Diagnosing & Fixing Shifted Polk MW Drivers

lawdogg

Have been tied up with work and have been remiss in answering PMs about fixing shifted magnets.

Hoping it isn't gauche or obtuse to bump my guide to the first page to make sure it's visible to those with questions.

May all our drivers stay aligned in 2024!

lawdogg

I've just confirmed that the same general process will work for a shielded MW7015 driver. These were used as the powered bass drivers in for example the CS1000P and ... they can shift. From what I understand they are VERY hard to find, so it may be useful to know they can be re-centered.

They are different than the old MW drivers in that (a) they are shielded with a bucking magnet and (b) the basket flange O.D. is larger than the magnet O.D.

I found that I could squeeze the basket frame in my same re-centering jig, and drill some new screw holes to re-center the magnet & pole piece relative to the frame.

Here you can see what's under the shielding, with my epoxy on the upper & lower magnet edges in the process of curing.



All back together waiting to go back to its owner.

pitdogg2

Very nice @lawdogg 👍

xschop

Good save. What are the physical dimensions of the magnet?

lawdogg

Dang I didn't measure it before I packed it back up. I should have. I don't want to guess - it was probably a hair smaller than the normal MW diameter.

Also, the voice coil former is vented. This led to an unfortunate scrunch upon shifting.



FWIW I did measure the voice coil: ~3.3ohms DCR.

subkron

@lawdogg Sorry for dredging this up, but what about the comment:

"(You can fix slightly crooked / sagging spiders using a spray bottle of water, two chopsticks, a rubber band, and a heat gun, but I won’t get into that here.)"

I have a sagging spider and am interested in trying to get it straightened out, any details?

Astrl55

... a spray bottle of water, two chopsticks, a rubber band, and a heat gun ...

Pretty sure these are the exact items MacGyver once used to thwart a terrorist attack.

lawdogg

subkron wrote: »

@lawdogg Sorry for dredging this up, but what about the comment:

"(You can fix slightly crooked / sagging spiders using a spray bottle of water, two chopsticks, a rubber band, and a heat gun, but I won’t get into that here.)"

I have a sagging spider and am interested in trying to get it straightened out, any details?

You put the chop sticks under the cone (parallel to each other on opposite sides), and the rubber band around them over the top of the cone - this pulls the cone up and articulates the spider upwards. Then, spray some water onto the spider to get it a little damp but not soaking. Use the heat gun (carefully) to evaporate the water while the spider is still being held up. Once dry, remove the chop sticks and if you did it correctly the spider will be level again.

Try at your own risk!

skipshot12

^ Geez o I love this site….

lawdogg

Ok another warning for folks ... shipping seized drivers. The problem is, when the voice coil is seized, the cone is effectively stuck in place. No matter how well you might pack the driver, unless you add padding AROUND THE UPPER BASKET AROUND THE SEIZED CONE, to prevent the upper rim of the cone from impact, you risk damaging the cone in transit.

Here's the result of an improperly packed seized driver:



The ability of the voice coil to travel up and down at EXACTLY 90 degrees while staying PRECISELY in the middle of the center axis is extremely important. There is even LESS clearance between the voice coil and the hole through the basket - almost none. When the cone gets deformed, the voice coil shifts away from the center axis AND doesn't move at exactly 90 degrees through the full stroke.

In the above example, the voice coil was so crooked it arrived stuck in the frame.



The only way I can fix this is by manually massaging by hand the cone back to shape. But once these cones crease, that's permanent and you can only hope you'll get it "good enough" to not rub.

Here it is almost - but not quite - back to center. Even this was enough to rub.



Ultimately I got it good enough to work - at least in testing - but we're not always that lucky.

PSA over!

xschop

Good write-up. I've found that these microcrystalline formations seem to build up over time (electrical properties/galvanic corrosion). Appear to be worse on base-plates that have more striations from the plate-punch process at factory. If you dremel/grind the galvanic coating off the ID, then it shouldn't form back. It's only several microns thick.

Gardenstater

Yes! Have you ever drilled out the staked rivets and repositioned the front plate?

lawdogg

I've never messed with those rivets, no. Luckily haven't had anything needing that kind of surgery (yet).

I also wanted to add, another way to reduce the risk of cone damage in shipping is to REMOVE the magnet assy and pack it separately from the basket / cone assy (making sure each is protected).