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jbl_man_uk
04-25-2006, 08:46 AM
I Have several of these 2445 here,im having a problem with distortion on 1 in particular,..ok,first thing,change the diaphramm.still bad.i have tried 3 new diaphramms in it,2 original genuine brand new ,and 1 "copy",also new,and all 3 times there is audible distortion at anything over a few watts...im crossing it over at 1Khz.The driver looks perfectly ok,nice clean voice coil gap,no visible dust or debris..and the driver has not been dropped,there is nothing out of line or astray :blink: ..any suggestions would be wellcome.thanks.

edgewound
04-25-2006, 08:50 AM
Try tapping on the phase plug to see if it's loose. Also, are you sure the gap is dead clean? Any bit's of tape adhesive, old coil wire, metal chip hiding? Anything? Check to see if the throat screen is loose, too, and make sure the internal wires to the diaphragm terminals are clearing the diaphragm, and the throat isn't cracked.

jbl_man_uk
04-26-2006, 02:52 AM
Okie dokie Edegwound,i will give those items a thorough check.thanks.

Mike Caldwell
04-26-2006, 04:42 AM
Hello
Have you tried swapping the driver with a known good one just to make sure that it is not something up stream of the driver causing the problem.


Mike Caldwell

jbl_man_uk
04-27-2006, 02:03 AM
Yes,i tried that Mike.i have 6 of them here,so i had a swap-about,but made no difference.for some wierd reason i only get the distortion with this 1 particular driver...im wondering if diaphramm is contacting the phase plug,maybe the phase plug is not properly seated"right home"?.it looks fine,no different to any of the others...but im sure the tolerence must be quite tight between the front face of the diaphramm and the rear face of the plug..maybe its just making contact somehow? :blink: its the only thing i can think of to cause it.

Mike Caldwell
04-27-2006, 05:49 AM
It does sound like there is something slightly out of alignment in the driver
it self since you have tried different diaphragms. Do you have a source to provide test tone frequencies? If so send the driver a tone somewhere around 400hz or 500hz with the driver back cover off and the diaphragm mounting screws all loose, see if slightly moving the diaphragm will help clean up the tone. Using a single frequency test tone is very revealing for any buzzes or distortion. Slight changes in the diaphragm position can help clean up the overtone/buzz. Does the voice coil gap look even all the way around or better yet take a stainless steel or brass feeler gauge around the gap and check for tight spots. If the gap is off center theres not much you can do to fix it short of sending the driver off to be de maged and re centered and then re magnetized. You could try this...by slightly enlarging the holes in the diaphragm that the guide pins on the driver fit into will allow more movement room for diaphragm alignment. I know Meyer Sound did that with the JBL drivers they used in their cabinets so they could hand align each driver.

Mike Caldwell

edgewound
04-27-2006, 09:25 AM
Mike's right about the alignment. Could be your gap is a little off, or has a burr on the edge of the top plate that you could sand off with emery cloth....and then clean it really, really good to get the metal dust out. Are you sure the voice coil terminals are aligned with the reliefs in the top plate that are 180 degrees apart? That's clearance for the VC welds...maybe you know that already.

The 2445 doesn't have alignment pins...it has a relief machined in the top plate for drop-in centering...but...It might take some manual tweaking to get it seated right before you tighten it down....that's where a sweep generator is necessary and voltage drive of about 2.83 volts for H(8 ohm) and 3.5 volts for J(16 ohm), starting at 500Hz.

As a last resort...I suppose you could shim the diaphragm ring higher in the seat to clear the phase plug, but that might cause other response problems.

Good luck...hope you solve the prob:)

Mike Caldwell
04-27-2006, 05:11 PM
Mike's right about the alignment. Could be your gap is a little off, or has a burr on the edge of the top plate that you could sand off with emery cloth....and then clean it really, really good to get the metal dust out. Are you sure the voice coil terminals are aligned with the reliefs in the top plate that are 180 degrees apart? That's clearance for the VC welds...maybe you know that already.

The 2445 doesn't have alignment pins...it has a relief machined in the top plate for drop-in centering...but...It might take some manual tweaking to get it seated right before you tighten it down....that's where a sweep generator is necessary and voltage drive of about 2.83 volts for H(8 ohm) and 3.5 volts for J(16 ohm), starting at 500Hz.

As a last resort...I suppose you could shim the diaphragm ring higher in the seat to clear the phase plug, but that might cause other response problems.

Good luck...hope you solve the prob:)

I was still in the old school driver train of thought on the alignment pins!
Meyer Sound used to tweak JBL 2441's with the alignment pin trick.

Mike Caldwell

jbl_man_uk
04-28-2006, 08:44 AM
Okie Dokie..i seem to have cured it...fitted a 2450 "ribbed" diaphramm from another driver,and all seems well now..i did check what u guys suggested,and yes im aware of the little cutouts 180* apart in the voice coil gap for the lead-in wires...only thing i can think of is that maybe the ribbed diaphramm is fractionally different deeper,so has a tiny bit more clearance on its front face,next to the phase plug?.....
Anyway.all sounds good now...i will make a note to fit the ribbed 2450 unit in any others when it comes to replace them i think.
Cheers. ian.London/UK.

Mr. Widget
04-28-2006, 09:31 AM
...i will make a note to fit the ribbed 2450 unit in any others when it comes to replace them i think.I don't think that is necessary... there are manufacturing tolerances at play here and it seems that you got lucky with that particular diaphragm... I don't think it would be wise to assume you should always fit 2450 diaphragms into 2445s.



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edgewound
04-28-2006, 10:13 AM
I don't think that is necessary... there are manufacturing tolerances at play here and it seems that you got lucky with that particular diaphragm... I don't it would be wise to assume you should always fit 2450 diaphragms into 2445s.



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jbl man uk,

Glad to hear it worked out.

2445's and 2450's...as well as 2446,2447, 2451, 2440's,2441's...all iterations should be physically interchangeable.

Not to forget the aluminum diaphragm 375's, too.

Mr. Widget
04-28-2006, 10:18 AM
My wife constantly... OK, frequently gives me grief for implying rather than saying... thanks Edge... that's what I was implying.;)


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