Magnet replacement on Mission 707 driver

[flyingtele]

Recently I acquired a pair of Mission 707 speakers, in need of a re-foam. However one of the two bass drivers needed more than a re-foam as its cone was siezed. Upon removal, of the cabinet I found this. Looks as if the speaker has at some point been dropped, as the magnet material is cracked right through and has a chunk missing. Here are the step by step photos of me replacing the magnet, as I thought some would be interested to see the process.

[flyingtele]

I removed the pole piece and then remove all the broken magnet material. The magnets in speakers are secured using an epoxy adhesive. This can be softened by applying heat, I use a pocket butane blowtorch, but you can also use a stove top (only if you have an understanding mrs!) or hot plate etc.
I wear a pair of welding gloves to handle the heated magnet. Once the epoxy is softened, you can wedge a sharp flat-blade screwdriver between the magnet and the pole piece to separate them. I make sure to scrape as much of the old adhesive off as I can while its still soft. These two photos are of the pole piece and basket with magnetic material removed.

[flyingtele]

Now I need to find some suitable replacement magnets. I have a pair of 5.25" kevlar Skytronic drivers lying about. I bought them because they were cheap and wanted to see if they were any good, but turned out to be crappy. They had something weird going on the mids, so I painted the cones black to add a bit of mass and they sounded a bit better but were very inefficient. So I thought I would sacrifice some working not-so-good speakers to repair some better non-working speakers. Sounds fair to me!
I removed the magnets in the same manner as the broken Mission, but just took more care not to break or chip the magnets. First photo shows the donor drivers (one is partially dismantled and missing its cone). Second pic shows the two magnets removed and cleaned up. The magnets are slightly thicker than the Missions but only by .3 mm, and are about 7mm larger in overall diameter. So still suitable, but I am changing both for the sake of keeping things consistent and matched.

[flyingtele]

Now I need to re-assemble the magnet assembly. I mix up some Araldite and glue the magnet to the top plate, which is attached to the basket. Once it is cured, I glue the pole piece to the back of the magnet. This is the hard part, because the magnet is still 'charged' it develops a very strong pull once the pole piece is inserted as it completes the magnetic circuit. And because of this, it is necessary to put shims around the gap to keep the gap width even right around, or else the pole piece will simply stick to the inner edge of the top plate. First pic shows the magnet glued to the top plate. In the second pic are the shims I made. I use calipers to measure the inner diameter of the top plate and the diameter of the pole piece. I then take the difference and divide by two to work out the correct gap width, and then need shims of the same thickness. I found some thin sheet aluminium that when folded over and a strip of cardboard inserted in the middle was the correct thickness.

[flyingtele]

Now I put the shims in the gap and bend them into a vague hook shape, and tape them in place. Then its simply a matter of applying some Araldite to the magnet and carefully inserting the pole piece, wile being careful not to be pinched by the magnet and making sure the shims stay in place. The two pics below show the shims in place and the pole piece inserted and the second pic is of the finished magnet assembly. Once the glue is cured then its just a matter of removing the shims and then putting the cone back in place.

[flyingtele]

Because the VC was stuck in the gap, it appears the cone suffered some damage, the are a few crease marks and where it attaches to the coil it has been creased and lost strength. I smoothed out the creases as well as I could, and applied glue to parts to regain some lost strength. After the glue had dried it was much stronger and had no noticeable weak spots any more. The photo shows where I applied the glue. The spiders on both drivers had lost shape and the cones appeared to be sunken in, so when I re-assemble I will be using new spiders.

[Andersonix]

Nice work so far! I like bringing old things back to life.

Perhaps this is the time to modify the cones into your own 'slit paper' cones (like a Revelator)?

[AEIOU]

I have plenty of suitable replacement magnets! Too bad you are so far away.

[flyingtele]

Originally Posted by Tosh

Nice work so far! I like bringing old things back to life.

Perhaps this is the time to modify the cones into your own 'slit paper' cones (like a Revelator)?

There will be plenty of time for that yet. First thing is to get all the bits to re-assemble them (new foams and new spiders), and then I'm sending them away to be de-gaussed and re-magnetised. When you open the magnetic circuit, heat, or knock the magnet it loses a bit of magnetism each time. 'Re-charging' the magnets will ensure maximum magnetic strength for good efficiency, and more importantly making sure both drivers are the same.

And to AEIOU, maybe you should send me some lol. I would look into sourcing some magnets but this is the first time I have replaced a cracked magnet. All other times I have done this is either to machine the gap width for a thicker coil, or to re-align the pole piece. Where did you get your magnets from? Also, are they pre-magnetised, or just the ceramic material?

[AEIOU]

Originally Posted by flyingtele

And to AEIOU, maybe you should send me some lol. I would look into sourcing some magnets but this is the first time I have replaced a cracked magnet. All other times I have done this is either to machine the gap width for a thicker coil, or to re-align the pole piece. Where did you get your magnets from? Also, are they pre-magnetised, or just the ceramic material?

They were all taken from, I removed them from dead, burned, blown, woofers and tweeters. Yes, all magnetized, but that stack is weak compared to some of the neodymium magnets I've acquired.
They represent about $600 worth of drivers!