Monitor 7b restoration

Anovak

A friend asked me to take these speakers and do what I could to restore them, said he'd bought them new in the early-mid 1970s and that while they still sounded good, something was not quite right. I removed the grills and checked the driver cone movement....frozen. Asked him how much he had been playing them lately and he said not much so I thought there was a chance that the voice coils might still be OK.

Brought them home to my place and started to do the work, much of which has been aided greatly by this forum and also one notable, kind and generous member who offered to send me a new magnet since one of them had a large chunk chipped off of it. In any case, I was able to re-center the magnets and glue them in place. The cones now move freely, and even with some heavy bass material there is no rubbing detected at all even when I pull the volume up to about as loud as most might listen at. So, success there.

The tweeters were also off-center so I played with them until I could put a test tone through them and achieve absolute clean response. Then I screwed up.....everything was so dirty that I wanted to do a nice cleaning, so I brought out some lint-free (or so I thought) cloth and some distilled water and went to work.....before I read the thread that suggested using only the real Windex.....and in the process because the dirt was so stubborn graduated from the distilled to some IPA and that was the big mistake. In trying to clean the tweeter dust caps, I immediately noticed that some of what looked like a coating was being removed. See the pictures. The question: now, I need to know what can be done about this faux pas if anything? I think it will mean taking the tweeter apart again and supporting the back side of the dust cap with something rounded and soft while applying whatever might be suggested to the outside of the cap where I removed some of it.

Also, I also was able to get new pegs for the boxes and sockets for the grills, the ones that Parts Express sells. This did require, however, drilling the holes in the grills out a bit to 9/16" but no big deal, everything fits really nicely and just waiting for some speaker cloth (Joann's Fabrics item # 0351 4023).

Any ideas and help will be appreciated. I also want to say "thanks" again for all the excellent discussions on this forum that made my job possible. I'm not the most skilled at such things but with the help here, basically got through it and the speakers sound absolutely amazing! They rival my Infinity Quantum 4's, which are far larger and do reach a bit lower in bass but hey, dual cone Watkins woofers will do that!

treitz3

Hello and good evening to you, sir!

I love it when people come out of the woodworks and join on in with the discussions! Congrat's on the restoration!

While I am not well versed enough to help you with your Polk Audio tweeter issues, there are a plethora of folks here (as I am sure you know by now) that are more than welcome to help you out or at the very least, point you in the right direction.

That said, please allow me to offer you a very warm welcome to Club Polk, Anovak.

Tom

Anovak

Mighty nice of you Tom! And thank you for the kind words.....I put a lot of time and effort into this as I'd like to present them back to my friend in as nice condition as possible. I'm hoping that there is a remedy for the tweeter dust cap issue but am prepared to just put the new grill cloth on them when I get it and forget about it.....kinda outta sight, outta mind, but that dang OCD just ain't lettin' me do it without at least giving it a shot! Hope someone can get me sorted on this thing!

Gardenstater

I have the same speakers but mine are 1979. Your friend's look to be 80 to 82...somewhere around there. I wish I knew what to tell you regarding the doping material used on the silk domes (not a dustcap). I wouldn't know what to suggest, at least not from personal experience and that is what you need to proceed.

It is good that they still work. I have had good success cleaning the silk domes with sticky tape (carefully) and that is the only method I will ever use.

treitz3

Anovak wrote: »

I put a lot of time and effort into this as I'd like to present them back to my friend in as nice condition as possible.

I am not sorry for saying this.....you, sir? ROCK!

That is a gesture and a work ethic that is so dearly missed in this world. Please allow me to offer you a good ol' tip of the hat to you sir!

Tom

Anovak

Thanks all, so kind of you Tom. I'll try the tape method for cleaning the domes, George, thanks for the suggestion. But before doing that I'd still like to know if there is indeed a way to re-coat that silk dome and with what?

treitz3

Like I said, I am not well versed on this subject.....but.....a replacement may be in order. There may also be an upgrade (or a replacement that is an upgrade) but please do not hold me to that.

It's probably a proprietary, specialized product that requires specialized machinery to produce/manufacture within spec..

Please refer to those whom you have read about and respect on the forum for further guidance on that one. @F1nut would be the type of cat you would want to weigh in for something like this.

