Speaker Foam Surround or Voice Coil Problem?

Archived from groups: rec.audio.tech (More info?)

Recently, I started hearing serious bass distortion on my vintage JBL
L112 speakers. I inspected them and found that a couple of sections of
the foam surround had disappeared (about 10-percent of the total
circumference) on both the left and right woofers (12" 128H).

Will re-foaming the speakers eliminate the distortion, or is it likely
that there is voice coil damage? And what exactly is voice coil
damage? Is it an alignment issue or does the coil of wire actually
wear out? I'm kind of surprised that the relatively small amount of
missing foam would cause so much distortion. I rarely drive the
speakers to more than a "moderately loud" level.

Suggestions welcome.

Thanks,
Art Harris

Archived from groups: rec.audio.tech (More info?)

"Boris Mohar" <borism@sympatico.ca> wrote in message
news:j7e5p09orp0pkrrf63882ggk3pjp2doevq@4ax.com...
> On 10 Nov 2004 11:32:20 -0800, n2ah@hotmail.com (Art Harris) wrote:
>
> >Recently, I started hearing serious bass distortion on my vintage JBL
> >L112 speakers. I inspected them and found that a couple of sections of
> >the foam surround had disappeared (about 10-percent of the total
> >circumference) on both the left and right woofers (12" 128H).
> >
> >Will re-foaming the speakers eliminate the distortion, or is it likely
> >that there is voice coil damage? And what exactly is voice coil
> >damage? Is it an alignment issue or does the coil of wire actually
> >wear out? I'm kind of surprised that the relatively small amount of
> >missing foam would cause so much distortion. I rarely drive the
> >speakers to more than a "moderately loud" level.
> >
> >Suggestions welcome.
> >
>
> The voice coil bobbin is rubbing on the center pole of the magnet. Remove
> the dust cap wit acetone and shim the voice coil with four pieces of
paper
> of suitable thickness. Credit card works sometimes. Replace the foam
and
> let he glue dry thoroughly. Only than remove shims and replace the dust
cap.
> Stay a light year distance away from any ferrous filings. There is plenty
of
> info on DIY foam on the web. I bought 12" foam rings for $4.00 in
Toronto.
>
>
Another method that frequently works:
Replace the foam. Foam replacement kits come with shim material to center
the voicecoil while glueing.
After you're done, remount the speakers rotated 180 degrees. The gradual sag
of the spider from the force of gravity is now reversed.