Dished spider on refoamed woofer

[swechsler]

I picked up this JBL 128H cheap on Craigslist, figure I'll probably end up flipping it but thought maybe I'd play around with it a little first.

The woofer was refoamed by a previous owner. The foam was correctly attached to the back of the cone, and the voice coil is correctly centered (it appears that shims were used since the dust cap was replaced). However, the cone naturally returns to a position lower than it should be. In this position the spider is concave and incoming excursion is very low. What I'm wondering is if this could be caused by the surround being installed incorrectly (although I don't see what could have been done wrong unless an incorrect surround was used), or is the spider permanently formed into that position. If the latter, is there any way that it can be fixed? How can I determine which it is, short of cutting the surround?

 

[teal'c]

Spiders are stiffer than foams, so I'll guess spider.
Have you put audio to it? I've got a spider like that and I keep trying to convince myself it might work OK after a refoam.

 

[thurber]

"Dished spider on refoamed woofer" sound like the name of a lost Syd Barrett album or something.

 

[Chris Brown]

My guess would be someone probably put-off refoaming and set the woofer down on a shelf or similar, magnet side down, and left it there for a while. The weight of the cone would stress the spider over time, especially with no foam to help it, and that would be the result. Not sure of any easy way to fix it other than to try and set the cone upside down for a while, supported only by the outer frame, and try to let the problem fix itself, maybe with some help from your fingers.

I hate to see pictures like this of botched dust-cap repairs after using shims on a re-foam job. Seems like another case of someone getting bad advice on how to do a re-foam.

 

[swechsler]

"Dished spider on refoamed woofer" sound like the name of a lost Syd Barrett album or something.

Maybe a recently discovered Picasso?

 

[SmitterDave]

The same thing happens to many drivers with the surrounds gone and hanging up or down. Snell Type A's all models Ohm F's even the woofers in the subs on my Acoustat Model 3MH's hanging down without a surround. once they have been hanging you WILL need to replace the spiders.

Easily repaired if you know how, need to order proper spiders and now new surrounds.

Good Luck.

 

[drumbum]

Before tearing apart to replace spider, judiciously use a blow dryer to heat sagged spider while holding cone out.

Heat it up good and let it cool extended.

Hope this helps.

 

[pdm4606]

Could be the cone is too deep. Maybe by design or error. It may be the wrong cone for the application. Or maybe the re-foam is on the wrong side. Photo looks ok from here.
That would require another re-foam.
Or, the voice coil is sticking, in this case sticking down. Have you tried to carefully move the cone at all? Does it move easily?
In some cases mounting the frame and giving it a shim or 2 could twist it back to original shape. Or as in this case the die cast frame is possibly stronger than a stamped one. A new AA battery put across the terminals momentarily may do something. Maybe move the coil up or down. But it should move. If it doesn't move you have another problem.

 

[swechsler]

The cone moves freely and doesn't scrape at all. I think the cone is original. I like drumbum's idea of carefully applying heat to the spider. Any caveats?

These JBL cast frames could probably support an elephant without bending. They are ridiculously overbuilt.

 

[Ramseybella]

Can you get a closer shots of that surround?
I don't know but it looks a little flat like not allowing the cone to seat properly.
You may want to contact a reputable speaker rebuilder and send him photos of this.