Rattle in LE85 drivers (JBL)

[FoolForARadio]

Hi. Last week I picked up some JBL drivers in furniture style cabinets that my friends from AK tell me is the S7 array of components.

http://i1116.photobucket.com/albums/...1/DSC_0265.jpg

Unfortunately, the horns rattle bad, especially with piano music. The foam in the back of the diaphragms has deteriorated and there are a couple of tiny holes in the surround of one driver, I haven't looked at the other one.

http://i1116.photobucket.com/albums/...littlehole.jpg

There's no fixing these, I suppose. I see several options out there, even from JBL. Is D16R2421 the right one, and are the $130 or so specimens on ebay the same part as the $350 ones you see at some sites? Should I have it done professionally?

And once I get through all this, there are adjustable wooden blocks covering up one rectangular hole in each baffle. Is this a port, should it be closed, partially open, or fully open?

Thanks...

[Mr. Widget]

Yep, S7.

Yep, metal fatigue. Your diaphragms are toast. You need to clean out the rotten foam... do this with the diaphragms in place to keep the voice coil gap protected and replace with some alternative damping pad... heavy felt or new foam I believe it should be about 3/8" thick, but I haven't opened one of these in quite a few years.

Replacing the diaphragms is simple but there are a couple of tricks to it. Personally I'd go with a reputable local JBL authorized service center as that way you get the full warranty, but it will cost the most. You can use a less expensive JBL titanium diaphragm or an aftermarket diaphragm, but the 2421 aluminum replacement will sound the best.

After all of that you may still want to add a tweeter... the top end of the S7 system is pretty well rolled off. And there are the LE15A woofers. Yours have the original Lans-a-loy surrounds. These stiffen over the years and if pushed after they are stiffened they will crack. The stiffening also affects the bass performance. You can restore the suppleness of the surround by carefully brushing on a small amount of DOT-3 brake fluid. The better solution is to have the surrounds replaced or do it yourself if you are handy. You can see how it is done here.

As for the port... hard to say why anyone does what they do... those woofers like a port tuning of 30Hz and a cabinet volume of about 5 cu ft. After you measure the actual cabinet volume you can find a box tuning chart or ask someone with speaker box design software to run the numbers for you.

The S7 is a very nice vintage system and if you put a bit of time and money into these you will have a system that should bring you a lot of pleasure and be good for quite a few years to come.


Widget

[Mike Caldwell]

With that tiny slit of a port your going to hear a lot of air huffing and puffing as the cabinet tries to reproduce any low end. Let me know the internal dimensions of the cabinet and I can run some tunings for you.

The hole in the diaphragm almost looks like something burned through it, being in the area of the voice coil lead could be possiable.....seen it before.

[FoolForARadio]

Dimensions are 15" deep, 22" wide, and 29" tall, maybe just a fraction less on some measurements. So pretty close to 5 cubic feet. The other diaphragm was in way worse shape. The foil and the surround were stuck to the magnet and the base plate. I think the deteriorating foam is corrosive and maybe creates an adhesive atmosphere.

[Mike Caldwell]

Hello

Those dimensions figured 5.53 cubic ft. I rounded it down to 4.80 to help offset the drivers, internal bracing etc.
35 hz tuning looked to be best compromise in low frequency extension and remaining flat to the cut off frequency.
Two round ports 4 in. dia and 8 1/2 inches long would work.

I ran it on a couple different programs and they were within a quarter inch of each other on the port length for the same tuning.

Mike C.

[FoolForARadio]

This is interesting. 2 ports per cabinet? Rear or forward? Where would you situate them? Would the port values change if I added a slot tweeter? Thanks...

[Mr. Widget]

This is interesting. 2 ports per cabinet? Rear or forward? Where would you situate them? Would the port values change if I added a slot tweeter? Thanks...

There is little difference between front or rear mounted ports... the differences are the potential added midrange coloration with front facing ports and the potential for placing the speaker too close to the rear wall with rear facing ports.

A slot tweeter is so small in terms of volume that it will have no effect. For the LE15A JBL recommends a 30Hz tuning for the 5 cu ft box, this would be two 4" tubes that are 11" long... that said, I agree with Mike and I think two 8.5" or 9" tubes should be just about right.


Widget

[Mike Caldwell]

If you wanted to use just one 4 inch port the length would drop down to 2 3/4 give or
take to 3 inchs in length for the same 35hz tuning. I would stay with two ports, within reason more port area is better equals less air noise.

[FoolForARadio]

I have refoamed them with Cobb surrounds. Mine were mounted behind the cones, so that's what I did. The fronts were cleaner, but the backs would have been ugly if I had left them uncovered. The Lansalloy was the dickens to get off the cone but I got them as clean as I could and let the glue do the rest. Way more volume, way better bass. I tried the brake fluid but clearly the refoam is the way to go. Also, I installed Radian diaphragms in the LE85 drivers. The rattle and distortion is gone and they sound great to me.

Next is the porting project, but not right away.

Now is there a substitute for the 077 slot tweeter and the extra crossover piece? And is there anything to be gained by new caps in the crossover?