Diagnosing & Fixing Shifted Polk MW Drivers

pitdogg2

Dunno how to ohm it.

Do you have a multimeter or digital multimeter?

xschop

If you can see the cone visibly move forward by placing a AA battery across the positive terminal to positive end of battery and neg to neg. The coil is still good. Just got to figure how to move it back in place without damaging it.

Scratch that. Only will happen if magnet back in place. Wally World has cheapie volt meters with ohms reading.

plastic_avatar

I have a nice meter, just never learned to use it =\

pitdogg2

Set it to ohms and if it reads zero it is toast. if you get a reading post it, we'll go from there.

polkguitar

acquired a sweet set of 5B's. one speaker is essentially non working - the tweeter is tweeting but the mid driver, 6502 - while measuring correct at 3.5k, it is nearly inaudible. Swapped said driver into the other speaker to rule out a crossover issue and the problem went with the driver ........ so, a search led me here and now suspect this driver is seized...is anyone currently able and willing to help save this driver ? location is US midwest...thanks much

F1nut

You obviously didn't read this thread from the start. The MW6502 spec is 6.54 ohms.

BTW, 3.5k would be 3500 ohms.

skipshot12

3.5k betty betty bad…..

polkguitar

i did see contradicting info but found 3.5k (4k) in several places on the net...so not sure which is correct.... i will measure the other - working- driver and see.....

lawdogg

If that DCR info I found is wrong - thanks for correcting me. I quoted an old post on the topic...

polkguitar

so they both measure the same ....... ?





also a couple other web infos. (came across others as well) suggesting the 5b circa '85-89 was the 6502 speaker with 4k resistance.







the set i have are '86 (maybe '88 hard to read) , and look like this set with inset speaker frames, no fuse, beveled edge, plastic grill posts etc

lawdogg

I guess I'd ask next - is it seized, or not seized? Have you tried pushing in the cone gently by hand?

polkguitar

it is seized - no cone movement...the other known good speak moves normally

F1nut

Checking my official reference material the MW6502 is indeed 3.502 ohms.

Stop with the "k"

lawdogg

If the VC measures the same as the good one, but it's seized, seems like a re-centering candidate to me.

Gardenstater

The MW6502 has a DC Resistance spec of 3.52 Ohms (no k). Nominal impedance of 4 Ohms is correct.

polkguitar

that all adds up then - the reason for wanting to fix is to try and keep it all original and didnt see a working 6502 for sale - there is the above modern 'clone' if all other options fail.....thanks for any help with the issue...

Gardenstater

Where did you look? I see a bunch for sale on eBay. Pick one up and repair your old one and you'll be able to use one of them as the backup. https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2380057.m570.l1313&_nkw=mw6502&_sacat=0

pitdogg2

polkguitar wrote: »

that all adds up then - the reason for wanting to fix is to try and keep it all original and didnt see a working 6502 for sale - there is the above modern 'clone' if all other options fail.....thanks for any help with the issue...

Those from Midwest Speaker I would steer clear of unless every option is completely exhausted. If you do decide to go that route buy two and change out both.



MW6502 is or was very common on the auction site. It's probably the most common driver polk made.

polkguitar

there are a few on fleabay, couple have cone edge rot of some kind, and there is one good one that is '91 - that should work, but the speakers would no longer be a stock set with consecutive dated drivers etc....but may pick that one up...thx

TomL

Hi LawDog,

Thank you for this step-by-step post. It's really well done.

Does a seised-driver make no sound at all? I dropped one of my 5A's and the result was a woofer that sounded anemic.....no lower frequencies just upper. I ordered an MW6500 on Ebay and it sounds exactly the same as the one I dropped. The seller said it worked "perfectly" but maybe he/she meant perfectly anemic. The box was not marked fragile or do not drop so maybe it was damaged in shipping.

Would your step-by-step fix the problem I'm having?

btw, I did check these damaged woofers on the other crossover and it was the same, so both speaker crossovers are fine.

Tom

Gardenstater

It would vary depending upon how badly shifted the pole piece is but one thing for sure, I would not be playing them anymore as soon as I got wind of a problem because there is too much chance of wearing off the enamel coating on the voice coil wire turns that way.

lawdogg

Can you check two things:
1 - does the cone move in & out freely without rubbing;
2 - does the voice coil measure the correct value?

The reason I ask is because I've seen drivers make noise but sound funny when ...

-They are seized but with good voice coils. They can generate high-frequency sounds but because it's locked up, it can't generate low-frequency sounds. Instead, the electrical current turns into heat which can unravel the voice coil.

-They have overheated voice coils that have melted insulated and shorted windings. These will measure low DCR and generate some sound, but it will not sound right.

(And of course crossover issues can cause the loss of highs, but I am assuming your crossover is OK since I'm assuming the driver sounded fine before you dropped it.)

robosdad

Have some Polk Monitor 10 and like them, and just got some SDA-1c today and thanks too this tutorial just fixed two of the three shifted magnets. Used Loctite red instead of the epoxy you describe only because I read your post after going hardware store shopping. Anyway, I just wanted to jump in and add thank you for posting all this. It was invaluable. Stuff like this is posted a few other places, but yours is the only complete one I’ve seen. At each step I would ask “yeah, but how do you ABC” and your next sentence was “…and here’s how to ABC”. Can’t wait until the glue cures so I can finally here these speakers!!

TL,DR = thank you for this invaluable tutorial

plastic_avatar

This is awesome.

lawdogg did good, and a newbie got help. The power of the Internet.

lawdogg

Nice!!! Glad to hear it. Enjoy those SDAs!

robosdad

Thanks again and a follow-up. I didn't take a photo of the inside bottom mid driver. Top inside driver is black to + (red painted dot), and white to ground. But all I have left for the bottom inside driver is two black wires? Huh? Did I mess up? If not, how do I know which one goes where?

Edit: Ack! Nevermind! I see now that one of the black wires has a small white band on it, and when I go back to my photos I see that I did have a photo of that one (white band marking that black wire for the ground terminal).

xschop

This is a great write-up. I would add that if the pole-piece to magnet is not glued concentric from factory by a difference of 20% error, I would disassemble and re-set/glue them back.

Recently @pitdogg2 made a statement that reinforces my experiences repairing these MWs. Being, every one I've dealt with that had the cones/surrounds torsionally twisted, also had their magnets/pole-pieces factory glued at bad concentricity. I believe this also negatively affects the driver's performance.

Gardenstater

And then there's mine, with the voice coils that are not perpendicular to the front plate. My alignment fixture has finally brought the one into alignment over time (like braces on a kid lol), but the one that had the cracked spider repaired by me with super glue is proving more stubborn. Not unexpected.

What does it look like when the cones/surrounds are torsionally twisted? Does it show up as a distortion in the surround?

xschop

A torsionally twisted surround looks slightly crinkled around the edges. If it's severe, it can tear the spider at the VC and even crease the cone surface, from what I've encountered. The worst ones had the most offset mag/pole-pieces from factory.

Wouldn't this big offset create an uneven Gauss/pull in the gap?

skipshot12

Hmmmm…
I saw my passive had shifted from over the years and turned 180° when reinstalling it.
Wondering if I should have done the same with the MW’s?