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View Full Version : How to measure magnetism of an alnico pro woofer(K-145 in this case)



maxserg
12-02-2015, 04:30 PM
I've had in my absence my two K-145 JBL woofer coils burnt because of a problem with my old Amcron (Crown) DC_-300.
DC current went verry hard I guess and the coils fried (I think) because they are stucked firmly when I push the cone.

So how hot went the magnet structure to go to an eventually loss of mag field and coil failure? Is there a way to measure the loss because I can get original cones from JBL (old stock brand new recone 8 ohms kits from that company. :) ) For the moment I won't tell you from where I can resource them! :)

But I want get them to original specs and the remag process is different from ferrite(I know that ferrite should recover from heat)

Thanks :)

NickH
12-03-2015, 01:51 PM
You can't heat alnico hot enough for it to loose its mag in a speaker.the temp at which a material looses its magnetic properties is called its curie temperature. The temperature for the magnet in a k145 is 1580 degrees Fahrenheit. The only way you could get a speaker that hot is to drop it in a fire, lol.


The only way to check the field strength in a mag gap is with a gauss meter. But you can't do it without tearing out the cone. Now I'm sure there's a way to measure the force a cone exerts with a specified amount of applied power to the vc. But you would need a force gauge and probably some pretty heavy math.


Good luck,
Nick

Wagner
12-03-2015, 04:57 PM
The only way to check the field strength in a mag gap is with a gauss meter. But you can't do it without tearing out the cone.
Good luck,
Nick
From his description of the failure, welded voice coils, sounds as if that's going to be the case anyway
Don't know if "edgewound" can do this for him or not
I'd ask, might get lucky.

1audiohack
12-03-2015, 06:12 PM
The mighty Edgewound does indeed have a thin probe Gauss meter.

Barry

maxserg
12-04-2015, 12:03 PM
I will have to recone it anyway so it seems that there is no problem anyway from nickH's opinion. :)

NickH
12-04-2015, 12:19 PM
From his description of the failure, welded voice coils, sounds as if that's going to be the case anyway
Don't know if "edgewound" can do this for him or not
I'd ask, might get lucky.


I agree with you completely. Just wanted to let him know though.

maxserg
12-07-2015, 12:47 PM
The only problem is that I live north of Montreal (in Quebec) ;)

NickH
12-07-2015, 07:51 PM
Have you done any online research to see if you've got any local reconers? If the shop has a magnetizer then they'll have a gauss meter. At least they should.


Nick

grumpy
12-08-2015, 09:35 AM
https://www.trifield.com/images/GM2h%5B2%5D.jpg

$675

Seems you would likely want to use it more than once (perhaps 100s of times)
before it would be worthwhile.

Earl K
12-08-2015, 11:33 AM
The only problem is that I live north of Montreal (in Quebec) ;)

Off of Hwy 30 in Chateauguay (https://www.google.ca/maps/place/Ch%C3%A2teauguay,+QC/@45.3532185,-73.8147828,12z/data=!3m1!4b1!4m2!3m1!1s0x4cc9150bf002489f:0x6552f 8359b6df45e) is an outfit that should be able to help you out .

Belisle Acoustic (https://www.google.ca/search?q=Belisle+Acoustic&oq=Belisle+Acoustic&aqs=chrome..69i57&sourceid=chrome&es_sm=93&ie=UTF-8) make their own woofers ( or at least they had done so in the past ) for their some of their retail speaker lines .

Transparence Professional (http://tr.ca/)

They ought to be able to measure the strength of your K145's magnetic assembly for you ( since they advertise that they will do remags & recones for just about any brand of speaker ).

:)

maxserg
12-09-2015, 09:18 AM
Tahnk Earl K That's where I was referencing in my first post! :)

Dave Zan
12-15-2015, 07:24 PM
I've had in my absence my two K-145 JBL woofer coils burnt because of a problem with my old Amcron (Crown) DC_-300.
DC current went verry hard...

So how hot went the magnet structure...?

I am a little late but I will comment because no one else has mentioned an important point.
You don't need to overheat Alnico to ruin the magnetization.
In fact you don't need to heat it at all, heat just makes it easier to ruin.
DC current alone can do it, and probably has because Alnico magnets are usually specified without much safety factor.
You may have been lucky and the DC was the correct polarity to increase the field in the Alnico rather than oppose it.
So you do need to have them checked.
If they are REALLY overheated then the structure of the Alnico is permanently wrecked but that's a separate problem and practically impossible for a speaker.

Best wishes
David