Magnet Fell Off MW6502

decato
decato Posts: 186
in Vintage Speakers
Hi all,
I purchased a pair of Polk Monitor 4s off eBay. They are dated 1989. The packaging was superb, and I saw no damage to the box. When I took the one cabinet out of the packaging, I heard a thumping from within as I set it on the table. I immediately knew what it was but couldn't believe it. The magnet didn't shift; it fell off the driver and was rolling around inside the cabinet. The coil is damaged, so this particular driver will go out in the trash this week.

It takes a lot of force to pry a shifted magnet off the driver, so I'm curious as to what happened. Did this phenomenon happen to any of you? Did UPS drop it off a several story building?

Thanks,
Brian
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Comments

  • pitdogg2
    pitdogg2 Posts: 25,412
    edited May 2023
    If the magnet was already compromised it could pop off if dropped and hit just right.

    Welcome back stranger 😜
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  • xschop
    xschop Posts: 5,000
    So the shipping box wasn't bent/damaged in any way?

    Got pics?
    Don't take experimental gene therapies from known eugenicists.
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  • decato
    decato Posts: 186
    pitdogg2 wrote: »
    Welcome back stranger 😜

    I've been lurking in the background!

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  • decato
    decato Posts: 186
    xschop wrote: »
    So the shipping box wasn't bent/damaged in any way?

    Got pics?

    No damage, at least no punctures or dents.

    Nah, I threw the driver in the trash and cut up the box. I had a few spare drivers, so I'm ok but still stumped.
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  • decato
    decato Posts: 186
    edited May 2023
    I pulled it out of the trash. Fortunately, there wasn't much in the trash bin!

    I completely destroyed the coil after I saw it was already damaged, so please ignore that part. However, you can see where the glue failed.

    zzhzfwpjpuaw.jpg

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  • bcwsrt
    bcwsrt Posts: 1,876
    I half-heartedly joked before I got around to J-B Weld-ing my 15s that whenever I listened to them at elevated levels I was nervous I would hear the "THUNK!!" of one of the magnets hitting the bottom of the cabinet. I didn't realize they could actually do that! 😮

    Brian

    One-owner Polk Audio RTA 15TL speakers refreshed w/ Sonicap, Vishay/Mills and Cardas components by "pitdogg2," "xschop" billet tweeter plates and BH5 | Stereo REL Acoustics T/5x subwoofers w/ Bassline Blue cables | Rogue Audio Cronus Magnum III integrated tube amp | Technics SL-1210G turntable w/ Ortofon 2M Black LVB 250 MM cart | Sony CDP-508ESD CD player (as a transport) | LampizatOr Baltic 4 tube DAC | Nordost & DH Labs cables/interconnects | APC H15 Power Conditioner | GIK Acoustics room treatments | Degritter RCM
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  • xschop
    xschop Posts: 5,000
    It's repairable.
    Don't take experimental gene therapies from known eugenicists.
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  • F1nut
    F1nut Posts: 50,494
    Wuhan voice coil to the rescue.
    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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  • decato
    decato Posts: 186
    Heh, it's going right back in the trash bin. Still, based on the comments posted thus far, it doesn't seem like magnets falling off is a common problem.
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  • xschop
    xschop Posts: 5,000
    Strange indeed. PM incoming.
    Don't take experimental gene therapies from known eugenicists.
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  • xschop
    xschop Posts: 5,000
    F1nut wrote: »
    Wuhan voice coil to the rescue.

    Wubay coils are about here.
    Don't take experimental gene therapies from known eugenicists.
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  • xschop
    xschop Posts: 5,000
    edited May 2023
    5h3pymy1858b.jpg

    These should fit perfect without any former trimming at 33mm length...

    7tqgylbja61q.jpg

    3.5 ohms DCR too

    The highest reading one oscillates between 3.6 and 3.7 Ohm on the multitester, but all register 0.20-0.22mH inductance...

    z3hh9jkdm3kq.jpg


    Post edited by xschop on
    Don't take experimental gene therapies from known eugenicists.
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  • decato
    decato Posts: 186
    I look forward to seeing your repair of the damaged driver.

    How does the inductance of the replacement coil compare to that of the original MW6502 coil?
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  • xschop
    xschop Posts: 5,000
    Im not 100% sure that the factory coil should be measured while installed. But got tester leads closest to coil. The other unknown is these 6502's are phase-plugged and focus-ringed, which may be reducing inductance?

    h2v0v11gzs2d.jpg
    Don't take experimental gene therapies from known eugenicists.
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  • xschop
    xschop Posts: 5,000
    edited May 2023
    This is another caveat I like about 6502's besides their outstanding midrange. The doping compound is always applied uniformly and the dust caps are concentric.

    n0xy907sdclk.jpg


    This one got fragged severely but definitely worth saving...

    t0iracm8019p.jpg

    9qjx67admadw.jpg

    I made another shorter Coil spud jig just for this little delicate cutting operation...

    o902fd37u7z9.jpg

    Cut the coil former slit with a razor sharp scalpel and then scroll it out...

