Cracked Rubber Surround on MW65xx Drivers

decato

Hi all,
I was browsing through eBay for some vintage Polk products, and I stumbled across an MW6502 driver in rough shape. The rubber is cracked where it attaches to the cone but appears fine everywhere else.



Seeing this prompted me to inspect my speakers more carefully. Unfortunately, I observed that several of my oldest speakers (1988-89) are starting to exhibit the same problem. The picture below shows the deterioration of the MW6502 in my Monitor 7C.



I'm not sure what's causing the rubber to crack. The rubber is still soft and pliable. No air leaks through the cracks. The speakers were not left in direct sunlight and they were kept indoors. Perhaps the cracking occurred shortly after production and remained somewhat stable. I am not the original owner of these speakers, so I cannot say for sure. I also picked up a pair of Monitor 10Bs but didn't notice they had cracked surrounds until I took them home.

Evidently, this problem exists on other drivers as well. There's a discussion on diyAudio about a Seas woofer that developed cracks in the surround shortly after it was manufactured. See the images below and the URL for the full discussion.



https://diyaudio.com/community/threads/proac-seas-woofer-surround-crack.116222/

I bought a product on Amazon for restoring cracked leather/faux leather/vinyl. I originally used it on a chair with decent results. The color was a little off, but it filled in the gaps and cracks nicely. I figured I'd have nothing to lose applying it to the Polk driver. Here is the product.


https://amazon.com/dp/B081KY1Q3P?psc=1&ref=ppx_yo2ov_dt_b_product_details

I gently applied a small amount with the tip of a toothpick. Here's how it looks.


Do any of you have the same problem? What are your thoughts?

VR3

I can't recall ever seeing something like this. You say there was no air leaks?

Gardenstater

Mine are all good. Ranging 1979 to 1990. On all of mine, the doping was extended up over the glue flange of the surround, right up to the convoluted section. Maybe that has helped to prevent the problem.

decato

VR3 wrote: »

I can't recall ever seeing something like this. You say there was no air leaks?

Right, no air leaks, which is good. The easiest test is to put your eye right above the area in suspect and push in the passive radiator. Your eye is so sensitive to the tiniest puff of air.

So far I see some cracks on my 7C, 10B, and SDA-SRS 1.2tl.

decato

Gardenstater wrote: »

On all of mine, the doping was extended up over the glue flange of the surround, right up to the convoluted section. Maybe that has helped to prevent the problem.

That's an interesting observation. I don't see any cracks where the doping material extended onto the rubber.

xschop

^^^Same observations here^^^
Seen microcracks on 6502's, 6510's, 6503's and IIRC on a 6511

decato

xschop wrote: »

Seen microcracks on 6502's, 6510's, 6503's and IIRC on a 6511

That's unfortunate. Did you attempt to repair the drivers?

xschop

At that time they weren't as bad as the first one in your pics, and didnt leak air either. Just annoying cosmetically. I have recently repaired a 6500's surround tear with butyl rubber caulk (believe was roofing repair tube I had left over)

Jazzhead

@westmassguy and I had an email conversation relating to this a while back. I was/am looking for an adhesive to safely re-glue the loose paper trim rings around my old 6500's. The big mystery is what the old Polk surrounds were made from...(I'll take no chances with something that will attack the surrounds). I've seen other websites that claim they are butyl rubber (does anyone know for sure?). The claims that these old surrounds are butyl makes sense because they last a long time without failure. I recall from my polymer chemistry classes decades ago that butyl is one of the more degradation resistant synthetic rubbers, but it can fail eventually from exposure to ozone and UV light. This thread started by @drumminman described how he used Rema SVS-VULC bike tube adhesive to repair a surround:

https://forum.polkaudio.com/discussion/163519/found-product-for-butyl-rubber-surround-repair

Bike tubes are also made from butyl rubber, so this also makes sense. That said, I have no long term proof of success, but perhaps someone can experiment. @xschop this sounds like a perfect experiment for you.

As far as speaker surrounds failing that were never exposed to sun, perhaps they were exposed to ozone from electrical device(s) etc. at some point.

xschop

I suspect some of the surrounds that are really cracked bad are some type of latex based rubber that is more prone to UV degradation and ozonation as you say.

Years back on hard-to-find Yamaha carb boots that had gotten hard from heat cycling and cracking, I'd soak them in a solution of wintergreen oil. Brought them back to suppleness.

pitdogg2

VR3 wrote: »

I can't recall ever seeing something like this. You say there was no air leaks?

I've seen it. I chalked it up to sun exposure or some sort of UV.

