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Seeking advice for repair of separated subwoofer Magnet

J

Jacobclem

New Member
JSUPn6A.jpg
I have no idea what I'm really looking at. I figured it might be easier to fix to a educated eye. I dropped the unit like two feet and the magnet was detached from cone. I'm a woodworker and some part of me wants to sand down on the magnet to remove residue and stick epoxy on it but I know that's like to damage magnet and I'm not sure sticking it back together means it'll work.

Please educate me friends I love audio equipment but I have no technical knowledge

edit: this is from an audio engine s8. It powers on and emits a loud clacking noise when unit is connected.
 
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It looks like the magnet itself is unbroken, so that's good. In theory, if you could install the magnet in the exact position it was before, the peaks and valleys of the original adhesive would match up. But that's the tricky part. If you cement the magnet back together and it's out of alignment then you've just got a mess. From your picture, it appears as if the voice coil former (the piece that the coil is wound on) is a fairly substantial material like aluminum or plastic, not paper, so that will make things a little easier. Also, there's quite a large gap between the magnet and the pole piece, although I suspect the diameter of the voice coil is only a little larger than the latter, so you'll still need to be careful. You don't want to lower the magnet down at an angle since that can damage the coil. If you can manage to get the magnet back on the frame, before you cement it, push the cone in and out several times. It should be completely silent. If you hear any sound at all, it means the coil is rubbing and you need to reposition the magnet.

Jacobclem said:
edit: this is from an audio engine s8. It powers on and emits a loud clacking noise when unit is connected.
Click to expand...
Does it do that without the magnet installed, or after you tried to put it back together?
 
curious if there is a cover on the back side of the magnet that can come off to get you access to the hole in the middle of the magnet. If it does, you can use paper shims to center it when gluing the magnet back in place. Not sure what the original adhesive would have been, thinking its probably not epoxy but I don't honestly know for sure. Either way, once the adhesive is set you can pull the shims out and put the back cover on again.
 
gadget73 said:
curious if there is a cover on the back side of the magnet that can come off to get you access to the hole in the middle of the magnet.
Click to expand...
It should be one solid piece, backing plate with pole piece. And if you shim a heavy magnet against a lightweight cone, I think you're more likely to flex the cone than to move the magnet.
 
Even if you just do it visually, if there was some way to confirm where the thing sits it would help. By the pic it doesn't look like the usual magnet structure I'm used to. It appears completely hollow in the middle instead of the solid piece of metal with a gap for the coil to sit in. Wasn't sure if that back cover where the vent is just glues to the magnet or what.
 
Vented pole pieces aren't uncommon. While it may not originally have been all one piece of metal, I suspect that there's no easy way to disassemble it further without destroying it.
 
There is a thread about centering a shifted magnet on a Polk midbass driver.
They used a piece of PVC pipe that was a larger diameter than the magnet.
Then there were 4 holes drilled around the pipe. Bolts were put into the holes with a nut on the outside of the tube, and one on the inside for each bolt if I remember right. Then the magnet was adjusted with that setup to center it so the voice coil didn't rub. I don't remember if the person removed the dust cap or not to shim the VC though. Maybe not.
It looked pretty involved.
It might be easier to just replace the subwoofer.
 