Tom

Anovak

Tom, that would be a good suggestion. Fact is, I found Jesse over on the AudioKarma forum and he directed me here. I'll see what he has to say about the domes.

F1nut

Sorry, I've got no idea what the coating is.

Anovak

Thanks, Jesse.....I wonder if there are still folks around who worked on these at Polk who might be around to shed some light on this?

F1nut

Apparently treated fabric dome tweeters are impregnated not coated. That is, the sheets of fabric are impregnated prior to the domes being cut out.

Since your tweeters are already compromised you might try removing all of the impregnated material, then apply a VERY thin rubberized coating. If that doesn't work those tweeters do come up on eBay.

BTW, Polk bought off the shelf tweeters from Peerless, so they wouldn't know what the impregnated material is.

Anovak

Aha, makes perfect sense Jesse. Since they are playing flawlessly, I'm going to leave them as they are for the moment unless someone chimes in who has had direct experience with doing this successfully and can pass along the process. I had also thought about removing the rest to at least make a more uniform mess of things but decided to stop while I was still however slightly ahead.....

Gardenstater

Here's a thread that I read a while ago. Lots of opinions and non-authoritative random information. Interesting read though. I think the doping and manufacturing of silk dome tweeters is sort of a proprietary secret. You'd be experimenting, which is fun but not predictable outcome. I agree with not removing any more of the coating and cutting your losses.

https://audiokarma.org/forums/index.php?threads/re-doping-a-silk-dome-tweeter.588100/

Gardenstater

This guy "enaldo" got good results on Audax tweeters by cleaning old coating off with isopropyl alcohol and applying a "vinyl plasticizer" sold as a coating for cloth surrounds. He has before and after measurements which show he got an amazing result. His before picture showed one of the tweeters was totally covered with hairs though lol.

https://www.audiokarma.org/forums/index.php?threads/re-coating-audax-dome-tweeters-w-measurements.559836/page-2

He also makes more comments in this thread:

https://audiokarma.org/forums/index.php?threads/re-doping-a-silk-dome-tweeter.588100/

Interestingly, the very early Monitor 7 used Audax tweeters, not Peerless.

This might be interesting as well. The first soft dome textile tweeter. Describes the resin thermoforming process and then coating with a liquid rubber compound for closing the weave and damping.

https://worldwide.espacenet.com/patent/search/family/023317193/publication/US3328537A?q=pn=US3328537A

Anovak

George, that is excellent information! I had just discovered "enaldo's" threads myself and have read top to bottom, including a few links that go to others' experiences and it appears that along with some other stuff, plain old tacky glue (such as Aleene's) diluted at about 5/1 might be a good fix as well. As I'm OCD, I would effort to remove the top plate of the tweeter, insert something round and soft, not too soft though, into the opening to support the back side of the dome, and then brush it on as evenly as I can. Thoughts?

Gardenstater

From what I read the Aleene's may not dry tacky whereas the material used to treat/seal cloth speaker surrounds does. He is from Buenos Aires and could not give exact product that would be helpful and the product he purchased does not even have a label! Bummer. Someone said that the cloth surround material is a PVA but I wouldn't assume that as gospel without verification.

I don't know if you need to remove the top plate, but you can. I would use a super soft brush and it should not harm the dome if you do. Go for the thinnest most even coat you can. If it was me I wouldn't try the Aleene's without trying it first on a similar piece of fabric. Any fabric you try it on will be different though because the soft textile domes have been impregnated with a resin in the thermoforming of the dome process.

Try contacting a speaker repair supplies company for the cloth surround treatment material would be my advice.

https://www.simplyspeakers.com/ ?

Gardenstater

I was advised by them just now that this would be the material:

https://www.simplyspeakers.com/speaker-repair-cone-edge-sealer-mi-1291fg.html

Gardenstater

Might be worth a try. May even want to thin it a bit and use an extremely soft artist brush to apply. Perhaps put the tweeter in the center of a lazy susan turntable. Found these somewhat reassuring QandA's:

F1nut

Aleene's does not dry to a tacky state.

Anovak

Did I miss something? I can't recall why the sticky makes any difference? Is there a reason why it might be preferable? Seems it only collects dust more readily.....

