Refoaming Bose 901s correctly??

[diyialwayssay]

Ok, so I have already read instructions on how to do this elsewhere, but I'm still confused about things, so am interested in getting firsthand advice from someone in here possibly. These are series 4 and I purchased them along with surrounds and glue from a seller on Craigslist. Unfortunately the surrounds are the cloth type. The series 4 used the foam type. but nonetheless, this is what I have, and they'll just have to do, I hope.

The foam on these were so bad, that the surrounds simply crumbled. So no problem removing the old surrounds. My first question is, shem or do not shem these? I've read it both ways. So not sure which way is best? Here's my next question. Since the foam had crumbled, this meant that all of the cones are basically sitting low in the basket, IOW towards the bottom. This is what I'm confused about. How high are the cones supposed to be sitting while waiting for the surrounds to cure after gluing everything? I've heard they should be sitting a bit above the basket. I wouldn't think this meant all the way up tho, right? If they need to come up some, how do I get them up there where they will stay while waiting for the surrounds to cure? IOW, what is the correct height of the cone, in relation to the basket, and how do I know where this is? I realize that I have to also keep these centered at the same time, which is why I wonder which way works the best. I'd prefer not to have to shem them if possible, since I don't have any new dust caps to replace with later.

Right now I'm working on one speaker(all 9 drivers)which are still intact in the speaker enclosure. Right now I have the inside of the surrounds glued to the edge of the 9 cones. I did this last night. Before I can finish the outside edge, which isn't glued yet, I need to understand the procedure for properly centering these, and what height the cones should be. Any help would be greatly apprecieted. Would like to finish those 9 drivers today if possible. Thanks

 

[cliffforrest]

Why not do a search on "refoaming"?

You'll get lots of hits and "how-to" links.

IME foam surrounds are just air seals to isolate front/rear. They do not contribute to the mechanical location of the voice coil assembly, which should be correct independant of the surround.

This may nor be true in all cases, of course.

 

[diyialwayssay]

Why not do a search on "refoaming"?

You'll get lots of hits and "how-to" links.

IME foam surrounds are just air seals to isolate front/rear. They do not contribute to the mechanical location of the voice coil assembly, which should be correct independant of the surround.

This may nor be true in all cases, of course.

Being new to here, I admit, I never thought of that first. Probably a million things about it. But I've been reading things elsewhere, and none of them seem to address these speakers in particular, such as how high the cones should be sitting, That's why I decided to join this forum after finding it via Google. Just thought someone might explain the process to me, someone who has refoamed these before with excellent results. But I will take your advice and a do a search, and see if I can find my questions answered that way, Thanks

 

[Cal Weldon]

Something's wrong if the cone is sagging all the way in when on their backs. Like Cliff says, the surround doesn't really contribute to the centering if the driver in and out, that's the job of the spider. Also you should not have to use shims on the coil. That's for when you are doing a recone. The spider should have it centered already. I'm worried about your cones though.

 

[diyialwayssay]

Something's wrong if the cone is sagging all the way in when on their backs. Like Cliff says, the surround doesn't really contribute to the centering if the driver in and out, that's the job of the spider. Also you should not have to use shims on the coil. That's for when you are doing a recone. The spider should have it centered already. I'm worried about your cones though.

Perhaps I should be a little clearer. The cones are nowhere towards the bottom of the excursion. The edge of the cone, where the surround is fastened, it's even, in heigth, with the edge where the outer edge of the surround connects to the edge of the basket, lip. If I were to guess, I'd estimate it's somewhere in the middle of the excursion. I just want to be sure it's at the height it's supposed to be, before fastening the other end with glue. I was under the impression that the cone should be a tad bit higher, as in higher than the outer edge where the surround gets glued, that narrow lip. I thought perhaps these may have sagged over time. Not certain tho, just cautious is all. When I gently push down on the cones, I hear a lot of scraping. I'm assuming it's off center or something. How do I get this scraping noise out, especially before I attach the other end to the lip I'm talking about above? I don't want to end up with VC rub and distortion when I'm thru. Am I making this more complicated than it is? I've never put surrounds on a driver before. Just hoping to do it right the first time, thus the reason for caution. Thanks

 

[diyialwayssay]

Let me explain what got me worrying about this in the 1st place. It was from an article I read last night that I was reading, in regards to refoaming Bose 901s.



