Diagnosing & Fixing Shifted Polk MW Drivers

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  • pitdogg2
    pitdogg2 Posts: 24,577
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    Dunno how to ohm it.
    Do you have a multimeter or digital multimeter?
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  • xschop
    xschop Posts: 4,701
    edited September 2022
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    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.
    Don't take experimental gene therapies from known eugenicists.
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  • plastic_avatar
    plastic_avatar Posts: 690
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    I have a nice meter, just never learned to use it =\
    The Thrifty Setups in Mah House Big thrifty stereo in the basement w/ my custom SDA-1C (built with help from kind forum members) * Beautiful 1966 MCM GE console upgraded w/ Bluetooth, Dual turntable, and Paradigm speakers in family room * Swanky 1980 Realistic system and great TEAC eq with dancing colored lights in the living room * Ye Olde college stereo in the garage
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  • pitdogg2
    pitdogg2 Posts: 24,577
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    Set it to ohms and if it reads zero it is toast. if you get a reading post it, we'll go from there.
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  • polkguitar
    polkguitar Posts: 9
    edited November 2022
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    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
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  • F1nut
    F1nut Posts: 49,801
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    You obviously didn't read this thread from the start. The MW6502 spec is 6.54 ohms.

    BTW, 3.5k would be 3500 ohms.
    Political Correctness'.........defined

    "A doctrine fostered by a delusional, illogical minority and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a t-u-r-d by the clean end."

    President of Club Polk

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  • skipshot12
    skipshot12 Posts: 1,002
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    3.5k betty betty bad…..
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  • polkguitar
    polkguitar Posts: 9
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    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.....

    bfa3afglr5z0.jpg
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  • lawdogg
    lawdogg Posts: 450
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    If that DCR info I found is wrong - thanks for correcting me. I quoted an old post on the topic...
    <3 my 3.1TLs

    I will fix your shifted magnets for free. :)
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  • polkguitar
    polkguitar Posts: 9
    edited November 2022
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    so they both measure the same ....... ?

    5ph5vb47eu0m.jpg



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

    iaez0lkgq6c9.jpg





    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

    hewgd7ryljrv.jpg

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  • lawdogg
    lawdogg Posts: 450
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    I guess I'd ask next - is it seized, or not seized? Have you tried pushing in the cone gently by hand?
    <3 my 3.1TLs

    I will fix your shifted magnets for free. :)
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  • polkguitar
    polkguitar Posts: 9
    edited November 2022
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    it is seized - no cone movement...the other known good speak moves normally




    emxakcc0kh38.jpg
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  • F1nut
    F1nut Posts: 49,801
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    Checking my official reference material the MW6502 is indeed 3.502 ohms.

    Stop with the "k"
    Political Correctness'.........defined

    "A doctrine fostered by a delusional, illogical minority and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a t-u-r-d by the clean end."

    President of Club Polk

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  • lawdogg
    lawdogg Posts: 450
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    If the VC measures the same as the good one, but it's seized, seems like a re-centering candidate to me.
    <3 my 3.1TLs

    I will fix your shifted magnets for free. :)
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  • Gardenstater
    Gardenstater Posts: 4,185
    edited November 2022
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    The MW6502 has a DC Resistance spec of 3.52 Ohms (no k). Nominal impedance of 4 Ohms is correct.
    George / NJ

    Polk 7B main speakers, std. mods+ (1979, orig owner)
    Martin Logan Dynamo sub w/6ft 14awg Power Cord
    Crown D150 amp
    Logitech Squeezebox Touch Streamer w/EDO applet
    iFi nano iDSD DAC
    iPurifier3
    iDefender w/ iPower PS
    Custom Steve Wilson 1m UPOCC Interconnect
    iFi Mercury 0.5m OFHC continuous cast copper USB cable
    Custom Ribbon Speaker Cables, 5ft long, 4N Copper, 14awg, ultra low inductance
    Custom Vibration Isolation Speaker Stands and Sub Platform
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  • polkguitar
    polkguitar Posts: 9
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    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...
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  • Gardenstater
    Gardenstater Posts: 4,185
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    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
    George / NJ

