In last years it happened to me to run across ruined speakers more often than we would think. I love those old heavy and huge black speakers typical of '90 years. I obtained this pair for free, I decided to repair it and this time I supported the process with a full photographic documentation and a detailed instructable for your pleasure ;-)
The re-foam process is the same for squawkers and subwoofers, but since I fixed both you can see the entire repairing dynamic for both cases. I also documented myself before planning my first speaker refurbishing attempt, and I found a good guide on decware.com. After some successful realizations I can tell that's a useful reading.
Step 1: Is it really to trash?
As you noticed from the first image the speakers appeared in a very bad situation. Actually you wouldn't tell, but this is an ordinary situation for a 20 years old woofer, indeed the edges (made of foam or rubber), after many years crumble. The tweeter's cone doesn't need to move as much as the woofers so there is no edge which could break. A speaker in this state usually emits a very low sound, and it's practically useless, many people throw it away, since a refurbishing professional service could be expensive. I will explain how to bring them back to life with a 15$ investment.
Step 2: The replacements
All you need are the substitutes for your speaker's surrounds. You can choose either foam or rubber edges from eBay. To choose the right size unfortunately is not so simple as to choose between S, M or L. You have to measure the external diameter of the cardboard cone, and the inner diameter of the steel frame. Practically you need the exact inner and outer diameters of the bumping central stripe of the surrounds. The inner and outer diameters of the full rubber/foam piece could vary a little bit. Measures are usually specified in inches, so if your meter uses metric system, I suggest you to convert it in imperial units. Some surrounds sellers let you choosing between flat and sloping cone border, so choose that feature after inspecting your speakers, that will affect the slope of the surround's inner stripe. I bought these and these replacements for my speakers.
Step 3: Cleaning process
Once obtained the replacements, first action is to remove old surrounds and clean cone and frame, so to obtain a smooth surface. As tool you can use a flat blade cutter, scratch out the foam and glue remains, ant pay attention to not cut the cardboard cone.
Step 4: Removing big domes
To center the cone during the gluing, wee need to obtain access to the central steel core. Since it's usually covered by a convex cardboard plate which protect the core from the dust, we have to take it off.
Dust covers have to be removed AFTER cleaning the surfaces from any dust and especially any iron filing remained after scratching the frame. Indeed magnet of the speaker will attract those filing, and they will obstruct the gap around the magnet, contrasting the cone movement.
With a cutter, paying attention to NOT CUT THE CONE, detach the dust cap from the cone. In the case of the subwoofer this parts are glued to the cone and with a small pressure they could be detached. We'll see that it's not so simple for the midrange speakers. You can try to leave a small hinge attached so to insert caps in the exact initial position.
Step 5: And little ones
Indeed for these smaller speakers, the dust caps are glued much more tighter, and I learned that trying to unglue them could ruin the cardboard cone surface. So I decided to cut them at an inner circumference, as you can see in pictures. But please try not to cut the power wires which lay at the bottom of the cone.
Step 6: Centering the cone
The paper sheet is needed to form some paper stripes, so that they can act as spacer to isolate the cylindrical core from the metal tube connected to the cone. This thickness is not constant for every speakers, and you have to try with more sheets and lengths.
I made a mistake here since with my 80 g/m2 sheets I had to make one and half turns to reach the exact thickness. That results in a not centered cone, because the spacer on one side is double than on the opposite side. By the way this mistake doesn't affect the speaker functioning in my case, but choose a different sheet thickness if you encounter in a similar circumstance, or use four shims, trying different thickness.
Step 7: Pour glue
As glue any vinylic glue is good, but I suggest you to check that it will become completely transparent, some of them remain white after drying. With the paper cylinder well into position place the foam surround over the cone, so to see if it fits right. You can also remove the speaker from their wood case, but I find useful to have a so handy stand.
Add glue all around both frame and cone, try to pour the right amount of glue and to diffuse it uniformly.
Step 8: Gluing edges
Now add the foam edge pushing it gently over the glue. Avoid forcing the cone on one side or the other, since it has to remain centered. With your finger spread the excess glue, don't worry if it appears to be too much.
Step 9: ...glued
Indeed when the glue will dry any excess will magically disappear. You can now remove the inner paper spiral. Now make a constant pressure on the cone with your hand and check that there is no friction at all between core and metal ring. If you feel friction, and you are sure it's not due to an unbalanced pressure from your hand, you can unglue the external stripe of the surround, and glue it again letting it free to position.
Step 10: Gaskets
Now set up all the foam gaskets cleaning them from any glue remains, by the way if your work is not accurate nobody will notice that ;-)
These gaskets are supposed to keep the front cover far from the surrounds, and to protect these from any accidental bump, anyway they are not essential.
Step 11: Gaskets glued on subwoofers
Add some vinylic glue on the outer stripe of the surrounds and also on the metal frame circumference, then place the four parts of the gaskets so that they match with the holes and between themselves. Push them over the glue and let them drying.
Step 12: ...and on squawkers
As you notice the foam gaskets for the midrange speakers are too tight and the make an interference with the new surrounds. This could be easily fixed cutting away a stripe of gasket from the inside circumference.
Step 13: Dust caps...
If you arrived to this step with no mistakes you can be proud of yourself, since the worst is left behind.
Gluing back in place the dust caps is not easy but could not affect the behavior of the speaker, so proceed serenely ;-)
I used a pair of adhesive tape to handle the domes better, but use paper tape, since in my case this ruined a little bit the cardboard.
Place some glue all around the disc border, don't leave any interruption along the glue trail.
Step 14: ..glued on subwoofers
Place the cone in his original position, push it gently trying not to leave big lacks of glue, and leave it drying. Any small split which appears without glue will be filled spreading glue with your finger. Place a weight (not too much of course!) over the cardboard dome until it's dry. Any further split in the gluing will be filled after.
Step 15: ...and glued on squawkers
The process to glue back in place the cover caps of the midrange speakers is identical to what you did for the subwoofers. Also in this case I suggest to use paper tape, you will have less damage to your caps when you'll take it off.
Step 16: Refinishing
When the glue around the caps is dry you can check there are no splits, and you can add some more glue just to be sure. Spread it with your finger and let it dry. The white will disappear if the glue is transparent enough, if not you can paint it with a black marker. Try not to add too much glue to keep low the weight of the cone and consequent inertia.
Step 17: Final breaking in
Assemble everything back and screw tighten each screw.
Your speakers are not ready to be placed around your hi-fi... you have to wire them but also to wait some more time before hearing them in all their new shape... anyway don't worry, all the exertion is done.
Indeed when you'll turn the hi-fi on, the sound will be horrific, that's because the new surrounds need to work for some time to loose their rigidity.
To do this process in the shorter time possible place speakers one face to the other, connect one speaker's wires reversed (positive wire on negative connection, and negative on positive), and let both speakers working with music at high volume... not too much high depending on your neighborhood ;-) By the way one speaker will cancel the sound of the other, and you will hear a low music.
After one hour, more or less, you can connect both speakers with right polarity and place them at your hi-fi sides.
Step 18: Ready to play!
Place also the two covers back in position, obviously after washing them. I had also to glue four new velcro pieces at their corners, because old velcro didn't work well anymore.
Now I can enjoy my new stereo system! Do you recognize another interesting project in the picture? ;-)