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Old 2nd Jul 2020, 6:10 am   #1
PWH
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Default How to separate a speaker magnet from its chassis

I have a blown, sealed back mid range unit which I would like to try and revive without ripping the cone out from the front. I have tried fishing around inside via the connectors to see if a wire was off but no, it is O/C. Obviously went to a good party.

It looks like the magnet is glued on. If I put it in the oven to try and melt the glue, would it ruin the magnet?

It is marked 10-500 - 8 ohm 25 W. Probably Plessey, Jensen or Coral from the 70's if anyone should have one.

Thanks
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Old 2nd Jul 2020, 8:54 am   #2
vidjoman
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Default Re: How to separate a speaker magnet from its chassis

Unfortunately for you the cone assembly will have been the last thing fitted, except for a dust cap. If you managed to 'melt' the glue and remove the magnet there will be no way of getting the coil back in the correct place when you re-assemble, other than hit and miss.
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Old 2nd Jul 2020, 9:08 am   #3
GrimJosef
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Default Re: How to separate a speaker magnet from its chassis

Worse than that, if you were to loosen or crack the glue joint the magnet would instantly slide sideways and attach itself to the speaker's central steel pole-piece trapping, and probably crushing, the voicecoil in what had been the narrow cylindrical gap it normally moves in. I speak from experience here (I didn't deliberately crack the glue - it happened when I brought the speaker from long-term storage in someone else's very cold shed into my warm house). I realised immediately that the speaker was ruined but I did spend a little while trying to see if I could prize the polepiece and magnet apart again. I couldn't. Not even by hammering old screwdrivers into the gap to act as wedges. I did manage to crack pieces off the (ceramic) magnet. But not to open up the gap.

The cone has to come out from the front.

Cheers,

GJ
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Old 2nd Jul 2020, 9:19 am   #4
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Default Re: How to separate a speaker magnet from its chassis

Oh well, worth a try. maybe one will turn up some day..
Thanks..
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Old 2nd Jul 2020, 9:24 am   #5
GrimJosef
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Default Re: How to separate a speaker magnet from its chassis

The usual approach is to sacrifice the flexible 'surround' at the outer edge of the cone (you could try splitting the glue joint between the surround and the chassis but the fragility of the surround makes this challenging) and to source a new one online. There is a wide range available. Otherwise, as you say, a complete driver may turn up, either on its own or in another speaker cabinet. In my case the good news was I found someone breaking a set of similar speakers to mine and selling the parts separately. The bad news was that the individual mid-range driver cost a three-digit sum (that was a fair market price) and I had to drive to Scotland to collect it !

Cheers,

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Last edited by GrimJosef; 2nd Jul 2020 at 9:33 am.
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Old 2nd Jul 2020, 9:40 am   #6
cathoderay57
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Default Re: How to separate a speaker magnet from its chassis

Hi PWH, you might as well have a go at removing the cone, since you have nothing to lose. I did this on an elliptical speaker and fixed the o/c speech coil. The set is still in daily use and the speaker has worked faultlessly since. Details in this thread https://www.vintage-radio.net/forum/...85#post1118085 Cheers, Jerry
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Old 3rd Jul 2020, 12:43 am   #7
PWH
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Default Re: How to separate a speaker magnet from its chassis

Quote:
Originally Posted by cathoderay57 View Post
Hi PWH, you might as well have a go at removing the cone, since you have nothing to lose.

I think you're right. I'll put it in the to do pile
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Old 3rd Jul 2020, 11:49 am   #8
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Default Re: How to separate a speaker magnet from its chassis

As has been said - don't try. You'll cause more damage than help any repair work!

That said, when I worked at Goodmans, we occasionally had the requirement to separate magnetised magnets from chassis on prototype builds and we had a special knife that had been filed down in the centre of the blade so that it was very thin. You positioned it between pole plate and magnet and whacked it with a hammer until the two separated!

This knife was almost a company heirloom and had been in use for over 30 years, so I was not popular on the day I whacked it with a hammer and the blade snapped in half...
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Old 6th Jul 2020, 11:41 pm   #9
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Default Re: How to separate a speaker magnet from its chassis

I had a go at repairing it but the coil was burned out. Being a sealed back unit is almost impossible to reconstruct. so if anyone has a spare please let me know
max diameter 120mm marked 10-500 - 8 ohm 25 W. Probably Plessey, Jensen or Coral from the 70's
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Old 7th Jul 2020, 4:11 pm   #10
Radio Wrangler
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Default Re: How to separate a speaker magnet from its chassis

And it has no separate surround to replace, it's formed as part of the cone.

Some very careful surgery and reconstruction might be able to be done.

In the past, I've taken dust caps out and inserted a lot of fine wires to shim the voicecoil former around the centre pole.

With the K0-40 sealed midrange, it has a dust cap that goes inside the former. I drilled tiny holes in the dust cap, around its edge, to take my shimming wires and then removed them when the new surround was glued firmmly in place. A few spots of PVA sealed the holes in an unobtrusive way.

It could be rebuilt with a modern surround, but what will that do to the speaker's characteristics? Well, not as much as an open voice coil does.

David
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Old 9th Jul 2020, 8:37 pm   #11
PWH
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Default Re: How to separate a speaker magnet from its chassis

Thanks David,
I think more trouble than its worth. The speaker is only a low cost mid-fi unit. I am sure one will turn up eventually.
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