Speakers have a rubber or foam "donut" around the edge of the cone that connects the cone to the frame, keeps the cone centered, and restricts the maximum excursion of the cone. These surrounds, especailly the foam ones, become brittle and crack or rot away over time making the speaker unusable. This is what happened to my Boston A100 10" two-way speakers, manufactured in 1985.
Forturnately they are repairable for not much money with a few tools and some patience. I used a kit from Parts Express, there are others out there that will also give good results.
Speaker Surround Repair Kit from Parts Express. About $20. Includes two sets of surrounds for different style cones, replacement dust covers, glue, glue applicators, voice coil bobbin shims and instructions.
When your surrounds have rotted away, what can be done?
The surrounds in most speakers can easily be replaced. The instructions included in the kits are easy to follow and will give good results if you follow them step by step.
Boston A100 speaker with one toasted surround. I''ve had these a long time. They still sounded good up until the surrounds dissapeared.
10" Bass driver removed. Sticker shows the date of manufacture as July 25, 1985.
Old surround and dust cap removed. You must scrape out all the old glue for the new surround to fit properly.
The voice coil bobbin is shimmed to keep it centered during the surround installation and while the glue dries. If it is off center, it will rub on the pole piece or magnet and sound bad or fail completely.
New surround glued up ready to install. The kit I used provided two different styles. You have to try them (before glueing) to see which works best.
All done. Surround glued in place and dried overnight. New dust cap glued in place. Ready for reinstallation in the enclosure.