All good advice above . . .
  • The magnet pole piece must be perfectly concentric with the voice coil--it must not scrape/touch the magnet/pole (or anything else) at any point when you press on the cone, that is, throughout the cone's entire "excursion"; iow, the coil must be free-floating. You can also test this using a D-cell battery to move the coil. This is similar to replacing a deteriorated "surround"--the foam or rubber ring--which joins the outer edge of the cone to the speaker frame (the "basket"). Some surround replacement kits include a strip of thin acetate sheet which you wrap around the voice coil as a shim to keep the coil concentric and not touch the pole piece when you position and glue the outer edge of the surround to the inner edge of the basket (kits include a special glue for this). Of course once you have it positioned properly and the glue sets you have to remove the shim (pull it out) from the rear; if there's a vent cover at the rear, as does yours, it will need to be removed (pried off) to do the shimming and then glued back in place.
  • Regarding cleaning off the old adhesive from the mating surfaces of magnet and pole assembly, I think you could scrap it off with a razor blade rather than sand it. (Deal with magnet grabbing the razor blade, I'd try it with a razor blade scraper.) I would use a good contact adhesive such as 3M brand high strength as used for car trim (got mine at Walmart, spray can, follow directions and get it right the first time) rather than use epoxy since epoxy might be brittle and, given a subwoofer's (a/k/a bass cube's) heavy vibrations, not hold over time. Just pure speculation I admit but if you're handy I think repairing it is worth a try. As noted above, whole magnet is good: had the magnet cracked you'd be out of luck since then you'd have two magnets and that wouldn't work.
Good luck, have fun!
 
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spray can adhesive for a subwoofer magnet? hell no.

yes to 2 part epoxy and yes to sanding the old glue off, i would CAREFULLY grind it off myself so that there is a good tooth for the epoxy.

i doubt i'd bother repairing it myself, the driver looks to be nothing special. i'd do a bunch of research and modeling and pick up a suitable replacement from parts express.

i think even starting from scratch with a new driver and building a box to match and harvesting the plate amp would be a better chance for success.
 
Looks like an easy fix from here. I'd clean the old glue (or leave it and use it as a guide for aligning), and use a slow (24hs) epoxy. Once glued, you have some hours to center the VC by feel, even play some test tones to check it's working before the epoxy cures.

Use the glue far from the magnet gap or the VC. You don't want some epoxy getting in there.

Anyway, there could be problems not visible from here, like a bent basket deformed by the magnet weight, or a broken (open coil) VC
 
elnaldo said:
Use the glue far from the magnet gap or the VC. You don't want some epoxy getting in there.

Anyway, there could be problems not visible from here, like a bent basket deformed by the magnet weight, or a broken (open coil) VC
Click to expand...

yeah, that's too many variables for me to waste my time on. just like a wrecked car, 99% of the time there is more damage than you can see. i'd chuck it.
 
I'd just fit the magnet in place, align it without glue, and check if it works. If it works fine, you can glue it later.
 
Welcome to AK your first post
This looks like a fairly large woofer maybe a 12 inch guy ,and a 2 inch voice coil ,the basket is probably stamped steel and the magnet looks like it at least a 5 inch diameter.so fairly heavy and powerful
The glue job when it was assembled was not well spread around and when pressed did not have enough pressure for it to spread around,sadly it is a lower quality job,
It can be fixed but you will need a big press that can attach it back again as the magnet is probably going to fight you to even try holding it down with the magnetic force against the pole piece
As others have mentioned aligning the voice coil gap will also be hard without the press
Btw what brand is this woofer and perhaps you can find a replacement easily and will be the cheapest way to fix it
Good luck
 
I don’t think that’s the voice coil we are seeing. It’s just a ring of steel. I pulled apart a fried MTX subwoofer magnet and it was multiple pieces like this one. I would just put it back together and I bet it works unless the drop caused other pieces to come loose.
 
That driver looks reparable. The biggest hurdle will be making a jig to center the magnet when epoxying it to the plate attached to the rear of the basket.

Magnets separating from the basket is a common problem with vintage Polk drivers (and some other manufacturers). The Polk Forum has a great thread on how to repair separated magnets, including what materials are best.
See https://forum.polkaudio.com/discussion/197152/diagnosing-fixing-shifted-polk-mw-drivers

The topic is also discussed here on AK. Search for POLK SHIFTED MAGNET in the Speaker sub-forum. For example, this thread looks interesting: https://audiokarma.org/forums/index.php?threads/my-polk-mw6500-shifted-magnet-speaker-repair.953406/
 
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