[Ron Ramirez]

Most speaker cones are held to the edge of the frame with glue. You will need to break that glue with a suitable solvent.

Here's how I've seen Phorum member Ed Locker do this:

Apply acetone, sparingly, all the way around the edge of the cone where it is glued to the frame. An eyedropper is very handy for this. It may take multiple applications, but eventually, the old glue will soften and you will then be able to remove the cone edge from the frame.

You need to do the same with the felt cover over the voice coil, if it has one. Now, you have to be very careful here as there is the chance that you may separate the cone from the voice coil, and you do not want to do that!

Once the felt cover is off and the edge of the cone is off the frame, you may proceed.

Take an old business card and cut narrow strips, long-ways, from the card.

Now take three or four of the strips you cut and insert them between the voice coil and the speaker pole piece. Space them evenly apart.

The voice coil needs to be inside the gap of the pole piece as much as it would normally be at rest.

With the voice coil shimmed, reglue the edge of the cone to the speaker frame. Contact cement should do the job.

Let the glue dry.

Once dry, remove the shims and your voice coil will then be re-centered if you did everything correctly.

Easy as pie...well, not quite...but it is still not a terribly difficult job.