Loudspeaker Rescue




Introduction: Loudspeaker Rescue

An singular loudspeaker on the side of the road was enough to make me turn the car around, retrieve the treasure, and leave behind just the plywood "free" sign as I drove away with fixer-upper of vintage speaker. Three feet tall, and sporting four woofers, a port, and some mystery crossover/loudness adjustment, this worn warrior was a few easy and inexpensive steps away from re-living its glory days:

             1) Un-denting the dust caps

              2) Re-foaming the woofer

              3) Repairing paper cones

In order to complete these steps we need the following materials:

-sewing needle or safety pin

-speaker re-foam kit that fits the woofer in question (I got mine from SpeakerWorks.com http://www.speakerworks.com/foam_surround_sizing_s/63.htm)

-coffee filters

-source of cotton fibers (cotton swabs, cotton balls, yarn, or whatever you have)

Step 1: Un-denting the Dust Caps

The dust cap in the center of the speaker protect the voice coil and motor that drive the speaker, so when a toddler pokes in a dust cap like it was bubble wrap, keep in mind the dust cap is just doing its job and keeping the crucial (and more expensive) parts underneath safe. It is also much easier to un-pop a dust cap than replace a motor structure.

This is where the sewing needle or safety pin comes in handy. Poke the  tip of the needle or pin into the dent in the cone, and position the needle sideways so as to allow one to pry/pull the dent out. Using the needle/pin to get inside the cap, one can push out smaller dents and wrinkles.

Alternatively, a vacuum cleaner hose can be carefully placed over the cone, and the vacuum may pull the dents out. This works for larger dust caps.

The dust cap won't look like new, there may still be creases in it and a small pin hole or two, but it will be in much better shape than it was.

Step 2: Re-foaming the Woofer

The major factor that hinders the Loudspeaker from playing music is the rotted surround on the 15" woofer. A speaker's surround is used to support the outside of the cone, and it keeps the cone's movement linear, and without wobble. If the cone wobbles too much as it vibrates, parts in the motor structure can scrape against each other and damage the speaker. Without the speaker surround, playing music is near impossible. 

So fixing the surround is the next step! Remove the entire woofer from the cabinet, to so it can be worked on more easily. Pry off the paper or plastic ring around the outermost edge of the woofer, and set it aside. It is glued in place, but it is okay if this ring is damaged slightly, but if one is careful, it can remain mostly intact. Remove excess glue and debris from the metal basket of the woofer. Carefully scraping away material with a razor blade, and using rubbing alcohol or nail polish remover to clean the surface very will is important, as the new surround will be glued directly to this metal basket. Please TAKE CARE to use the razor blade, rubbing alcohol and nail polish remover CAREFULLY and SAFELY, making note of all warnings and directions for those tools. After cleaning the metal basket, clean the cone of the woofer very gently, trying to avoid using solvents and sharp cutting tools on the cone, instead rub off the old surround with your finger or a cloth. Once both the basket and cone are mostly clean, the new surround can be mounted to the woofer. 

A tip to note is that if one side of the cone is clean, and the other isn't, for example if the original surround was mounted to the back of the paper cone, and the front of the cone remains clean, it may be alright to mount the surround to the clean side. In my case, the old surround was mounted to the back of the speaker cone, and I mounted the new one to the front. This will likely make no difference in how the speaker sounds in the end, but will make sure the new surround adheres to the cone correctly, which is very important.

In order to make sure the new surround is mounted correctly, the dust cap is cut off of the woofer, in a manner so that it can be attached again later. Be VERY CAREFUL not to cut too deeply into the dust cap, as you could damage the cone or wires underneath. With the dust cap removed, the motor structure of the woofer is exposed. The cone is attached to a cylinder called the voice coil, and there is a concentric piece inside that cylinder. It is crucial that the voice coil cylinder does not touch the inner concentric piece after the new surround is installed. To avoid it, one must "shim the voice coil" by sliding pieces of paper evenly into the gap between the voice coil and the concentric piece, until it is snug and the cone does not wobble side to side. This ensures a linear alignment of the voice coil when the surround is glued on.

