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Picked up a pair of JBL L112's - Need advice on new foam surrounds...
- Thread starter VintageUT
- Start date
VintageUT
New Member
It sounds like the music is not clear and it’s soft.
From what you described, (cone freely moving when you push the center, and clear tone with the 30hz tone) it doesn’t sound like the voice coil is actually rubbing.
It’s possible that the something could have gotten into the voice coil I guess... or that the voice coil could be partially melted or damaged somehow. (Hopefully not)
Have you ruled out your source as a culprit? Same source on the same ch etc?
It’s possible that the something could have gotten into the voice coil I guess... or that the voice coil could be partially melted or damaged somehow. (Hopefully not)
Have you ruled out your source as a culprit? Same source on the same ch etc?
Yes, verify that it is not amp or source.
I doubt coil damage, based on resistance readings. Something in the gap could cause your problem.
Can you hear the distortion with the woofer out of the cabinet and connected directly to amp? Don't push it too hard.
Are you sure the distortion is the woofer, vs other drivers?
While removing the cap does require care, why would you think removing it and replacing it will ruin the original look? The disintegrated foam may well be gumming up your voice coil gap and limiting cone movement.
I doubt coil damage, based on resistance readings. Something in the gap could cause your problem.
Can you hear the distortion with the woofer out of the cabinet and connected directly to amp? Don't push it too hard.
Are you sure the distortion is the woofer, vs other drivers?
While removing the cap does require care, why would you think removing it and replacing it will ruin the original look? The disintegrated foam may well be gumming up your voice coil gap and limiting cone movement.
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GD70
Lunatic Member
It took some time to get the glue wet and soft enough to separate from the basket. Just kept working it until it was free. I had bought a replacement woofer set off fleabay with replaced surrounds when I first thought that woofer was shot. Installed them, but the bass wasn't quite as good as the replaced surrounds were too stiff. That's what made me try to redo my original,, and luckily I was successful.
VintageUT
New Member
Yes, I’ve ruled these two items out. Really wondering if the foam cover inside of the dust cap fell into the coil.
VintageUT
New Member
Yes, it has the distortion out of the cabinet. It really seems I need to remove the cap as there may be foam in the coil gap. Any tips on removing the cap for a beginner?
First make sure you have a good mental picture of how the cap and cone are related to one another from a phyical standpoint.
Your blade (exacto knife) must be very sharp, and must stay at an angle close to that of the cone. Do NOT cut downwards; that is, do not cut with the blade held vertically...you will cut the cone ifyou do.
The blade should be at only a very slightly steeper angle than the cone.
Because they are aluminum, I am not sure if they can be cut and then glued back in place.
If not, you will have to order the correct replacements. If you can glue them back, leave a small section uncut, to serve as a hinge with which to fold back the cap. This makes lining it up for gluing pretty easy.
Watch some refoaming videos posted by people who prefer shimming. Ths will illustrate my comments.
Your blade (exacto knife) must be very sharp, and must stay at an angle close to that of the cone. Do NOT cut downwards; that is, do not cut with the blade held vertically...you will cut the cone ifyou do.
The blade should be at only a very slightly steeper angle than the cone.
Because they are aluminum, I am not sure if they can be cut and then glued back in place.
If not, you will have to order the correct replacements. If you can glue them back, leave a small section uncut, to serve as a hinge with which to fold back the cap. This makes lining it up for gluing pretty easy.
Watch some refoaming videos posted by people who prefer shimming. Ths will illustrate my comments.
To be clear...you would be cutting at the base of the cap's dome. You are NOT trying to cut under its lip (the flange at the outer edge of the cap, which is glued to the cone). The lip remains glued to the cone.
I use a piece of shim material or business card stock, overlaid with a piece of wide masking tape (folded sticky side out over the edge of the shim material or card stock) to clean out the gap. Repeat until tape comes out clean. Follow with canned air, then vacuum. This has always worked well for me...altho there well may be other methods that are equally effective, or even better.
I use a piece of shim material or business card stock, overlaid with a piece of wide masking tape (folded sticky side out over the edge of the shim material or card stock) to clean out the gap. Repeat until tape comes out clean. Follow with canned air, then vacuum. This has always worked well for me...altho there well may be other methods that are equally effective, or even better.
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z-adamson
Addicted Member
This is why trying to get around shimming is not wise.