F1nut

The vintage Polk surrounds are butyl rubber.

decato

Assuming it is UV or ozone damage, it seems somewhat mysterious as to why the rubber cracked only where the surround is glued to the cone. It's also strange that the passive radiator is still in perfect condition. On the Monitor 7C, the passive radiator is in such close proximity to the driver that one would expect both to fail simultaneously in the presence of UV/ozone.

I went through my archives and found an ad from the October 1989 issue of Audio magazine. Polk claims to have used butyl rubber. Here's a scan of that page of the ad.

Gardenstater

Maybe they used an adhesive to attach the surround to the cone that, combined with the UV and ozone, did the dirty work at that spot. When they applied the doping material to cover that area that could've given it the needed additional protection. It would mean they must've used a different adhesive at the surround to steel basket joint, which I admit is a little weird.

Jazzhead

Thanks @F1nut and @decato for confirming the old surrounds are butyl. Because these surrounds are so thin and fragile, I'd try this out on a cracked "last resort" driver first ... but, based on this info, another last ditch repair would be to carefully, sparingly, apply vulcanizing cement designed for butyl bike tubes (like SVS-Vulk). All the chemistry should "get along" and result in a "best possible", flexible bond. One could Q-tip the area first with a little acetone (will not react with butyl) to clean and degrease first.

decato

Thanks, @Jazzhead. I can try the vulcanizing cement on one of the other drivers and see how it compares to the leather/vinyl repair product I used.

odcics2

So, does anyone make butyl rubber surrounds for Polk midwoofers?

As time goes on, yes, they can fail. It's a stressed part that moves.

There are thousands of these drivers out there in the 6.5" size.
An opportunity to make some money for some vendor.

Another question: What about a small hole in the surround? Fixable?
I have a pair of 7A speakers that need work.

xschop

A couple years ago I found an eBay seller that had posted dimensions matching the 6502s, 6510s, and 6511s I had at the time.

The MAIN problem I had was trying to find an aqueous solution that would help soften/dissolve the cone-to-surround glue without damaging the paper cone.

odcics2

I use a single edge razor blade to remove old glue/foam from 'paper' woofers. Has to be new for best results.

odcics2

Bought some teflon coated blades years ago. I’ve had good luck using them.

decato

After speaking with various engineers, I'm starting to think that this form of degradation is the result of repeatedly pushing the driver hard. These tears might actually be stress fractures caused from high excursion.

Gardenstater

I feel like mine got cheated considering how loud I used to blast Pink Floyd DSOTM in college. All I got out of it was a cracked spider lol.

odcics2

Like anything mechanical, rubber wears out.
At least butyl lasts longer than typical foam surrounds!

audioluvr

Once upon a time I had a cat that tried climbing my SDA 2B's back in the 90's. and made a small tear in them maybe a quarter inch. (don't worry the cat got beat within an inch of it's life and never attempted that again) I took black polyurethane and a very fine art brush to reseal the tear and as far as I know those speakers are still playing here somewhere in Polk Audio Land. I also know that people who use ozone generators for room air purifiers are at risk of destroying anything rubber in their homes. I used to own a fire damage restoration company and have seen the carnage first hand. It's great for removing smoke smell but ozone will turn rubber into goo which eventually dries out and becomes brittle.

odcics2

I have an early 7A with a small hole in the midwoofer surround.
Grills are toast too!
I’ll try your fix.

odcics2

Fixable?

pitdogg2

Believe it or not folks in the past tried an inner tube repair kit. Cut a small patch apply the glue to both let dry and then apply the patch to the hole and press in to the best of your abilities.
The patch went on the inside (backside) of the surround

I'm not guaranteeing anything it may or may not work. At this point what do you have to lose?

Gardenstater

I'm thinking maybe you could use the vulcanizing rubber cement from a speaker repair site designed for surrounds, but since you have such a large opening you could try cutting a small square of tissue paper and impregnate it with the cement by putty knife and then stick it on from behind as stated.

VR3

That surround has smaller rips elsewhere as well

Jazzhead

There are eBay vendors selling butyl surrounds. You could buy the smallest, cheapest surround to cut a patch from, and then use the SVS-vulc cement to adhere it for an inexpensive hole repair. As stated above, ozone can definitely wreck rubber. In my last job we had dozens of drysuits for running whitewater and the air in the building (they were stored near some HVAC air handlers) would trash the neck and wrist gaskets in no time. The gaskets are a huge pain to replace (probably like gluing surrounds), so I started soaking gaskets in 303 (great stuff) strong them in bags, inside large sealed containers.

VR3

One thing about the old Polk surrounds is that they are extremely thin compared to more modern day rubber surrounds. The surround can completely change a driver so if you install one that is much heavier than the original that can make quite the difference.