MW6510 shifted magnet repair

geoff727

I've seen other threads containing tips on how to repair a driver with a shifted magnet (actually a shifted pole piece), but never one with pictures, and never one specific to the drivers that many of us are using, the Polk MW series.

I recently sold a pair of MW6510's on ebay, to another forum member. Despite my careful packing, one of them still ended up with a shifted pole piece and frozen voice coil. I know that others have had unsuccessful attempts to repair these, so I decided to learn for myself why, and document with some pictures, for anyone wanting to try in the future. If the uploads don't work, I'll post some jpeg's.

First of all, the parts in the pictures are labelled as follows: FP=front plate, PP=pole piece, BP=backplate. The back plate and center pole piece are one assembly. Standard speaker parts.

When I disassembled the driver, I assumed (wrongly) that the front plate was held to the driver basket with adhesive. It's not. It's held on with 4 metal posts.... like rivets. I was tapping away to separate the basket and magnet assembly and put a ding in my tool. Separating these parts also resulted in the basket and driver cone distorting. Driver ruined at this point!







As you can see, there's a gasket between the basket and the front plate.





The front plate, magnet, and back plate/pole piece can come apart by gently tapping something wedged in between the parts.



In reality, this is what I should have done from the start. However, remember that the pole piece "wants" to be in the magnetic gap, and bringing it straight out of the gap without it slamming to one side of the gap is EXTREMELY difficult. In fact, one needs android precision to do it. If you try this, just know that the pole piece will probably be rubbing on the inside of the voice coil former as you pull it out of the gap. Obviously, the voice coil is pinched by this thing anyway, otherwise you wouldn't be taking it apart in the first place.

The best way to do this is to separate the back plate from the magnet with very gentle rubber mallet taps on the wedged piece, and progressivley using 'stand-offs' to gradually increase the distance between the back plate/pole piece and the magnetic gap. Be careful not to get your fingers in between the two. One small slip and the BP and magnet will slam back together with about 100 lbs. of force. It may not be that exactly, but your fingers will think so.



When I got the driver apart, you can see that the voice coil had one small spot of damage. This was due to my clumsiness, not the fact that the pole piece had shifted.



Looking at how the back plate, front plate, and magnet are held together, you can see the glue rings in between the 3. This is obviously the glue that fails when the driver gets whacked. That allows the pole peice to slam to one side of the magnetic gap, pinching the voice coil and coil former in position. Even though the glue may have failed, though, the strong magnetic attraction forces still keep the magnet and BP/PP assembly sandwiched together.





Now, say you didn't do what I did taking the whole magnetic assembly off. Instead, you successfully separated the back plate from the magnet and brought the pole piece out (TRICKY AND DANGEROUS TO FINGERS!). The real questions are....

1. Can you separate the magnet, then reglue it to the front plate to give it a stronger bond? Ya, probably.

2. Can you reglue the back plate to the magnet, inserting the pole piece into the magnetic gap to get a perfect ~1.5mm tolerance all the way around for the voice coil/coil former to go down into? For a non-android like me, definitely not.

As soon as you insert the pole piece into the magnetic gap, the natural tendencies of the magnetic field are to make the pole piece snap to one point of the inside of the gap. And, everyone knows what it's like trying to get two magnets to come together in the first place. Things get wobbly and inaccurate.



So, that's about 10,000 words just to say.....get a new driver. Oh well, I thought it might interest somebody.

Cheers all,
Geoff

geoff727

Here's the jpeg's.

kawizx9r

Very nice write up, although I read on older threads (and one recent one) on here that the separation was not between the basket and front plate (because the 4 metal posts do their part and hold the piece together firmly) but instead glue failure as illustrated in your picture "10.JPG" those are the common problems of "frozen drivers/etc" because of the PP assembly shifting. I might be way wrong but this is all stuff I've gathered in many a readings

I still say this is very informative, and am glad you took the time to post it.

geoff727

kawizx9r wrote: »

the separation was not between the basket and front plate (because the 4 metal posts do their part and hold the piece together firmly) but instead glue failure as illustrated in your picture "10.JPG"

Yes, that's right. I was interested in seeing the exact failure point. Now I know.

polkie4life

geoff727 wrote: »

Yes, that's right. I was interested in seeing the exact failure point. Now I know.