Part of a refoam when shimming is cleaning out the voice coil gap.
VintageUT
New Member
VintageUT
New Member
Yep, I’m new to this and have definitely learned that shimming and cleaning the VC gap is the right way to do it.
Chris Brown
Super Member
I've never heard about foam disintegration (under the dust cap) being an issue with 128H woofers. Foam disintegrating underneath some JBL dome tweeters is an issue in some cases but I've never heard about it being an issue for a woofer. It's not been an issue with any of the 8 128H woofers I've done in the last 6 years. There are many times when I run my 128H woofers up to near Xmax, so if there was any borderline-disintegrated foam under there, hard to believe it would survive long. With that said, none of our speakers are getting any newer. New issues emerge. If you do cut off the dust cap, make sure to take pictures and let us know what you find.
If it were me, I'd personally be more willing to do the test tone method twice before I would ever consider cutting off the dust cap. Here is an example of what you might call a "best case scenario", a JBL woofer that was actually professionally restored at an authorized JBL repair facility: https://www.ebay.com/itm/362811136844
Even with that "pro" dust cap glue job, it still sticks out like a sore thumb. Once you cut that cap, there is no going back. Good luck.
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Wouldn't a 128H (from an L65 Jubal) be similar in construction to a 126A other than the magnet went ferrite and surrounds the coil? So where is the foam in these woofers?
http://www.lansingheritage.org/html/jbl/reference/general/1975-sum.htm
I've had the edges on my L112 woofers redone by Speaker Pros and Bao had no problem doing a perfect job and I can't see that he cut off the dust caps.
http://www.lansingheritage.org/html/jbl/reference/general/1975-sum.htm
I've had the edges on my L112 woofers redone by Speaker Pros and Bao had no problem doing a perfect job and I can't see that he cut off the dust caps.
I agree that cutting the dustcaps is a big step, and that the replacements - even the proper aluminum ones - must be a bit larger so as to extend past the lip of the old caps for glueing. I also don't remember there being any foam inside/under the dustcap when I did my 128H woofers.
It really does come down to identifying any other cause of the distortion before taking this step. It is certainly true that debris in the gap could restrict the movement of the coil and cause the problem the OP is having. But it may well be something else.
I would also say that regardless of the claim in the ebay listing, that does not look like a professional job to me.
OP, you might want to contact someone like Bill Legall at Miller Sound and see if he has any ideas as to what the problem might be before opening them up. Bill knows more about speaker repair than just about anyone else in the USA. That said, I don't know if he is still active...he's getting on in years, and may have retired by now.
Alternatively, you might send member GordonW a PM to see if he might have thoughts on how to proceed. He is another highly-regarded expert on these topics.
It really does come down to identifying any other cause of the distortion before taking this step. It is certainly true that debris in the gap could restrict the movement of the coil and cause the problem the OP is having. But it may well be something else.
I would also say that regardless of the claim in the ebay listing, that does not look like a professional job to me.
OP, you might want to contact someone like Bill Legall at Miller Sound and see if he has any ideas as to what the problem might be before opening them up. Bill knows more about speaker repair than just about anyone else in the USA. That said, I don't know if he is still active...he's getting on in years, and may have retired by now.
Alternatively, you might send member GordonW a PM to see if he might have thoughts on how to proceed. He is another highly-regarded expert on these topics.
Same here. I’ve done many pairs of L112s and never had this problem, and never shimmed the woofer. Personally, I do NOT think it’s necessary, and is almost certainly a waste of time, and an opportunity to do more harm than is necessary. But to each their own.
Why not cut the outer surround (leaving the inner part of the surround still glued to the underside of the cone) and test at low volume to see if the distortion is still present. If not, then you know you just need to remove the surround and order another surround and try again. If there’s still distortion, you know it’s not due to the voice coil rubbing, and you can start diagnosing other causes.
To each their own... but if they don't do it my way they are idiots.
It is NOT "necessary" for refoaming, although it does assure proper centering. In the OP's case, his problem does not appear to be an off-center coil, as he did not describe it as rubbing. If he rules out other causes, wouldn't he need to get at the coil gap to check it for debris or binding?
On a related note,and as an L112/128H expert, can you comment on the OP's finding of two stacked dustcaps? Could this somehow be a clue?