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wrager
01-24-2012, 01:48 PM
I have three 4622's as LCR's. I noticed when level matching, one is -7dB compared to the other two. Do I need a new phram?
I have a spare 2431. How different would it be? I know the later 4722's used the 2432...

Lee in Montreal
01-24-2012, 01:56 PM
Sounds symptomatic of an aftermarket diaphragm versus original. :eek:

Lee

wrager
01-24-2012, 03:21 PM
Intersting Lee, never considered that. Would there be any visual signs, original vs. after market?

Lee in Montreal
01-24-2012, 03:25 PM
Intersting Lee, never considered that. Would there be any visual signs, original vs. after market?

Open your drivers and inspect the diaphragms to see if they are different. I had a similar problem with 2402s purchsed off eBay that were supposedly "mint". One side was onviosuly much, much lower than the other. I opened they up and surely enough, one driver had an aftermarket diaphragm. Seller sent me an original JBL unit... Problem solved. It will take 5 minutes to open them up.

wrager
01-24-2012, 03:30 PM
Will do and thanks! Anything special when doing this? Any "watchouts?"

Lee in Montreal
01-24-2012, 03:56 PM
BTW If you have a spare one, prhaps you can swap the spare driver and see how it compares with the one that sounds low.

Lee

wrager
01-24-2012, 05:11 PM
Unfortunately the spare is a 31 from an AE/AM6215, not a 30. I am curious to see (hear) the 31 vs. the 30. I loved the sound of the AM6215 but never compared side by side, or measured.

Mr. Widget
01-24-2012, 05:52 PM
Could be an aftermarket diaphragm, but it could also be a damaged diaphragm or a shifted magnet or phase plug. There are many things that can and often do go wrong with compression drivers, especially these newer 3" JBLs. They do not travel well when loose.

Widget

edgewound
01-24-2012, 10:22 PM
I don't think there are any aftermarket diaphragms for the 243x style drivers. I haven't seen any, anyway.

The voice coils in these drivers' diaphragms are encapsulated in a Kapton sleeve so they're pretty well insulated from the gap walls.

But...The diaphragm will shatter while the voice coil still stays intact...

Sooo...My guess is a glitterfied diaphragm.....maybe.

herve M
01-25-2012, 04:44 AM
I have the same problem.A pair of jbl 2430 purchased directly from jbl in 2002.
One 2430 plays louder in1khz -3KHz.
I exchanged the two diaphragm, a problem identical.The diaphragm is not an issue.
The voice coil is written in marker"+0" and the second"+4".Bizarre.The 2430 writing "+4" play louder

Hervé.

wrager
01-25-2012, 10:39 AM
Wow, good additional info by all. I'll try to take some measurements of each this weekend for comparison.
What is a "glitterfied diaphragm" edgewound?

1audiohack
01-25-2012, 11:13 AM
Just a test tip for drivers you don't want to disassemble.

If you have the equipment to perform impedance sweep measurements, you often find if the voice coil is at all in contact with the top plate or the pole it will have smaller than normal peaks because of the frictional dampening therefore looking more like the sweep of an inductor. If the diaphragm is broken or the voice coil former is separating from the diaphragm, it will have higher than normal impedance peaks due to very little motional dampening, sometimes over 100 Ohms.
That combined with a frequency response test, you won't even have to pick up a screwdriver (if you don't want to) to figure it out.

All the best.

Lee in Montreal
01-25-2012, 12:18 PM
Perhaps simply comparing resistances between both drivers can help you diagnosis a problem. You won't have to open the drivers ;)

edgewound
01-25-2012, 02:22 PM
Wow, good additional info by all. I'll try to take some measurements of each this weekend for comparison.
What is a "glitterfied diaphragm" edgewound?

A "glitterfied diaphragm" is one that has made serious contact with the phase plug and shattered...basically turning into glitter.

1audiohack might also be onto something as well if the voice coil has separated from the diaphragm at all...but you might typically be able to hear a raspyness coming from it... if that's the case.

You might also check for signs of the top plate shifting. This would be indicated by the outer edges of the layers of front exit snout, magnet and top plate not lining up perfectly around the perimeter. It seems some of these units' magnets are plated with nickel that can separate from the neodymium disc and cause all kinds of problems....some of which is flakes of nickel migrating into the gap.

subwoof
01-27-2012, 08:02 PM
I had 2 of the 2435's with shifted phase plugs sent to me from our west coast backdoor supplier. all were replaced but I never measured one.

Due to the VERY small actual diaphragm movement, a pinched coil from a shift will still play but at a noticeable reduction in volume and of course, some serious dips in response.

This was a common issue way back when the 2440/2350 horns fell off the stacks and the top plate moved ever so little...got pretty good with a hammer and 2X4 at those outdoor shows..:)

sub

Rudy Kleimann
10-27-2012, 11:19 AM
...a pinched coil from a (phase plug) shift will still play but at a noticeable reduction in volume and of course, some serious dips in response.

This was a common issue way back when the 2440/2350 horns fell off the stacks and the top plate moved ever so little...got pretty good with a hammer and 2X4 at those outdoor shows..:)

sub

Love it! :rotfl:Beat it 'til it sounds right!!!

I bought a 2425 that ad a rough life before me- same problem. Had to rip the diaphragm apart to get it out. Had nothing to lose, so on a whim, I got the plastic-faced mallet after it and after a few well-placed raps to the side of it got the gap eyeballed concentric again. Pulled the bug screen, using a little MEK to soften the cement, then a little time in vinegar (not too long) to remove the oxidized Zinc electroplating and other crud, followed by a toothbrush and pipe cleaner scrub/rinse/soak in baking soda and water to polish up the metal and neutralize the mild acidity from the zinc, and it looks like it MIGHT live to see another day. A few minutes at the "JBL Lab Annex" in Vegas will tell the story...

I had done the same clean-up trick on a couple of "drunk drivers"- 2416 and 2417 compression drivers from stage monitors that had drinks spilled in them and discovered weeks later when they sounded bad. The 2416 phase plug was pitted up (sugar, liquor, and water aren't nice to several different metals left soaking for days/weeks), but the damn thing worked when I finished-good enough for stage work.:applaud: The 2417H wasn't so bad- it cleaned up well, and in my shop, sounded better down low (500-2KHz) on sine waves than my other two virgin 2417's:bouncy:.
I kidded 1audiohack that I had "ported and polished" ;) the signal path of that driver, improving its' performance, and was considering doing the same to all of my compression drivers:D