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View Full Version : HELP!! 128H1 kit into 122A frame???



GordonW
09-13-2003, 10:24 AM
OK, I'm trying to track down a solution for my guy here, with the blown 122A woofer (L65A)...

Since the 122A kit (C8R122A) is long gone, and the JBL recommended retrofit 129H kit (C8R129H) is out of stock for the forseeable future, at the advice of Giskard, I've been looking at putting a 128H-1 kit (C8R128H1) into this basket. I even looked up the T-S parameters, and discovered that if I was to engineer a 10-11 gram "mass ring" onto the 128H-1 kit, it would be almost a DEAD RINGER in in-box performance to the 122A (Bl is identical on both, and Mms = 100g on the 122A, 90g on the 128H/H1)...

HOWEVER...

I did some research. As expensive as these kits are, two things make me hesitant to order this kit sight-unseen:

1) the voice coil gap guage is DIFFERENT in the 128H/128H1 basket. The 122A has a gap guage of .053", the 128H has a .057 gap. My question is: Does the 128H use a thicker voice coil winding (ie, bigger wire, more layers, etc), or is it just opened up for power handling or manufacturing ease, due to the increased flux density available from the ceramic magnet on the 128H??

2) Apparently, according to one lookup book I was referred to by our local JBL parts guy (Image reconing book), the voice coil FORMER LENGTH (ie, distance from the cone apex to the top of the voice coil winding, specifically) on the 128 may be DIFFERENT than the 122A, making the voice coil NOT center properly, vertically, in the gap at rest! Is the 128H basket made with a different magnet gap placement WRT the cone, compared to the 122A??

Can anyone directly address these issues? Is the 122A basket that much different from a 128H basket , that the 128H-1 kit won't work???? I can't find any "officlal" dimensions to tell me one way or the other!!!
:banghead: :banghead: :banghead:

Regards,
Gordon.

4313B
09-13-2003, 11:33 AM
I'll go rip the cone out of a blown 128H and see what it's voice coil's physical dimensions are.

Mr. Widget
09-13-2003, 11:45 AM
Originally posted by Giskard
I'll go rip the cone out of a blown 128H and see what it's voice coil's physical dimensions are.

Great! ...and keep track of that info so that we can eventually create that list... remember.:D :D

GordonW
09-13-2003, 11:14 PM
Cool, when I'm back at work on Tuesday, I'll rip out the moving assembly of the dead 122A and measure it too...

One way or another, this cat is gonna get skinned!

Regards,
Gordon.

GordonW
09-16-2003, 09:38 AM
OK, here's the dimensions from the 122A:

Voice Coil inner diameter: 3 inches exactly (as close as I can measure)
Voice Coil Former Length: 1 5/16" (1.3125")
Voice Coil Winding Length: 3/4" (.750")
Voice Coil winding type: Edge Wound, appears to be one layer
Distance from Bottom of Spider to Voice Coil Bottom: 1 1/16" (1.0625")
Motor Top Plate Thickness: 9/32" (.281")

This means there is about 1/4 (.25") of voice coil former sticking through the spider and cone neck, through to the top of the cone.

One thing I'm wondering, tho... if the 128H1 kit IS a 1 3/8" voice coil former length, what is to stop me from unbolting the magnet assmembly from the 122A basket, and installing a 1/16" SHIM made of thermally conductive material (to ensure heat transfer from magnet to basket), between the magnet motor and the basket?? That should get me back in dimensional good shape, assuming the VC winding height, composition or thickness isn't out of whack... and it would also be easily undo-able, if anyone in the future decided to rebuild with a 129H kit...

Regards,
Gordon.

4313B
09-16-2003, 09:48 AM
Originally posted by GordonW
OK, here's the dimensions from the 122A:

Voice Coil inner diameter: 3 inches exactly (as close as I can measure)
Voice Coil Former Length: 1 5/16" (1.3125")
Voice Coil Winding Length: 3/4" (.750")
Voice Coil winding type: Edge Wound, appears to be one layer
Distance from Bottom of Spider to Voice Coil Bottom: 1 1/16" (1.0625")
Motor Top Plate Thickness: 9/32" (.281")

This means there is about 1/4 (.25") of voice coil former sticking through the spider and cone neck, through to the top of the cone.

One thing I'm wondering, tho... if the 128H1 kit IS a 1 3/8" voice coil former length, what is to stop me from unbolting the magnet assmembly from the 122A basket, and installing a 1/16" SHIM made of thermally conductive material (to ensure heat transfer from magnet to basket), between the magnet motor and the basket?? That should get me back in dimensional good shape, assuming the VC winding height, composition or thickness isn't out of whack... and it would also be easily undo-able, if anyone in the future decided to rebuild with a 129H kit...

Regards,
Gordon.
How about I just ship this 128H cone to you? That way you can measure it the same way you measured your 122A cone. Send me a PM if you want it and please post the results.
About the gap, JBL states the 128H gap as 0.057" and it indeed is. JBL also states the LE10H gap as 0.053" and I have six sitting here in front of me with 0.057" gaps.

GordonW
09-16-2003, 11:34 AM
PM sent, thanks!

Regards,
Gordon.

GordonW
09-23-2003, 10:27 PM
Hey, got the 128 cone today.

Great news! Turns out, dimensionally IDENTICAL, as far as voice coil is concerned, to the 122A! The only difference I can see, is that the 128H cone is about 1/8" bigger in diameter at the outside, which I assume is compensated for by a slightly narrower surround, as the outside of the baskets are the same AFAIK. No reason why a 128H-1 cone kit shouldn't work... especially if I do like my local JBL pro parts guy suggested, and make a 10 gram mass ring out of solid-core solder, and attach it just under the dustcap... should sound almost identical to a 122A, at that point, even an efficiency and low-end extension match to within a couple percent!

Regards,
Gordon.

4313B
09-23-2003, 10:41 PM
Cool! :)