowing a few milli-Gause from the Earth, I suppose 

-but here I am with one phone of a set much weaker than the other; neither pulls the ol' screwdriver like they used to.  OK, it's been decades.  Used to do a lot of experimenting with diaphragm to magnet gaps, etc. and think I used to know how to remagnitize. This was probably a whole "Electronics Illustrated" articel at one time.

Brandes "Admiral" Matched Tone.

modified-
Should have looked first, I did find this.
http://www.eht.com/oldradio/clubinfo/monthly/Nov-96/HEADPHON.html

[WBear2GCR]

Well let's have a jpeg of the "innards"??

My first impulse is why bother remagging the old magnets?
I think the trick would be to use a new neodymium magnet to supplement the old... they're small and light, and cheap. That's how I'd go.

The proper remagnetizing rig is a big coil heavy wire, and a cap discharge bank, iirc... big pulse..WHAMMMM! Tada, magnetized.

They use the same trick for bending cans, nickels and the like using a magnetic field...

But the neodymium "booster" magnet is easier and cheaper, imho.

A bit depends on the physical layout inside, imho

              _-_-bear

[Rob K2CU]

You can usually resue a nice pair of mini high power magnets from the "Sonic" type toothbrush replacable heads. Every time we replace a head, I rescue the tiny magnets. Taped tp the side of a pice of brass brwsing wire they make gereat scre/nut retrievers!

[k4kyv]

This is a common problem with magnetic headphones and horn type speakers.  The metallic diaphragm makes an imperfect "keeper" for the magnet, and it eventually loses strength.

A magnet may become weakened from loss of flux. Thus when storing magnets, one should always try to avoid excess leakage of magnetic flux. A horseshoe magnet should always be stored with a keeper, a soft iron bar used to join the magnetic poles. By using the keeper while the magnet is being stored, the magnetic flux will continuously circulate through the magnet and not leak off into space.

You might do a web search.  I have seen instructions somewhere exactly how to do this.  As I recall, it requires removing the magnet from the rest of the phone (usually easily done by removing a couple of screws), wrapping some wire round the magnet, charging an oil-filled capacitor similar to what would be used in a transmitter power supply, and discharging the capacitor through the coil.

I would use some limiting resistance in series with the coil, since placing a dead short across a fully charged HV capacitor may produce enough current surge to internally damage the capacitor, and could possibly cause personal injury. For the same reasons I wouldn't advise doing this using an electrolytic capacitor. I would first try the procedure with a nail and test with a cheap magnetic compass, to make sure I didn't end up reversing the magnetic polarity.  Figure out the size of wire, charging voltage and number of turns by trial and error, again perhaps using the nail before applying it to the real McCoy.  And several "hits" may magnetise it more strongly than just one.

[WBear2GCR]

...magnets are funny things... interesting that anyone thinks that magnetism can "leak off"... hmmm... the physics folks say that it ain't a flow of anything, it's a force, and so it can't do work... or something like that... which is why the idea that you can magnetize anything by "zapping" it with an instant pulse of energy which then makes it magnetic for a long long long time always has puzzled me... I don't buy any description I have read yet.

No one has yet been able to explain the correlation or lack of correlation between the energy in that magnetizing pulse and the resulting "magnetism's" force/strength, etc.

And then what about electromagnets... what the heck are those silly electrons up to?

Ah well... <scratches head as it fries internally>

                   _-_-bear

[Ed/KB1HYS]

In a peice of iron, there are elements called "domains" each domain has an associated magnetic field. In a non-magnetized iron bar the domains are randomly placed in the crystal latice that is the iron material.  When you provide some sort of external force, ie: a magnetic pulse or physical shock, the domains can "line up" with their poles facing the same way as dictated by the surrounding magnetic field. (either the earths or the field generated by the magnetizing pulse).  Over time these domains randomize again which is what causes a simple iron magnet to loose it's strength.  In other materials the domains are more firmly locked into alingment so they can't randomize as easily.
as for remagnetizing an old set of cans, you can take the cover off (the part next to your ear) and rub a magnet across the poles of the iron core.

More details on rejuvenating some old cans here:

http://www.eht.com/oldradio/clubinfo/monthly/Nov-96/HEADPHON.html

[W3RSW]

I think I'll try the 12 v. battery, 18 turn electromagnet and find the correct pole with a compass, full immersion trick first, but ...

Uh, I'm having real trouble, short of destruction of getting the damn plastic caps off the aluminum shells.  Tried the ice cube trick on the back of the aluminum shell, light taping with a hammer. (the Ruski method).  If the shells are bakelite and not some thermoplastic substance, penetrating oil might help.  But don't want to melt the damn things.

 Ice cube trick is supposed to create dissimilar radii due to temp. diff. if you don't cool the backs too long, then the caps cool off too. 

Ideas to loosen the caps.?   I mean it looks like we're talking pipe wrench torque here; nothing less seems to budge them including super grippers, etc.

Ok, how does a magnet lose strength?  The keeper is a good method to forstall the inevitable, but I think that the once aligned crystal lattice of iron (in this case) molocules gradually resume their random directions over many years.

A lot of minerals can be magnetic... even paramagnetic salts.