I had the same thing happen to one of my sub drivers when i shipped my speakers. I was lucky enough to have a speaker maker in close proximity to repair it for me.
He made comment that the glue is applied in two parts, one being an activator. The glue was still tacky so his thought was that there was not enough of one of them applied. That and the fact they were never designed to be dropped. :mad:
In the end he needed to make another voice coil as it had damaged it, and a jig to align it and reglue it.

zarrdoss

So put some kind of epoxy on both sides of the magnet to stop this from happening?

Pauly

Got any other spares FS or are those yours on ebay now?


Pauly

gdb

I will buy the pieces from you if you haven't tossed them yet !!Sent a PM;)

geoff727

Pauly wrote: »

Got any other spares FS or are those yours on ebay now?

I do have a pair on ebay now. My last two, in fact. The nicest ones I had. I packed them differently, though. I sliced some styrofoam pieces in layers to fit around the magnet structure and basket with a nice tight fit. If this doesn't pretect them, I don't know what will.

Pauly

geoff727 wrote: »

I do have a pair on ebay now. My last two, in fact. The nicest ones I had. I packed them differently, though. I sliced some styrofoam pieces in layers to fit around the magnet structure and basket with a nice tight fit. If this doesn't pretect them, I don't know what will.

Yeah i got my eye on them. I seen you had a pair last week or so as well.


Wish me luck

Pauly

geoff727

Same pair, actually. I had to re-list them. Buyer had an emergency come up.

Pauly

geoff727 wrote: »

Same pair, actually. I had to re-list them. Buyer had an emergency come up.

Guess i get a second shot at them then


Pauly

inspiredsports

Nice photos and commentary .

Their pricing is not cheap, but about a year ago the folks at http://www.midwestspeaker.com repaired a shifted magnet for me on one MW6503 driver for one of my SDA-2B's.

I'm fairly adept at do-it-yourself, but destroyed a similar MW6509 from my SDA SRS 2's and decided to leave it to someone with the proper tooling for the rarer MW6503. After repair with new glue, the magnet will not shift during shipment whereas buying used on eBay seems to invite that problem.

When mailing old drivers, thick layers of bubble wrap around the magnet is recommended, and fastening drivers face-to-face with 1/4" nuts & bolts was done when shipping 2 drivers, or to a piece of 7"X7" thick cardboard or luan plywood when shipping just 1 at a time.

kawizx9r

Thought I should bring back an informative thread. Saw another thread that mentioned magnet-shifts but this one was much more thorough

wbcooley

Thanks for the info and pics. Was able to repair mid/bass driver on monitor 10a. Never jarred or dropped these speakers, but instead increased amp power from 100 to 210 watts and cranked them up. Sound was amazing for days until 30 year old brittle glue couldnt take it anymore. Took back plate and magnet off with no damage to voice coil, cleaned all surfaces and reglued magnet to front plate. Cut out dust cover to see what i was doing and pushed speaker from bottom to clear voice coil from post contact when putting back plate on. Used strips of xray film as spacers between post and hole in front plate and with a little tapping i was able to center pole. Glued edge of back plate to magnet only, released voice coil into gap glued dust cover in place. I was back in business. I would suggest glue to the edges of both plates to magnet for those who have not had problems and dont want any. Great thing to do before shipping speakers

Schurkey

Similar thread, with photos including photos of the home-made alignment tool:
http://www.polkaudio.com/forums/showthread.php?135716-Let-s-Try-to-Fix-a-MW6511-Damaged-in-Shipping...