The foam speaker surround kit came with a small bottle of glue, and an application brush. These were used to apply a thin layer of glue around the outside of the cone, so that there was a 1/4 inch wide border of glue around the cone. Apply a thin layer of the same glue to the inside lip of the new surround, and let them both sit for a few minutes, until the glue becomes tacky. Being careful to center the surround on the cone, place it on the cone so faces with glue mate with each other. Begin to press the two together evenly and uniformly around the perimeter of the cone. Make sure the surround is placed as it was designed to be on the cone, for example, the surround I used was meant to be red side up, with the half-roll round-side-up. Let the glue dry for a good while (several hours, or as according to the glue's directions).

Apply the glue to the speaker basket and outside lip of the foam surround, let it become tacky, and then press the two together uniformly around the perimeter of the woofer. PUSH LIGHTLY on the cone, and ensure that there are no scraping sounds or feelings, and that the voice coil remains properly shimmed and centered. place the paper or plastic ring back on the edge of the speaker, and place it face down (with weight... say a bowl of apples...) so that there is pressure on the newly glued outside edge of the surround. Allow it to dry for 24 hours.

Glue the dust cap back in place, using some tape to hold it while the glue dries, if necessary, and also glue the paper or plastic ring back in place. Wait 24 hours and HORAY! A repaired woofer! Re-install the woofer into the speaker cabinet.

Step 3: Repairing the Paper Cones

The last part of the loudspeaker that needs repair is the small tweeter, or high frequency speaker driver, which has a tear in the paper cone, which has also separated from the accordion-style paper surround. These two issues need to be addressed before the little tweeter will sound right.

The glue supplied with the woofer re-foam surround kit has the valuable property of being somewhat flexible when dry, and it bonds readily to paper, making it an exemplary candidate for repairing paper cones. The idea is to use a diluted solution of this glue (about one to one), along with coffee filter paper and cotton fibers to create a kind of paper mache patch for a torn paper cone. The diluted glue soaks into the paper and actually bonds both sides of the tear back together, with the light-weight patch for added support. If the cone is torn at the edge, where it meets the paper accordion-style surround, cotton fibers can be used instead of the coffee filter patch. The cotton is used where the cone meets the surround because it is more durable over time than the paper, and also more flexible at the high-motion flexing point where the cone and surround meet.

Remove the tweeter from the loudspeaker enclosure to make it easier to work on. Pour a small amount of glue into a small tray or the bottom of a plastic cup. A blob about the size of a dime is enough to repair the small tweeter in the picture. Add the same amount of water, and mix thoroughly. Cut a piece of coffee filter large enough to cover the tear in the cone generously, and keep it hand for use in a moment. Brush the diluted glue over the tear in the cone, and align the parts of the cone so the edges of the tear are touching each other. Do the same with any other tears in the cone or where the cone joins the surround.

Take the coffee filter patch that was cut just a moment ago, and dip it in the diluted glue. Wipe off any extra glue, so the patch isn't dripping wet, but is more than moist, and place it paper mache style over tears in the cone. If the tear is bad, and needs stability, a second less wet patch can be applied, all the while being careful not to knock the parts of the cone out of alignment, so the edges of the tear remain touching. Pull off a very small pinch of cotton, so the strands and fibers stretch. Pull out those fibers and place them, several fibers at a time, along the edge where the cone meets the surround. Let the diluted glue that is there absorb the fibers and adhere to them. Try and get the fibers to adhere to both the surround and the cone. Next, let the whole thing dry in a safe place for 24 hours. Check to make sure the tears were all successfully sealed, and your done! Re-install the tweeter into the enclosure!

Step 4: ENJOY!

Hook up the repaired speaker to an amplifier or receiver of your choice, and listen to Ben Howard or Mumford and Sons' new albums, or maybe some Ellie Goulding or Of Monsters and Men, or whatever music suits your fancy. The real point is that you enjoy your newly restored loudspeaker.



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    34 Discussions

    well done! I just posted a similar instructable, I found my speakers on the side of the road too, mine are smaller, but still very hard to carry on my bicycle... LOL! Did you have to let the speaker play some time before hearing a good quality sound?