    Polk 7B main speakers, std. mods+ (1979, orig owner)
    Martin Logan Dynamo sub w/6ft 14awg Power Cord
    Crown D150 amp
    Logitech Squeezebox Touch Streamer w/EDO applet
    iFi nano iDSD DAC
    iPurifier3
    iDefender w/ iPower PS
    Custom Steve Wilson 1m UPOCC Interconnect
    iFi Mercury 0.5m OFHC continuous cast copper USB cable
    Custom Ribbon Speaker Cables, 5ft long, 4N Copper, 14awg, ultra low inductance
    Custom Vibration Isolation Speaker Stands and Sub Platform
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  • pitdogg2
    pitdogg2 Posts: 24,577
    edited November 2022
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    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.
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  • polkguitar
    polkguitar Posts: 9
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    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
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  • TomL
    TomL Posts: 10
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    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
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  • Gardenstater
    Gardenstater Posts: 4,185
    edited December 2022
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    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.
    George / NJ

    Polk 7B main speakers, std. mods+ (1979, orig owner)
    Martin Logan Dynamo sub w/6ft 14awg Power Cord
    Crown D150 amp
    Logitech Squeezebox Touch Streamer w/EDO applet
    iFi nano iDSD DAC
    iPurifier3
    iDefender w/ iPower PS
    Custom Steve Wilson 1m UPOCC Interconnect
    iFi Mercury 0.5m OFHC continuous cast copper USB cable
    Custom Ribbon Speaker Cables, 5ft long, 4N Copper, 14awg, ultra low inductance
    Custom Vibration Isolation Speaker Stands and Sub Platform
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  • lawdogg
    lawdogg Posts: 450
    edited December 2022
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    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.)

    Post edited by lawdogg on
    <3 my 3.1TLs

    I will fix your shifted magnets for free. :)
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  • robosdad
    robosdad Posts: 48
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    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
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  • plastic_avatar
    plastic_avatar Posts: 690
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    This is awesome.

    lawdogg did good, and a newbie got help. The power of the Internet.
    The Thrifty Setups in Mah House Big thrifty stereo in the basement w/ my custom SDA-1C (built with help from kind forum members) * Beautiful 1966 MCM GE console upgraded w/ Bluetooth, Dual turntable, and Paradigm speakers in family room * Swanky 1980 Realistic system and great TEAC eq with dancing colored lights in the living room * Ye Olde college stereo in the garage
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  • lawdogg
    lawdogg Posts: 450
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    Nice!!! Glad to hear it. Enjoy those SDAs!
    <3 my 3.1TLs

    I will fix your shifted magnets for free. :)
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  • robosdad
    robosdad Posts: 48
    edited January 2023
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    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).
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  • xschop
    xschop Posts: 4,701
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    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.
    Don't take experimental gene therapies from known eugenicists.
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  • Gardenstater
    Gardenstater Posts: 4,185
    edited January 2023
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    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?
    Post edited by Gardenstater on
    George / NJ

    Polk 7B main speakers, std. mods+ (1979, orig owner)
    Martin Logan Dynamo sub w/6ft 14awg Power Cord
    Crown D150 amp
    Logitech Squeezebox Touch Streamer w/EDO applet
    iFi nano iDSD DAC
    iPurifier3
    iDefender w/ iPower PS
    Custom Steve Wilson 1m UPOCC Interconnect
    iFi Mercury 0.5m OFHC continuous cast copper USB cable
    Custom Ribbon Speaker Cables, 5ft long, 4N Copper, 14awg, ultra low inductance
    Custom Vibration Isolation Speaker Stands and Sub Platform
    · Share on FacebookShare on Twitter
  • xschop
    xschop Posts: 4,701
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    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?
    Don't take experimental gene therapies from known eugenicists.
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  • skipshot12
    skipshot12 Posts: 1,002
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    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?
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