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Frozen Woofer Cone - magnet slip

K

kychucky

Member
Yamaha NS 690 woofer. Good continuity and ohm reading, some sound but not much, like a weather radio. The cone does not move.

I have removed the speaker and can see through the centering hole on the back of the magnet and see misalignment down through the hole.

I can see that the magnet sections are still connected with what appears to be a contact cement. Glue oozing out from each of the 3 sections. It seems like its melted out of alignment.

Question is how do I get the magnet off to realign?
 
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Hi kychuky. I'm not familiar with your woofer but here goes. First, is the glue hard and set up, or is it runny? Has the shift occurred between the back plate and the magnet, or between the magnet and the inner plate? Can you shift the plate/magnet if you push sideways? If you can shift it, you should be able to pull the plate or magnet off using brute force or carefully wedge and shim your way around the magnet until it's loose enough to pull off. I suggest that you do a search here for "shifted magnet". You should find a few threads that deal with making a jig to recenter and glue the magnet. It's not terribly difficult. I've done two of them myself.

Walt
 
OK I have the first steel section off - the part that centers on the VC. There is corrosion. Should I remove the magnet doughnut as well? Just to be sure, clean it and glue it fresh..??
 
kychucky said:
OK I have the first steel section off - the part that centers on the VC. There is corrosion. Should I remove the magnet doughnut as well? Just to be sure, clean it and glue it fresh..??
Click to expand...
Do not remove the Ferrite ''doughnut'', as it's very easy to break/ crack.
You should epoxy it around it's perimeter, so it won't shift.
 
Ive seen the first one but not the second. - S

Q: so you just drop it down into your gig? Seems like the voice coil is at risk.
 
No, I didn't just drop them into the jig. On the first one the magnet stayed attached to the backplate. I kept my fingertips between the magnet and the frontplate and carefully lowered it into the voice coil. Install the jig so that one of the screws or a mark on the outside of the jig lines up with a mark on the woofer basket or a wire connector, and snug it down. Now, using the upper row of screws work the topplate until the pole is centered in the voice coil. You can judge position by carefully moving the speaker cone in and out by hand while feeling if it is rubbing against anything. When it is centered, I would connect it, and at a low volume, run a signal or music through it to double check its position. If it's right, keep the backplate snug in the jig, carefully loosen two screws opposite your locating mark just enough so that you can lift the jig over the magnet, and pull the jig off. This is your trial run. You can apply epoxy to your magnet, keeping the layer thin and away from the hole. Now carefully lower the jig back over the magnet and snug up the two screws that you had loosened to get the jig off, being sure to realign your locator marks. Double check for free movement. Everything should be centered. Fine tune if necessary.

An alternative to removing the jig after the trial run is to carefully brush a small bead of epoxy around the magnet/backplate joint while the jig is centered up and let it harden. Remove the jig and get the areas that were blocked by the screws. I preferred the 30 minute epoxy over the thread locker used in the second article.The epoxy gives you more time to make adjustments if you are putting the adhesive between the magnet and backplate. If doing the one-step method, it really won't matter.

Walt
 
I ended up making a jig out of 2x4 s. Not pritty but worked. I was able to pull off the ferrite magnet easily. I noticed it was the component that moved so I just heaved it off. Cleaned it an reglued that, easy. The pole piece was harder. Not much clearance and a lot of magnetic pull. What I ended up doing was pushing the speaker cone in as far as I could. This way I could get the pole piece started into the coil before the magnetic pull got out of hand. Then I set up my jig and just moved the cone by hand so it didnt make any rubbing sounds then ran a signal throught it. I applied my epoxy around the outside to secure. I figured by piling up the epoxy around the outside it would be easier to get off if something didnt work out. So far so good.
 
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