Please help to remove foam residue on cones

[Gene Cho]

Hey, fellow members,

After hearing some buzzing from my Bose 901 III speakers, I ambitiously decided to challenge woofer surround repair myself, which I've never done before.

I've spent many many hours to watch videos, read instructions and others' experiences, etc.. and finally this morning, I opened the front grill on my Bose 901 and started the refoam job.

First, I have to tell you that the surrounds of my 901 are not rotted or dry-from visual inspection and touching them, they're still soft and most surrounds look OK-but I still went ahead as I really wanted to remove the buzzing and improve the sound.

But the process didn't go well as the residue on the cone was too soft and VERY hard to remove. When I tried to remove it, it looked the cone could be damaged.

In a youtube video from Simply Speakers, they recommended to use alcohol, but the unit they showed had dry foam, not soft like mine.

I tried to use my fingers to roll it, but as it is too soft and the glue is still so strong, it didn't work.


Should I try alcohol or does anyone have any suggestion?

Thank you in advance!

 

[62vauxhall]

Abrasion - sandpaper or scraping with a knife blade. With that many drivers, I don't envy your task.

I'd be concerned that chemicals (alcohol) might absorb into the cone's edge and mess up the pulp. Experimenting with alcohol plus abrasion in a small area is worth a shot as a first treatment but it won't soften the glue.

There is TV & Radio Service Solvent which might work as far as solvents go but scraping or scrubbing away the softened residue will still be required.

 

[cademan]

If the surrounds are ok, how do you know the buzzing noise is being caused by the speakers? Did you swap them left to right? Is it happening in both left & right speakers?

Those 901's cannot have all those speakers blown at once. It's unheard of unless the drivers were all damaged due to massive distortion or DC at the speakers.

 

[Oerets]

I just use a razor blade to scrap off the old foam from the cone.

The buzzing may be a spider loose from the cone.


Barney

 

[Gene Cho]

Abrasion - sandpaper or scraping with a knife blade.

Thanks for the tip! I'll try that. Does anyone know a safe solvent product for Bose 901 cones?

If the surrounds are ok, how do you know the buzzing noise is being caused by the speakers? Did you swap them left to right? Is it happening in both left & right speakers?

I've changed amplifiers and the noise is the same. There are two types of noises from right speaker. One is occasionally occurring static noise from one (or more?) of the driver units, and the other is rattling noise only from very low frequency. The second one occurs mostly when I watch video on certain scenes, not from normal music sources.

The left speaker also has noise occasionally, but it's obviously a VC touching noise I believe, as with pushing and aligning one driver unit, the noise is gone for a couple of months.

The buzzing may be a spider loose from the cone.

Barney

Hey, Barney, I don't really have knowledge about this. I have almost no experience in repairing audio stuff-I spent hours to research and found these could be mostly VC problems or rotted (from inside) surrounds, as from outside, the foams look OK, except for a few places that are a bit apart from cones and may need to be re-glued.

If it's a spider problem, does it involve soldering?

If this requires soldering, it's actually out of my hands. :-(

Thank you!

 

[62vauxhall]

A spider is another component of the cone's suspension system. It is attached to the cone's apex, very near the voice coil and only visible with the drive unit out of the cabinet.

Those noises you describe make me lean towards deteriorated surrounds being the cause. It could be a spider problem (although that is not super common) or it could be a delaminated voice coil. Both of those would lead me to believe there was major yahoo-ness from a previous owner - too much power or too little power turned up too loud.

To do any assessment, all the drivers must come out of the boxes. A good place to start is measuring resistance of all the voice coils. Ideally, they should all be pretty close. If otherwise, then transplanting a replacement might be cheaper than fixing the old one....or not.

Re-foaming drivers is pretty much a routine necessity with old speakers but it is a bit tedious. Unfortunately, in the case of 901's there's a total of about 19 lineal feet of old surround to get off.

 

[jcamero]

I've use clear fingernail polish remover and a Qtip with great results, but you need to scrape off as much of the old foam as possible first.

 

[Blue Shadow]

I use a dremel with wire brush and brush the foam off the edge of the driver. A mistake in the spin direction (from inside to off the cone) will let the brush catch the cone and damage it. Not for those that can't control power tools 100%. I now have a slower turning, higher horsepower unit I will be using in the future, same small wire brush.