refoaming without pulling dustcaps ?

[SEAWOLF97]

I keep hearing about great refoamers that can do it without removing the dustcap to place shims. How do you do it ?? All I can think of is that they pull the magnet and shim from the rear. Is this it ?:hmm:

[Robh3606]

Hello SEAWOLF 97

No. You can't always do that. Many times the screws attaching the frames are under the spider. Typically when you do a refoam you take a leap of faith that the foam is the only thing shot and the spider is still in reasonably good condition. If that's true the spider will keep the voice coil centered as long as the cone is not tilted in the gap. If you look at refoam kits there are 2 schools of thought. One is to cut open the dust cap, shim the coil and do the new surround. The other school is to leave the dust cap be and use a low level LF tone to self center the coil in the gap. You use the magnetic field to help align things. Me I leave the dust cap in place. Since your not going to change the spider anyway why open up the voice coil gap and risk damage to the cone by cutting off the cap?? Never made sense to me so all the refoams I have done have been the leap of faith school where you change the foam only. So far it has worked for me just fine on the drivers I have refoamed. That said you have to remember that the only repairs JBL will sanction and guarantee must be done by an authorized Repair center using complete JBL cone kits. The refoams are a new lease on life without the expense of the cone kits which can be considerable depending on the driver. The right way is a recone, that's the bottom line providing your budjet will allow it and if the kit is available. I have refoamed a bunch of LF drivers but I have also had new kits installed when I could.

Rob:)

[clh1997]

The other school is to leave the dust cap be and use a low level LF tone to self center the coil in the gap. You use the magnetic field to help align things.

Very useful information - thx Rob.
I’m about to re-edge a pair of JBL 122A (from the L166 Horizon) and will prefer to leave the dust caps on. How necessary is the low frequency/low level tone centering?

Mine seems OK centered and are moving w/o friction of the coil.
Can I easily disturb this during mounting of the new edge?
And how can the LF/LL-tone help me avoid this - must be impossible to re-center w the tone first the new edge is glued on?

Which frequency and level did you use?


Thx in advance!

[SEAWOLF97]

Hello SEAWOLF 97

The other school is to leave the dust cap be and use a low level LF tone to self center the coil in the gap. Rob:)

Thanx for your answer Rob. Can you elaborate on the tone procedure ???

[Robh3606]

Bo wrote an excellent step by step post that seems to get lost. I made it a Sticky in the DIY forum. This will explain the whole process. Before you start I always sweep them with a low level tone to see if the VC is rubbing or not. I use a Denon Test CD with a sweep from 200Hz down to 20Hz. It also has some warbled LF tones that are used to set subwoofer level that are great to use as well. Not loud just low level to center the coil. I will just put it on repeat and let it sweep until the glue set's up. When it's getting close I will turn it up a bit from 60Hz down to make sure it's aligned and not tilted in the gap so it won't rub during large excursions.

Rob:)

http://audioheritage.csdco.com/vbulletin/showthread.php?t=469

[grumpy]

Rick Cobbs kits also come with a tone CD and instructions (when last I went down that path).

[SEAWOLF97]

pulled the dustcaps and used shims. 100% perfect (so far):applaud: