Repairing Damaged Speakers / Drivers
This article will give some background information on speaker damage and repairs, and run through the correction of damaged dustcaps on a small driver, from removing the deformation to patching the damage. Weíll also briefly touch on a speaker refoam job. The same principles apply to patching other cones, domes, and surrounds: similar materials, and only as much material and adhesive as necessary.
Tremble, ye who hear these words. They represent the hideous destructive capacity of a child. Even things that interest and please them are mercilessly beaten and destroyed. And what looks more interesting than that little unprotected dome in the middle of your little, unprotected midrange.... or that little eyeball at the top! What's that shiny metal foil? Even many adults are driven to touch the parts of a speaker: to these people a speaker is a magical item which they are compelled to touch. I am one of these people. I have to plug it in, test it out. I'm not hard on them, but I touch many speakers, handle them, take them apart. Speakers are the most important part of a system, and many times an excellent speaker that someone would like to keep around gets a curious finger, an overzealous volume control, or simply needs a little tender loving care.
In instances of speaker damage, there may not always exist a drop-in replacement. Speaker drivers change all the time, and few manufacturers stock replacement drivers for 20 year old models. Even JBL, known for the availability of replacement components, cannot provide replacement cones for some models (including Lansaplas coated woofers such as those used in the infamous L100) because of restrictions on the material, low demand, and other factors. Many times, the most effective solution to maintain the speaker's original characteristics is to need to repair driver damage. Driver damage doesn't just consist of poked-in speakers, there's also blown speakers, speakers whose surrounds have dry-rotted and need replacement, speakers with rips and gouges and other problems.
Depending upon the severity, a person may have to replace both drivers in a stereo speaker pair (heaven forbid it's surround!!!) with the closest new equivalent they can find, and hopefully be satisfied by the results (some crossover retuning is usually necessary here as well). This is a worst case scenario, and can be avoided by finding an original replacement driver, or repairing the driver damage. For some items such as small tweeters, some damage is absolutely fatal. Neither the blown voicecoil nor the damaged dome nor the torn surround can be properly serviced without total replacement. For some tweeters, woofers and midranges you have a little more working space, and more options. You can replace the entire cone assembly, including surround, spider, voicecoil, and cone on some models (including most compression drivers used in horns) or only the surround on others. Replacement of the whole moving assembly is more involved for woofers and midranges, requiring a deft touch and careful centering of the voicecoil in the magnetic gap, whereas replacement tweeter or compression driver assemblies are usually 'drop in' using a guidepin or some other alignment mechanism, though proper protocol requires test tone sweeps and gentle tapping into optimal alignment.
1) Blown Speaker
2) Bad Surround
A Foam Surround
Once the cone and frame are clean, you must do a 'dry fití. You donít want to start gluing until youíve confirmed that the foam is a proper fit. Pay attention to what the stock arrangement was. Foams are sometimes mounted to the front, sometimes mounted to the rear of the cone.
You will then be applying glue. Get a feel for the consistency of the glue. Normal glue sold with speaker surrounds is PVA, and is non-toxic and water based. You want to spread an even layer on the cone first, and make sure itís thick enough to have no dry spots, but thin enough to avoid large 'clumpsí. Before doing any glue application, spread a little out onto a sheet of paper and get a feel for the stuff. Youíll also want to make sure youíve removed as much of the old foam as possible.
Once youíve done this, apply glue to the area that the surround will attach to, spread evenly.
Youíll then apply glue to the surround in the portion where it connects to the cone, evenly. Give it a couple minutes to start drying, then pinch it gently but firmly into place, working your way around, make sure you get good adhesion, and keep pinching for a few. I usually check on it to make sure itís properly adhered every 5 minutes for 20 minutes or so. Hopefully you did a good job, DO NOT USE TOO MUCH glue, and got it into place so that you donít have any gaps or air bubbles. A little glue coming out on the edges of the surround (and I do mean a LITTLE) is ideal. Careful not to overdo it with the glue, as if you let too much out itíll look awful (but is mostly a visual issue)
Let this cure for a few hours, then youíre ready to work some more. Youíll want to use your test tone now. Hook it up to the speaker, let it play at modest volume (cone will be visibly moving, but not much more than visual motion- be careful not to overdo it. Start at mute and work your way up. Once youíre satisfied that there are no rattles and buzzes (which would indicate a loose winding or other problem), mute the tone again. Then repeat the glue process, substituting the frame for the cone. Try to visually center the foam on the frame, but donít stress about it too much. Once itís attached but BEFORE letting it cure, turn the test tone on again. Listen for rattles, scraping, or buzzing, look for asymmetrical "rocking" motion, or asymmetrical deformation of the surround, and gently slide the surround into the best position on the frame to eliminate these issues. Then turn the tone down very low (barely audible) and repeat the curing process, checking every few minutes while the glue dries to make sure itís attaching properly.
After a couple hours, test once again with the tone, make sure youíre getting clean response, and youíre all set, assuming you did it right and have no extraneous nastiness. When you gently push and pull the cone, there should be only minimal resistance (within a few mm in and out), and no sensation of rubbing or scraping. If there is, make sure youíre not inducing tilt during your test. If itís rubbing or scraping, you didnít get the centering right and you need to start all over.
Now youíll glue any gaskets or trim rings back in place. Just glue 'em, stick 'em, and let 'em dry. Easy as pie! You can see that the rubber cement poorly chosen to patch the cone previously has been replaced with whiteout, a simple process of peeling and painting (thereís also some spots on this coated cone that got a little extra white-out to emulate the original Lansalas coating).
All done! Let it dry overnight and your speaker is back to performing as it should!
On those rare instances when you can repair a surround (rubber, paper, corrugated cloth surrounds with small damage), you must find appropriate materials and adhesives for the material. Internet searches have a lot of information available, you'll have to do your due diligence and figure out what you need to do by matching your materials to those of a speaker cone. For a treated cloth surround with a cut in it, one might use a small snippet of a woman's stocking might make a good patch, with a small amount of rubber cement. Remember, less is more, and use lightweight adhesives. A general hobby store (In the USA, Michaels hobby stores are common) is an excellent sources of repair materials, as well as parts for generic hifi tweaking. Some of the same guidelines apply to surround repair as cone repair. See the section on dustcaps and cone patching below:
3) Weak Woofers
■ More on AlNiCo: The alloy used for most of the decades in which speakers were built was power limited. Too much current through the voicecoil and you risked demagnetizing the speaker driver (the magnet assembly, in particular) to some extent (typically maxes out at 3dB loss in sensitivity, and an according shift in Qes, Qts and Vas). This throws off the bass alignment and the whole speaker design goes out of whack. Modern speakers don't have this problem, but have problems that AlNiCo didn't. AlNiCo is a very stable magnetic material, stronger than ferrite, and is electrically and thermally conductive, much moreso than ferrite ceramic. This leads to good heat dissipation properties and stable magnetic flux. Speakers transitioned to ceramic magnets when AlNiCo became nearly entirely unavailable due to the trade embargo against Zaire which supplied the vast majority of Cobalt (the Co inAlNiCo), and now have features like 'shorting rings' (aka faraday rings) and shaped pole pieces and top plates to help minimize the problems that ceramics have vs. AlNiCo. Manufacturers are using AlNiCo more and more again, though the material is relatively expensive, several major manufacturers have implemented AlNiCo products back into their lines, usually at a premium, but it's had a relatively long following for guitar pickups and speakers. Lowther, Fostex, and Seas are all popular suppliers of DIY drivers, and all have (or have recently had) AlNiCo products in the 'full range driver' category. JBL recently reintroduced AlNiCo with an extremely complex motor design with multiple shorting rings for the woofer in one of their premium products. The 15" costs about $2500.