Anovak

Unless sticky also means still "wet" meaning it doesn't dry out and remains there longer....

Gardenstater

It's a good question. My 7Bs MW drivers and Peerless tweeter were sticky when I bought them in 1979 and now, 43 years later they are not. I believe slightly sticky is desireable for damping of breakup modes in cones and domes.

txcoastal1

Anovak wrote: »

Did I miss something? I can't recall why the sticky makes any difference? Is there a reason why it might be preferable? Seems it only collects dust more readily.....

The flies man, the flies

vulcan

I recommend to leave the tweeter domes as is, if they are playing flawlessly. Any repairs attempted
could interfere with sound quality. Also, you may find some good OEM replacements in the future.
Good job on the restoration and welcome to Club Polk!

Gardenstater

vulcan wrote: »

I recommend to leave the tweeter domes as is, if they are playing flawlessly. Any repairs attempted
could interfere with sound quality. Also, you may find some good OEM replacements in the future.
Good job on the restoration and welcome to Club Polk!

I think most Peerless coatings are not as effective as they were when they were new, at damping. I would find it hard to believe, especially from the graphs in that audiokarma thread I posted and he looked at too, that there is not deterioration in sound quality with the coating partially removed. Just my 2c though.

Gardenstater

Reading the Hecht Soft Dome patent, it is very clear that the coating is the 2nd crucial part in the composite soft dome, and fills the interstices (air passing) of the cloth with a non resonant rubber-like material.

Anovak

The longer we go at this, the more information comes to the fore and that will not only be particularly useful for me, but for folks in the future who are confronted with this proposition and I'm very grateful to you all.

Thanks for the kind words, Vulcan, and my initial leanings were to do precisely as you recommend. The key here is that they do sound really good and are apparrantly flawless as they are. However, all of this information George has discovered (you da man, George!!) does massage my whims to explore the idea of a restoration of said domes if only to address the appearance. I know, a bit vain....however let's not forget the OCD part operating here!!

I just don't want to get to a point where there is degradation of the sound quality in my pursuit of the perfect visual presentation. Now if we only knew exactly what that "rubber, or rubber-like material" is that they were using? Good thing I am in no particular rush to get this done, so being patient will, I trust, in the end be an asset to the process.

Great stuff, guys! Let me provide one more picture of the problem for reference:

Gardenstater

I am probably OCD also lol, so we have that in common. This piqued my interest to do research because, although they sound great, my OEM Peerless are not testing as smooth as they used to. I have to find an old test to show how they used to graph out, and I am not exactly using proper testing methods like DarqueKnight does, being that I just have an app on an aging Android phone. I now have a bit of a dip in the freq. response that I am wondering if it may be because of the aging coating on my Peerless. This was taken about 12" from baffle midway between MW and tweeter, on my left speaker with the R receiving no signal, with whitenoise. I should try doing a sweep but I have to figure out how to generate the test file to send to my speakers, from my computer:

Gardenstater

Anovak wrote: »

Now if we only knew exactly what that "rubber, or rubber-like material" is that they were using?

Therein lies the rub as they say . I think that enaldo's experiment makes me somewhat less fearful to try this on my tweeters, or maybe one of the extras that I have in "inventory", which are untested at this point. The Audax tweeter was definitely one of the Peerless tweeter's .....peers (lol).........at the time.

The first company to do this may have been Fisher and according to this the Hecht patent was licensed by many companies including Peerless.

Anovak

One thing that is becoming more clear to me, and especially from looking at the Fisher tweeters above, is that the coating extends all the way to the frame of the opening, not just the dome. That is good to know. In removing the tweeter from the cabinet and separating the magnet from the frame, I'm reminded that one can certainly support the dome from underneath whilst applying said unknown rubber material to the outside/top.

I'm wondering how something like this might work?

https://www.permatex.com/products/gasketing/black-gasket-makers/permatex-ultra-black-rtv-silicone-gasket-maker-5-oz/

plastic_avatar

I love waking up to new knowledge in the morning.

[goes to re-read this thread]

[stops]

[re-reads just a little]

[stops]

- Does this still have the original crossover bits?

- You don't own Polks yourself?