    2x912gc2xaic.jpg






    Don't take experimental gene therapies from known eugenicists.
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  • xschop
    xschop Posts: 5,000
    edited May 2023
    IMO Polk got these right also because the Base Plate IDs are always between 1.085 and 1.090". This one has the galvanic corrosion (albeit lighter than what I've seen on 6500's, 6510's 6511's...

    8drvquyqxl6m.jpg

    Light drum sanding to remove corrosion and these will never coil rub again...

    5zycuch2mchj.jpg

    Not sure what the markings mean on Base Plate...

    ftg8244gpfjy.jpg

    Wubay Coil OD's are all 1.070-1.072"

    Dual-layer too looking under mag glass...

    trwepzurcbz2.jpg

    Before gluing in coil, I apply a thin layer Aleene's Tacky Glue between spider edge and cone edge ID's...

    zrl3m4x9341q.jpg






    Post edited by xschop on
    Don't take experimental gene therapies from known eugenicists.
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  • xschop
    xschop Posts: 5,000
    edited May 2023
    Glue setting with 0.010" thick shims...

    c49nwgnskgdr.jpg

    y30fllbol1gd.jpg

    You want to make sure the length of the coil is exactly centered with Base Plate thickness while the spider is in a straight/flat plane front-to-back

    a73m5t0svtst.jpg

    nhxosfgw2k9f.jpg




    or1fuek9veb9.jpg

    Don't take experimental gene therapies from known eugenicists.
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  • xschop
    xschop Posts: 5,000
    edited May 2023
    Magnets usually come centered with pole-pieces from factory on 6502's also. I won't even bother removing and re-setting when I go to add a focus-ring and phase-plug.

    i5oveg83i4u2.jpg

    Another bonus besides being the correct Polk 26mm ID's, they appear to be easily modified/un-wound a few tenths of an ohm to get them to the Polk published DCRs of the 6511's...

    yffluzz1wtxh.jpg

    Post edited by xschop on
    Don't take experimental gene therapies from known eugenicists.
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  • decato
    decato Posts: 186
    It's coming along nicely!
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  • Gardenstater
    Gardenstater Posts: 4,456
    edited May 2023
    Definitely a larger VC I.D. than on the 6500. I have 1.008" PVC plug gage that is a slip fit to mine, so they are around 25.6mm I.D.

    Nice pictures and tutorial! Looks like you got an Xmax that is even greater than 1/8" there.

    Oh.....and the BD0602-A being the Polk part# for the driver, the D0654-1 must just be the part# for the baseplate? Wonder what the company was that was fabricating them at the point where they were being made in Mexico. I guess Mexico was not using the EIA Source Codes.

    This morning's research reveals one way to measure BxL. I wouldn't have thought it was safe to put that much DC current through a VC but I guess it is ok for short period of time:

    zjfbm92hlnkj.jpg
    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
    Onkyo A-8017 integrated
    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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  • xschop
    xschop Posts: 5,000
    edited May 2023
    That's an interesting method I'll have to try sometime.
    I just remembered that my little tester was causing a pulsing signal to be emitted from the cone as it was performing it's test.

    And yes these particular coil lengths are 0.55" long compared to most factory ones I've measured at @ 0.51" coil length. So only 20 thousandths extra length either side of the base plate stroke.

    Just measured this one's ID at 1.018"

    I don't pull any punches and have to say these are stronger and better built than what left the factory. 100% concentric with no deformities in the former or wavy flanges.

    Don't take experimental gene therapies from known eugenicists.
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  • xschop
    xschop Posts: 5,000
    edited May 2023
    Glue completely dried, removed shims and soldered leads back in location, trimmed excess, then painted them and the exposed bare cone area. They need a thorough cleaning after paint dries.

    cpd8ti5e7daq.jpg

    nt6vflo8xb13.jpg


    .....As of late, been trialing a rubber rejuvenating admixture since I believe the doping compound to be latex...

    fzp9bl3c28sw.jpg


    Don't take experimental gene therapies from known eugenicists.
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  • Gardenstater
    Gardenstater Posts: 4,456
    edited May 2023
    Does your rejuvenating process leave the surface sticky like they were when new? Another question is whether the stickiness is a key feature of the damping effectiveness or not.
    George / NJ

    Polk 7B main speakers, std. mods+ (1979, orig owner)
    Martin Logan Dynamo sub w/6ft 14awg Power Cord
    Onkyo A-8017 integrated
    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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  • xschop
    xschop Posts: 5,000
    edited May 2023
    I originally started applying the solution to the butyl surrounds that are dry and micro-cracking near cone edge. I noticed it also seems to be making the doping a little more tacky, yes.
    Don't take experimental gene therapies from known eugenicists.
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  • decato
    decato Posts: 186
    What product did you use on the rubber surround and cone? I have accumulated some drivers that are in rough shape.
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  • xschop
    xschop Posts: 5,000
    1 part wintergreen oil to 1 part 70% isopropyl alc.
    Don't take experimental gene therapies from known eugenicists.
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  • decato
    decato Posts: 186
    Interesting. Thanks!
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