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Zilch
10-21-2006, 01:04 AM
Pair of 2420s from ca. 1972, both sounded bad.

One had an obvious failed diaphragm.

Other looked good, 'til illuminated from behind:

[Note fragments of deteriorated foam from the back cap on it, too.... :( ]

Rolf
10-21-2006, 06:14 AM
34 years old Zilch. Is it not normal they go to pieces, like the foam on the woofers??


Pair of 2420s from ca. 1972, both sounded bad.

One had an obvious failed diaphragm.

Other looked good, 'til illuminated from behind:

[Note fragments of deteriorated foam from the back cap on it, too.... :( ]

57BELAIRE
10-21-2006, 07:55 AM
I wonder how many compression drivers (JBL) have had perfectly good diaphragms replaced when in fact it was the crumbling damping foam
lodged in the gap causing the problem?

When removing the back cover for diaphram inspection, closely examine the foam first to make sure it is intact....and DON'T TOUCH IT!....unless you are going to replace it.

Zilch
10-21-2006, 09:49 AM
The deteriorated foam can't get to the gap if the diaphragm is intact.

[Stuff from the throat can, tho....]

I don't know the relative merits of felt vs. foam, but longevity would be one in the felt column.

moldyoldy
10-22-2006, 01:01 AM
I Saw the Light!

A bead of primered cyanco's worth a try. If it fails in another spot you'll know it's shot.

edgewound
10-22-2006, 01:44 PM
Pair of 2420s from ca. 1972, both sounded bad.

One had an obvious failed diaphragm.

Other looked good, 'til illuminated from behind:

[Note fragments of deteriorated foam from the back cap on it, too.... :( ]

Metal fatigue at the fulcrum point.

Paint some thinned down brown bostik glue around the perimeter of the aluminum surround up against the mounting ring. Use several coats for good measure on both sides.

What have you got to lose there at "Zilchlabs Spa & Grille Cloth Emporium"?

**Disclaimer**: This is not a JBL Factory Sanctioned repair procedure.:D ;)

Zilch
10-22-2006, 02:09 PM
Metal fatigue at the fulcrum point.
Admittedly there was an element of wishful thinking in this. As we know, the lifetime of vintage aluminum diaphragms is finite. The chances of finding a pair of 2420s that weren't rode hard in monitors is certainly less than that for LE85s in home systems.

The upside is they're usually cheaper, and don't have wax seals to "preserve," thus being good candidates for rehab. I think I'll move some D8R2421 or aquaplas'd titanium diaphragms into them, with new felt damping pads. :thmbsup:

Thom
10-23-2006, 11:10 AM
I think some ebay sellers save these for those " these are sold as is, I have no way to test them but they look like new and the vc is not open"

Zilch
10-23-2006, 12:16 PM
Yeah, I been bit by a couple of those.

They know it's crap. That's why they're selling it.

It's a rare seller up front with it. "This is really total crap. Buy it anyway...." :p