4) Damaged Cone / Dome
a) Pucker up sweetie: it's time to suck. Make sure you have the space to yourself and get close to your speaker. You can often gently 'suck out' a poked in dome or dustcap. This is generally the most effective and safest way to pop out an indentation. Some people do this with vacuum cleaner attachments, but you need to be extremely careful to limit the amount of suction applied. You can kink the hose, or put a wad of rag in it to soften the suction, but be very careful in any instance.
b) Tape it: attach some tape to the indentation, being careful not to make it worse, and try to pull it out that way. Use a mild adhesive tape, like blue painters tape, scotch tape, or masking tape. You want to apply the least amount of force you can while still getting the job done. A stronger adhesive would be prone to tearing the speaker, or leaving a residue.
c) Poke 'n pull: this is the worst case scenario for a badly poked in dustcap or tweeter dome. In the case of a tweeter, it's a horrible situation, you'll likely wind up with a compromised product when you're done poking and pulling and patching. For the patching, you do as you would in the speaker cone itself- try to use similar materials. Paper makes this easy, as you can use tissue and PVA glue (yellow carpenters glue, lightly diluted with water, or "modgepodge").
So, in many cases, you'll need to repair a cut or puncture in the cone or dome, as in method 3 for fixing a poked-in dustcap or tweeter dome. The idea is to apply as little new material to the assembly as possible, and match the characteristics of what is there. The same method used for patching these is applicable to speaker surrounds and cones, however, depending upon the driver's base materials; you must adapt your choice of patches and adhesives accordingly! Rolling papers are a fantastic solution for paper cones.
Down To Business!
Note the hole in the dome, carefully cut to allow the insertion of my little 'hook' implement (small boomerang-shaped piece of copper).
This driver had a severely poked-in dustcap. I began by attempting methods 1 and 2, but was unsuccessful in returning these dustcaps to their appropriate shape. In this instance, we go to method 3, and carefully cut or poke a hole in the dustcap/dome. This hole should fit a small, sturdy piece of wire or other implement with which you can gently restore the original profile (shape) of the dustcap/dome by applying pressure from the inside. Careful, you're working with a strong magnet close to your tools, so be sure you're not going to lose it and make matters worse!
Once this is accomplished, we proceed to the patching of the hole we just created. This applies to surround repair (when possible) and cone repair (again, where possible), the only difference is which materials match the surround or cone. In this instance, we're using paper cones, so the obvious choice for patching paper, is, paper. A small section of tissue paper (rolling papers work here too), to be precise. Here we also see a pipe cleaner for application and a disposable lid used for thinning the glue with water. Note: the size of the piece depends upon how much area and stiffness you require from your repair. The piece shown here is somewhat larger than the final piece, which was shaped down. The oversized piece photographs better.
Since we have an appropriate patch material, we needed to choose an adhesive. In this instance, poly vinyl acetate glue is appropriate. This is a water soluble glue that binds well with paper (indeed, this is the basic formulation of 'wood glue', also sold as puzzle coat under the brand name ModgePodge). For a poly cone, someone might wish to use a rubber cement or silicone glue, and in minimal quantity, with a small piece of soft plastic or rubber for a patch. For metal cones, a layup of 2 part epoxy would work, in the rare instance when a metal cone could be cut or punctured without 'creasing' the cone (which is a death toll, as it will completely change the driverís behavior and likely distort like crazy). In all instances, use only as much glue as necessary to lock the patch in place. In some damage, you can re-join separated pieces of cone/dome/dustcap/surroud with only the adhesive and a deft touch. In all other instances, be sure to get the patch material as close to 'disappearing' on the surface as possible, in other words, it shouldn't leave any more of a discontinuity than necessary.
For this cone, we thin the glue with water to a likely consistency, and apply it to the patch and cone, carefully avoiding drips. Once all is said and done, you have a very serviceable repair, hardly distinguishable from the original cones beyond a few inches. Note the second pair of drivers, Radioshack 40-1197, a very common DIY driver. These were purchased knowing they had the indentation of the dustcap, and with the knowledge that they could be repaired.
So, there you have it! The skills involved in speaker repair are tremendously useful, not only in the instance of damaged drivers, but also when 'dumpster diving' or hunting for classic/vintage speakers. Without this knowledge, I wouldn't have been able to 'save' some pretty decent speakers, from those little Radio Shack drivers, to some larger scale toys. Many of the classic and vintage speakers are of very high quality. Classic Altec and JBL are generally excellent (and well known), but there are fantastic drivers from Phillips, Foster, and others, as well. Keep your eyes peeled, there are many gems! So, good luck to you who endeavor to repair your own speakers, and hopefully, this article saves some speakers and/or money.