Burnt Voice Coil MW6500 and MW6511

xschop

I'm still looking for these two MWs to dissect, take measurements, and rebuild voice coils. If you have any laying around, just PM.

Tony M

xschop

Thanks Tony. Now just missing a 6511 to experiment on

xschop

The Wuhan coils showed early...

xschop

The 6500's defect lies in the baseplate ID being 1.070" whereas all other MWs are at 1.085". There is so little VC to Baseplate clearance that when the suspensions start to sag any little bit, the coil starts rubbing...

I fixed it by chucking the baseplate in the lathe (it actually ran concentric by @ only 2 thousandths out-of-round) and used a 1/2" boring bar to open it up to 1.086" like all other MWs, to prevent any rubbing in future.

pitdogg2

Interesting use of latex, it definitely will not spawn offspring now ...

😜😜

xschop

Besides the baseplate ID being too small from factory. This dust cap is not the light woven open-mesh fabric like other MWs, but a denser, harder felt type fabric. I'd say about 2x the weight of the mesh versions.

Gardenstater

I noticed that mine are very heavy and only a few pinholes when held up to the light. Also, my 7B's have always taken a long time on the leakdown test.

xschop

So the phase plug mod will eliminate that weight.
And re-setting their magnet/pole-piece by penlight is a no-go.

Gardenstater

I should've said thick, rather than heavy. They only weigh in at a whopping 0.030 oz.

pitdogg2

All early mw6500, mw6600 and mw6501 had thicker felt like dust caps. When I finished my SDA 1 signatures and had rings, reglued the cabinet I could get 6-7 second hang time on the passive test. They were tight 😂.

xschop

I could glue the cap on inverted. 😁

xschop

What's the consensus on minimal leak time?
Since the PR design of these vintage Polks use "fluid coupling", at what frequency is this coupling no longer effective?
My point is, seeing how Polk went to a very light, air-permissive dust cap would be indicative that only cabinet and gasket sealing is more important than the voice coil gap itself where air flows back and forth.

<6500 6502>

xschop

Coil rebuild pics because it did happen...

Main objectives are to ensure the voice coil wire winds protrude past base plate only 1/8" while spider sits in a flat plane. This is because the coil winding length is 0.50", the base plate is 0.25" thick. At spider relaxed position, this allows even/central placement of coil on forward and aft excursions....










Make sure wire leads are positioned evenly next to old ones...





I used a 25.5 ID coil on this rebuild, so .020" centering shims are needed. If you use a 26mm ID coil, you'll need 0.010" shims while glue sets up...

pitdogg2

There are other variables that could be involved with the more open dust caps. One could be heat.

Gardenstater

Will you need to turn down the pole piece shaft to fit this Wuhan coil with sufficient clearance? I'm thinking that if you went with a one piece (or shrink fit 2 pc.) pole piece made out of a low carbon steel you might be able to get back any BL you may have lost by going with a larger air gap than stock, which was admittedly super close fitting.

xschop

I've already turned down the pole piece by .007 extra clearance and reset It. The simple choice is either that, or still have a non-functioning MW6500 due to the sloppy build/suspension of theae early MWs.

And by accident, I now have an inverted lightweight woven cloth 6500 that functions perfectly.
I went to glue it on, and adding slight pressure, it inverted, so I just left it as it looks neat.

Gardenstater

xschop wrote: »

I've already turned down the pole piece by .007 extra clearance and reset It. The simple choice is either that, or still have a non-functioning MW6500 due to the sloppy build/suspension of theae early MWs.

And by accident, I now have an inverted lightweight woven cloth 6500 that functions perfectly.
I went to glue it on, and adding slight pressure, it inverted, so I just left it as it looks neat.

Rodger that......so pole piece is now 0.988" (from 0.995")? Since you don't have any early speakers that took the MW6500 are you going to try it in a speaker that takes the MW6510 temporarily, to see how it sounds?

Oh and what adhesive did you use to secure the VC to cone and spider?

xschop

I used Eileens, just as I did for the 6502 that I rebuilt.
I temp tried it in my 5Bs, functions properly, just isn't crossed over correctly in them.

I'll karma these off or trade for a set of 16uf Black Dragons lol.

Gardenstater

Thanks for the tutorial and you certainly proved it can be done!

xschop

Won't be long, die hard vintage Polkies will have to adopt the Wuhans.

Gardenstater

Let me know if you find some 6.500 DCR, 25.7 mm I.D. ones and I'll buy a bunch.

xschop

https://www.ebay.com/itm/2-pcs-of-25-9mm-26mm-H-35mm-Black-aluminous-Bass-Woofer-Speaker-VoiceCoil-8Ohm-/122662036782?mkcid=16&mkevt=1&_trksid=p2349624.m46890.l49286&mkrid=711-127632-2357-0

Buy them in pairs. Would be interesting to see how you'd unwind them foe exact 6.5 DCR.

Gardenstater

xschop wrote: »

https://www.ebay.com/itm/2-pcs-of-25-9mm-26mm-H-35mm-Black-aluminous-Bass-Woofer-Speaker-VoiceCoil-8Ohm-/122662036782?mkcid=16&mkevt=1&_trksid=p2349624.m46890.l49286&mkrid=711-127632-2357-0

Buy them in pairs. Would be interesting to see how you'd unwind them foe exact 6.5 DCR.

The more I have thought about that, the less feasible I think it would be. It's one thing to do that with an inductor but this is different. I think it is strictly a matter of the number of turns on each layer and the wire gage and composition of the wire they have chosen. Maybe you'd have to basically unwind the wire from the former and start over again so that you have the same length of coil and the same number of turns on each layer. Would probably have to use different magnet wire.

Is it possible to unwind 5 turns (say) from the outer layer first, and then unwind 5 from the inner? If it is 0.008" dia. wire that would give you a coil length that is only 0.040" shorter in length. It would bring the DCR pretty close, with a *possible* drawback being that it would change the inductance a bit.

xschop

Stop pontificating, and start purchasing lol.

pitdogg2

Why is it I remember years in the past a discussion about Polk and edge wound voice coils?? ...

What are those?

Gardenstater

Edge wound would be a way of getting more turns in per length of the coil, for the same awg of wire. It would mean the wire was more rectangular in cross section, rather then round.

xschop

Added some black paint aground dust cap. Doing a tone sweep 2k. down to 1, YouTube quality....

xschop

Interesting that without the butyl mat, at loud voulume, they rang around 1500 Hz, quieted down, then again rang @ 500Hz, then quiet all way down to 5 HZ.
After the butyl mat, all ringing went away at loud volume....



@VR3's felts would be an eyesight upgrade to these peeling 6500s...

Gardenstater

That is interesting. They didn't ring at 1kHz? I was thinking multiples of 500 Hz maybe.

I do want to try to remove the cardboard trim rings. Danger will be to not cut or nick the surround in the slightest. An aluminum flashing guard perhaps. I wonder how much glue they used. I guess I will find out.....

xschop

It peels off in 1/16" layers. You can scribe across the circumference and peel easily.

Mind you I'm using the 5B xover that calls for a 4 ohm woofer. It also may be the inner frame foam taking care of 1K Hz?