    2 replies

    I didn't have to let it play for long, it sounded great right after the repair, though typically new surrounds take time to break in completely. At this point in time the speaker has been seeing regular use for a year and all the repairs have held up very well, and it still sounds great!

    thanks for posting I never knew that the subs could be refoamed

    Great post, restoring free stuff is my favorite. Curious if the ohms match up to what the box says. I've got a pair that look similar and one has a toasted x-over that is on the repair list.

    Several years ago I traded a hybrid waterbed for a pair of Bose 901 Series IV speakers. They were in excellent condition, or so I thought. After using them for a while I decided the sound just wasn't right so I took the grill covers off only to find the foam on almost all the speakers was disintegrating. Luckily everything else was still in good shape. I bought a repair kit (set of foams & glue) off Ebay & replaced them & the speakers still sound like brand new. It was an experience that took about 2 weeks, scraping the old foam & adhesive off with an exacto knife (there are 8 speakers in each unit), but was well worth the effort.

    Nice that you saved another old speaker from certain death. Good 'ible!

    Nice job saving these from the landfill.
    Speakers are easy to fix if you have a source for the new surrounds.
    It's more important to have patience than skill to do this.

    Methyl Ethyl Ketone (MEK) will loosen the glue used on the original surround and dust cap making it unnecessary to cut them off. You can get this in the solvents area of the paint dept of your favorite hardware store or home center. Do this in a well ventilated area, this goes for the new glue too. Don't breathe the fumes.

    If you're replacing the midrange drivers, make sure to get ones that have a sealed back, especially if it's a sealed enclosure. Otherwise the pressure developed by the woofer will blow them out.

    1 reply

    Does anyone here know how to test voice coils to check if they are ' blown ' or burnt out ? I assumed the way to do it was to check the resistance of each ( Voice coil ) with a DMM to see if it matches the stated spec. And does it have to be exactly the stated resistance, or is there a range, as in 2 ohms ( 1.8 - 2.2 )

    1 reply

    If the voice coil was burnt out, I imagine the resistance would be orders of magnitude higher than spec, or there would be no electrical connection at all, and wouldn't be readable. It does not have to be exactly spec, there will likely be an acceptable deviation range, and if the spec is for impedance (also measured in ohms) the measured resistance of the coil will likely be lower. If there is a raspy or metallic scraping sound when the woofer plays, there is likely physical damage to the motor structure, (or a cone/coil misalignment).

    They actually are not dust caps but reflectors for the mid range frequency sound. On speakers, only the lowest frequency sounds propagate to the outer edges. The mid frequencies loose too much energy to reach the outer edge. They use the center cap to improve the dispersion of the mid range frequencies. The best sound location for speakers are corners of a room - where the two walls and a floor or a ceiling meet. The room acts like a horn to boost the bass and focus the sound in one direction.

    Thanks for posting the instructable.

    1 reply

    Great instructable! Besides straight pins I have also use a bit of good sticky tape to restore dust caps.

    Yes, blanchae, believe it or not, the industry refers to it as the 'dust cap'. One time I was able to fix a dented dust cap with a vacuum cleaner. As you might expect, I put the end of the suction hose up to the dust cap until the suction grabbed hold and then pulled. The dust cap has to be fairly big, though. The best solution though is to install a heavy metal grille so it doesn't get dented in the first place.

    I use a vacuum cleaner to pop out the speaker cones. Kids always press them in.

    Good instructable! I did the same thing with two huge vintage Pioneer speakers that I bought for $5 each at a thrift store and they rock the house. Love those speaker kits. They make them for all sizes if you want to do the smaller ones and sell the dust caps separate if you want to replace the crushed ones, or just cut them off, push out and reattach and an alternative.

    I've always used a loop of sticky tape, wrapped sticky side out, around my finger to "pull" a pushed dust cap out. Also blue-tack, if manipulated / kneaded enough so that it is as sticky as possible, will also work.
    Seems unnecessary to pierce the cap with a pin.

    When my son was little he thought it was funny to poke at the Dust Caps. I always used a piece of Scotch Tape to pull them back out.