Sloped front JBL?

[JBLAddict]

Originally Posted by BMWCCA

And from looking at those photos of the remainder of the surround on the back of that 708G-1, I'm not sure that what we've assumed were butyl surrounds on the 708G are really that at all and not just normal poly-foam with a smoother finish.

It does look like the surround on the smaller 704G-1 in the L7 is actually some composite with reinforcement built into the surround. The 708 looks more simple, for some reason.

Somewhere in the L-series thread, Chris Hagen (the designer of this series) cited the trade name of the polymer JBL developed specifically for the surrounds on this series. He cautioned against using replacements that didn't have the same characteristics.....of course this is from a purist's vantage point

[BMWCCA]

Originally Posted by JBLAddict

Somewhere in the L-series thread, Chris Hagen (the designer of this series) cited the trade name of the polymer JBL developed specifically for the surrounds on this series. He cautioned against using replacements that didn't have the same characteristics.....of course this is from a purist's vantage point

Yeah, it's right here: http://audioheritage.org/vbulletin/s...&postcount=219

Originally Posted by CHagen7

VERY IMPORTANT is to use a factory-authorized re-coner with factory-authorized parts! JBL used a special rubber for the 708G-1 (as well as the 4" mid and 6-1/2" for the L-1)surround, called Poron. It is related to neoprene, but has better damping characteristics. If you go to a re-coner who does not use factory parts, you will end up with something different. I wouldn't do this because the L series worked so well.

Now, having said that, JBL may have changed to a normal foam surround for service purposes. But it is STILL better to pay more for the factory-authorized service because when they decided that they would change to a normal foam, they certainly performed the necessary tests to make sure that the driver is still reasonably close in performance. Joe Blow reconer down the street who can't get factory parts can't guarantee what you'll get, other than a speaker that doesn't make bad noise and makes more bass again.

That's all fine and good except JBL never supplied these parts, ever, as far as I can tell. Add to that that the surround on the 708G-1 appears to be of a completely different material than the 704G-1, and of course the LE120H-1 uses what appears to be a conventional poly-foam. I'll check the rest of the driver line but the point appears moot since JBL doesn't stock or supply them and never did. Maybe Chris remembers who the supplier was, not that it would help.

[BMWCCA]

Upon closer examination, both pairs of my L7s appear to have a simple monolithic rubbery surround material on their 708G-1s like the defective ones from the L3 in this thread. The 704G-1s appear to have more of a texture that would suggest some reinforcing material with a grain not unlike a fine fiberglass mat inside the surround. Eye-glasses-off examination of the drivers on my L5s shows this "reinforcement" on all drivers, including the 708G-1 and the 706G-1. Odd, isn't it? I'll try and capture an image that imparts what I'm seeing with my eyes but I'm not sure I have that macro-capability with my cameras. No drivers on the three pairs I checked have any apparent deterioration or separation of the surrounds on any drivers.

Quite the mystery, one I hope the original poster can solve to his friend's satisfaction.

[JBLAddict]

Originally Posted by BMWCCA

Upon closer examination, both pairs of my L7s appear to have a simple monolithic rubbery surround material on their 708G-1s like the defective ones from the L3 in this thread. The 704G-1s appear to have more of a texture that would suggest some reinforcing material with a grain not unlike a fine fiberglass mat inside the surround. Eye-glasses-off examination of the drivers on my L5s shows this "reinforcement" on all drivers, including the 708G-1 and the 706G-1. Odd, isn't it? I'll try and capture an image that imparts what I'm seeing with my eyes but I'm not sure I have that macro-capability with my cameras. No drivers on the three pairs I checked have any apparent deterioration or separation of the surrounds on any drivers.

Quite the mystery, one I hope the original poster can solve to his friend's satisfaction.

When I first acquired my L7s many months ago, I thought I was seeing things with the differences in the foam on my L5's 708 and 706. All this time I figured, either the L7's 708 had been refoamed (which I hoped wasn't the case), or JBL made a change along the way between 92 and 96. It's always bothered me.......good to know another L7/L5 owner has the same thing, though would still like to know the reason why.

[Rudy Kleimann]

To clean the backside of the cone, make a support shim with a piece of styrofoam (or anything similar) about 1/2" thick. Cut a circle just small enough to clear the frame, but large enough to overhang the edge of the cone. A slightly snug fit in the frame is best.

Then, place the driver face-down and lightly push it down until the styrofoam contacts the cone and the frame is contacting the work surface. Assuming the 1/2" thickness of the styrofoam disc is appropriate, the cone will rest against the foam, perhaps being pushed in slightly from its normal resting position, but not so deep as to bottom out the voice coil in the magnet or stretch the spider to a ridiculous degree. Now, scrub away on the rear edge cone with a toothbrush to remove the old surround without fear of deforming the cone or bending the voice coil.

Follow up with a cleaning solvent like MEK (a favorite of many LHS guys) to remove the last of the old surround (test styro or other material supporting the cone with the MEK and remove it first if the MEK attacks it) and dissolve the old glue or at least level it out. Use it sparingly to avoid saturating the paper cone and wear gloves to keep the MEK out of your system, lest you end up with a nervous tick, a third eye in your forehead, or God only knows what else it might do to you. Work in a well-ventilated area.

Remove the dust cap (MEK to soften the glue here too, provided it doesn't attack the dustcap, or mark the cone and cap then exacto-knife the cap right along the glue line) and shim the voice coil around the pole piece. Narrow strips of thin plastic slid in side-by-side, just thick enough to spring-load the coil against the pole piece without necessarily completely filling the gap works very well. Get it to a near-interference fit. Whatever you use, make sure it won't leave any behind in the magnet. Leave it shimmed until after the surround is set and the glue is dry, then reglue the dust cap.. Some guys pull the shims and shoot low-power LF tones to the woofer while the glue is a little soft to get it moving and check for voice coil rub, the ide being you can play with the surround position on the frame or the cone itself before the glue sets. I don't do this particular trick myself... your mileage may vary.