[WBear2GCR]

are we gonna get some nice clear closeup jpegs?

[k4kyv]

Here's another one

http://www.nostalgiaair.org/References/Articles/TheFlash/Flash06.htm

[KL7OF]

got a welding machine?   coil up the lead (or a coil of wire)  + to one end, - to the other...This serves as a 24 volt  electromagnet that you can vary the current to suit......works great for all jobs big and small...Back in the day of mechanical wrist watches, it was a common thing for a weldor's watch to stop after being magnatized....An old buzzard welder showed me the trick to demagnatizing by coiling up some welding lead, putting a small current thru it and drawing the stopped watch thru the coil until the watch started up...Can also be used for magnatzing....might work for you...Good luck..
 

[W3RSW]

Bear, got the caps off so here's a description.

aluminum cage so magnet and pole pieces screwed directly to it without getting any magnetic 'short.'

The magnet is approx. 1/4 in. stock. originally round bar but flattened on top and bottom so it's kind of an oval cross section but with flat top and bottom surfaces.  Total length of magnet is about 1 1/2 in. and is curved about 1/4 of a complete circle and held in by screws through the back of the cage and brackets inside.  Much more roomy in there than I remembered.  A new neobidium magnet might fit.  But it will be easy to remove the old ones and re-magnitize them too. 

Working on getting pix.  But looks very standard for 2k to 4k headphones.

[W3RSW]

Thanks for the thoughts everyone,

Here ya go Bear.

[N3DRB The Derb]

I bet the HB crystal set gangsters would know how real quick. Wheres your galena?  Those headphones just the thing for your Grebe CR-9 too.

[k4kyv]

got a welding machine?   coil up the lead (or a coil of wire)  + to one end, - to the other...This serves as a 24 volt  electromagnet that you can vary the current to suit......works great for all jobs big and small...Back in the day of mechanical wrist watches, it was a common thing for a weldor's watch to stop after being magnatized....An old buzzard welder showed me the trick to demagnatizing by coiling up some welding lead, putting a small current thru it and drawing the stopped watch thru the coil until the watch started up...Can also be used for magnatzing....might work for you...Good luck..

Unless the welder puts out DC, that would demagnetise the iron. All the welding machines I have ever seen are nothing but high current low voltage transformers with variable taps and heavy leads.  The welders' watches may have become magnetised at the pulse when current was first applied or when the circuit was broken.  Trying to do that intentionally would be  literally a hit-or-miss proposition.

I often demagnetise screwdrivers (beats me how they end up magnetised in the first place when I haven't been working near magnets or conductors carrying high currents), by running the blade through the loop at the tip of my Weller soldering gun.

I remember the old "bulk erasers" at the radio  station for degaussing reel-to-reel tapes.  Nothing but a coil of wire with 110 volts a.c. applied to it.

[WU2D]

I like the super magnet idea but I would simply put a pair right on top of the existing magnets overnight so that they "recharge" the weak ones. Dunno

Mike WU2D

[W3RSW]

Well my first experiment after determining polarity of the phone magnets by using the compass is:

-have an old Warfdale tweeter , "Super 3" labeled with "13,000 lines" of magnetic force. Now that baby sucks a screwdriver from across the room.  The magnet has the same dia. as the 3" diaphragm. 
    I'll determine polarity of that beast and if physically possible without dissassembling the speaker, rub the phone magnets in the proscibed manner, ( uttering the proper chant, "OH WAA TAA GOO SIAM") whilst tapping them sharply but gently with a hammer.

***********

Don, I think screwdrivers are magnitized overtime by the Earth's field, when your using them, by throwing them in the tool box or on the bench ... numerous times but statistically a lot in the north-south direction when they land hard, bang around other tools, particularly heavier pliers and wrenches.  Actually I like some of mine magnetic so they hold screws.  Use them to sort too.

[KD6VXI]

I often demagnetise screwdrivers (beats me how they end up magnetised in the first place when I haven't been working near magnets or conductors carrying high currents), by running the blade through the loop at the tip of my Weller soldering gun.

Any RF around the shack?  ALL my  screwdrivers that end up near a QRO amplifier end up magnatised in one way or another (varying strengths of pulling power).

When floppy disc's where common, I had a set of screwdrivers for working around RF, and a set for working around the PC's.  Otherwise, the same screwdriver would end up erasing the floppies near it, or at least scrambling them.

It was so easy to magnetise one, just putting it near the tank coil of a 4 watt CB radio would cause magnetic attraction

--Shane

[W3RSW]

OK, did it.
The phone magnets are definitely stronger than they were but made no diff. in sound output.  That's subjective, of course, what with 2 or 3 db to make much of a difference to 'old ears.'

It's beginning to dawn on me that the loudness of crystal radios in my youth don't seem so loud these days. 

From what I'm reading, Brandes Admirals of the later era weren't the greatest anyway.

The Warfdale was clearly polarized N to S from the back of the speaker directly through the cone's centerline or center of radius to the front.  Very strong.

[W1EUJ]

Use old sound-powered telephone/headsets instead. They are more sensitive.

[W3RSW]

I've heard that.  Have to check out the impdances, transformers, etc.