Centering the foam surround is CRITICAL. the cone sticks up past the basket slightly, so I placed the ring, then pushed the cone down slightly and made sure that the ring is centered on the basket, then when I relieved pressure on the cone it would center itself.
Bose 901 Speaker Restoration at SOLID-orange

It was from this point in the article that I began to become confused.

Then further down the article it stated the following.

Like the inner edge, I let this set for 5-7 minutes before affixing both surfaces, they wont touch since the cone naturally sits up higher than the basket.
Bose 901 Speaker Restoration at SOLID-orange


It says they won't touch, etc. This is not true on my drivers. The foam is indeed resting on this outer lip, it's not glued yet, but it's resting on this edge that should be lower than the outer edge of the surround. I'm assuming, since the inner edge of the surrounds are already glued, that the outer edge should then be hanging in mid air so to speak, not resting on anything. But it's just the opposite, leading me to believe the cone is not naturally sitting as high as it should. That's what got me to wondering if the cones are not naturally sitting up higher than the basket? All I know is, after reading this article, I've become nothing but confused.

 

[sklimek]

Since the foam had crumbled, this meant that all of the cones are basically sitting low in the basket, IOW towards the bottom. This is what I'm confused about. How high are the cones supposed to be sitting while waiting for the surrounds to cure after gluing everything? I've heard they should be sitting a bit above the basket. I wouldn't think this meant all the way up tho, right? If they need to come up some, how do I get them up there where they will stay while waiting for the surrounds to cure?
Thanks

You may want to check out this step by step tutorial for replacing your foam surrounds. Centering was with a 30Hz signal from an oscillator or a Walkman on repeat for voice-coil centering by listening for any 'scratchy' sounds.

I followed this tutorial for replacing my foam surrounds on my two JBL 4313B's and it worked great!

Resurround Step-by-step

Stan

 

[Bobby Mcgee]

Old post, but for any future searches.... I think I understand. You should first glue your new surround to the edge of the cone and allow it to dry. Next, pull the cone and surround up and glue it to the rim of the basket. This will pull the cone up to where it belongs. It they want to fall back down at this point then something is wrong.

 

[Magz]

Nah, I just did 39 series III 901 drivers with cloth surrounds. Just string a few in series to give a decent impedance, hook them up to a signal generator with a test tone (can be fancy like the Agilent arbitrary function generator I used or a receiver playing music), put glue on the cone edge and basket at the same time and gently press the surround into place around both the cone and basket - done.

39 out of 39 worked perfectly without scraping.

 

[Magz]

BTW, I found a test tone frequency of 250 Hz worked very well to center the cone. I ramped it up and down from 100 to 5000 after gluing just to be sure there wasn't any rubbing/scraping. I found that refoaming in batches of 8, connected in series using clip leads, worked best for me - that gave an impedance of 8 ohms so not too low to drive and a manageable number to work on so I didn't lose concentration.

These will be used in a future line array project, FYI.

 

[chris661]

Interesting. Is 250Hz a balance between excessive excursion down low but still obvious enough if something's scraping?

FWIW, the contribution of the surround towards keeping everything aligned will vary driver-to-driver. Soft surrounds in general do very little. For high-power PA units, the surround might be contributing as much as the spider. Maybe moreso, since its at the end of a long lever.

Chris

 

[Magz]

Interesting. Is 250Hz a balance between excessive excursion down low but still obvious enough if something's scraping?

Chris

Pretty much. The drivers don't generate much SPL below 200 or so unless they're in an enclosure, so it's much easier to hear scraping above that frequency. I still ramped them up and down in frequency to check for anything obvious. One of the easier refoams I've done.