[jbl-ahhh]

Hey, that method for removing the old surround definitely sounds worth a try, but I already got mine off. With the driver magnet side up, I managed to use a small screwdriver to pry one spot of the old surround away from the underside of the cone enough to get a finger in between surround and cone and then gently, slowly ease the surround off of the cone all the way around, a little at a time. I used heat from a hair dryer to loosen the glue but I'm not sure it did any good because in one of the pics you can see that a very thin layer of cone got peeled off with the surround. Not ideal, but the cone seems fine.

Once the surround was free from the cone, I flipped the driver over to get the surround off of the basket. I just used a small screwdriver underneath the surround to scrape it off. The surround lip is only 1/8" wide, so it comes off easily.

Next, by pinching the surround between my thumb and index finger and pulling very lightly and slowly, the glue bead along the outer edge of the cone came free - see pics. I used a finger pressing on the cone to counteract the pulling force. Came off clean as a whistle!

The search for exact fit replacement surrounds goes on. Looks like I'll have to go with "close" and then trim them to fit. That narrow 1/8" outer lip must be quite unusual.

[Rudy Kleimann]

WOW! That's the narrowest lip on any surround I've ever seen! That's going to be a tricky one. How much are the recone kits? Most JBL FASC charge $25-35 labor to install. Sometimes, however, new woofers are the only factory option, which is sometimes pretty high $$.

If your only viable option is made-to-fit surrounds, find or make a jig to cut the rim of the surround to fit. Raid the kitchen pantry for things like pots and pans or covers, tupperware, margarine tubs, old metal coffee cans, anything that will snugly fit the recess in the frame and fit over the roll of the surround. maybe even the next smaller woofer frame will be right, if you happen to have one handy. If nothing else, get some sheet metal and roll it into the correct diameter tube. Just be sure the surround is pulled out evenly all around, with the right amount of surround flange left inside the edge. Then get after it with a fresh exacto knife (easiest to use for the best job) or a razor blade on a cutting board. You can use the same to

On the glue, less is more, as long as it's evenly applied. You sure don't want overflow creeping up into the rolled area of the surround.

Just a few ideas that came to mind, FWIW.

[mikebake]

My idea is to get something more substantial for a dance studio; like a 15 and a horn.

[Rudy Kleimann]

Like JBL MR 825's?

I have 1pr in fairly good and 1pr in very good cosmetic condition (all drivers are original in excellent condition) that would do a good job and prove handy for the studio. Lightly used, mostly for Karaoke. I don't use them much anymore after moving up to 2226/2447 -equipped cabs and would sell them at a fair price.

Offers?

[CHagen7]

Hi all! It sounded like you might appreciate it if I'd drop a note here:

Yes, I know I said before that you should use EXACTLY that surround. But, times being what they are, and sometimes you just want the thing to work, and sometimes a material is so difficult to deal with that repair parts are scarce or non-existent (I have no proof of this, although I know LE-120 parts are custom to order, and my $3800 Citation processor was no longer supported after 6 years, so...).

You end up making do. To get these working I would get the closest fit surround kit that I could, based on measurements from the other woofer. Cut or have cut masonite circles that re the same size as the OD and ID of the surround. Attach them at their centers and use their sharp edge and a BRAND-NEW-BLADED exacto knife to trim the surrounds so they fit. Then go to town.

Yes, the OD land is VERY narrow - make SURE to coat it with adhesive.

Last note - the damping properties of the outer material of the surround turned out to make it difficult to form. So we ended up making a laminate of the outer material and conventional foam. This allowed it to hold shape.

Chris

[BMWCCA]

Originally Posted by CHagen7

Last note - the damping properties of the outer material of the surround turned out to make it difficult to form. So we ended up making a laminate of the outer material and conventional foam. This allowed it to hold shape.

Well that would explain why the surround material looks so odd in Ahh's photos. Hmm.

Thanks Chris. More information is always better even if it's not a solution. Any idea who made the surrounds/cones?

[jbl-ahhh]

Like Chris said, I ordered some surrounds that aren't the exact size but are very close. I'll only have to trim the OD down to size.

Chris, I'm not sure I follow your suggestion about the masonite. Do you mean cut one to the OD size (which is 7 1/8") and a second to the ID size (which is 5 1/2") and then sandwich a surround in between them when I cut it?

[jbl-ahhh]

I've got the first surround trimmed down to size and as I'm doing a dry fit something occurs to me. Since the outer lip of the surround is so narrow and butts up against a vertical portion of the basket, the surround has no where to go, no room for slight adjustments. So if I glue surround to cone first -- the normal way to do it -- and then find that some adjustment is needed, what would I do? So does it make sense to glue the outer lip to the basket first, so any necessary adjusting could be done while gluing to the cone? I don't like some things about the idea, but I also don't like not being able to make adjustments. Anybody ever try it?

[jbl-ahhh]

Well, this was my first ever re-edge goof. Did it have to be on someone else's speaker? Ugh. I'm guessing I should have used thicker shims. This morning the final glue was dry and as I check things out I hear the dreaded, slight, high pitched squealing/scraping noise when I play a 30Hz tone. If I push lightly on the surround roll in one spot the noise stops.

Has anyone ever found anything that helps this after the glue is dry? Other than plastic explosives and leaving town, that is! Cursing hasn't done a bit of good. I'd better try some fist shaking.
The noise might not be heard while playing actual music, but I don't know if it will get worse.