Re-Foaming Speakers

My mom is moving to a smaller apartment, and she no longer wanted her old living room stereo. Mom offered it to me, and so, out of the blue, here comes another stereo post.

I certainly do not need more audio equipment. Our basement is beginning to look like an electronics store. I was interested in the speakers, though. They were produced by Boston Acoustics. At the time, the A70s were lauded for their value and performance.

I was disappointed when I pulled off the speaker grills. The woofers' foam surrounds were crumbling to dust. You can see where my finger poked through the foam near the 2 o'clock position. I barely touched it.

According to the serial number tag, these speakers were purchased 31 years ago. No wonder the foam was falling apart.

I didn't know a lot about speaker construction, and I assumed that these were ruined. I did a search for replacement woofers, and found foam edge repair kits instead. There were quite a few kits on eBay. Maybe I could try fixing the foam surrounds myself. Why not try? 

I did some research, and I believe the foam surround has two jobs to do. One, the foam acts as a support that allows the cone to move out and back the required amount. When the foam goes bad the speaker can perform like a car with bad shock absorbers. Two, the foam surround holds the cone on "center" so the attached voice coil will move freely and won't rub against the magnet. A speaker can "work" without foam surrounds, but the damage from the rubbing will ruin it.

(diagram from https://www.cuidevices.com)

Simplyspeakers.com has a lot of repair videos on YouTube, and they were also running an eBay sale, so I bought a kit from them for $20.

I watched this video several times to learn the process. ----> link

The first step is to remove the old foam from the speaker's frame and cone. You can see the replacement foam on the right.

The next step, gluing the foam suround to the cone, wasn't too difficult. The kit's glue did set up quicker than expected. There wasn't much wiggle time before the glue took hold. 

I was much more worried about the last step - gluing the foam to the metal frame. If I glued the surround in the wrong spot, the voice coil would be forced to rub against the magnet, and I'd have to start again.

I didn't trust myself to find the "sweet spot" by hand, so I used a little science. 

I needed an amp or a receiver, a speaker wire, a RCA to 3.5mm adapter, and a laptop. The idea is to play a low frequency tone that the human ear can't hear through the speaker. The coil will make a rattling sound until it's in the correct position. At that point the speaker will become silent because is it smoothly playing a tone that can't be heard. (Make sure the receiver's balance is turned all the way to the side you are using, and keep all volumes - tone program, laptop, and receiver - on low.)

This is the video that helped me find the "center" for the the cone. -----> link

I used this online tone generator to play the low frequency. I set it at a 50 Hz sine wave. ------> link

I connected the speaker to the receiver and briefly did a "dry run" without any glue. I used a long speaker wire that would allow me to turn the speaker as needed. I discovered that the speaker went silent when I slid the foam towards one side of the frame, so I marked that side with some tape.

I used rolled up paper towels to lift the cone up. I wanted more room to get the nozzle of the glue tube under the flap of foam.

I placed the paper towel rolls through the lower openings in the frame. I avoided the opening with the wires.

I must have been more nervous than I realized because my hand was shaking the glue tube. The speakers are meant to be played with the grills on, so if I would have made a mess it wouldn't have been too obvious...

When the bead of glue circled the frame, I removed the paper towels, held the cone up from underneath with my hand, turned on the tone, and lowered the foam onto the frame. I then edged the foam towards the tape marks. When the speaker went silent, I pressed the foam down into the glue in spots using fingertip pressure. It didn't take long before I could smooth the foam out with my finger, turn off the tone, and then walk away. The glue needs at least an hour to cure. That was it. Job done.

Before:

After:

Yes, I did the other speaker, too.

I tested the A70s with my retro stereo, and they worked! I'm really happy I didn't put these on the curb. I love thrift store stuff, but it was fun fixing something that has a family history.