708G voice coil repair

I recently got a really nice pair of L-5's except that one of the 708G's may be a goner - the meter shows it to be an open circuit. I have removed the dust cap and carefully scraped a bit of the coating off the leads going down the side of the voice coil former to the actual voice coil winding (see bad photo). From the scraped point out to the wire terminals measures a solid connection, so the problem must be somewhere in the voice coil winding. When I expose as much of the winding as possible by pressing up from the underside of the cone, I don't see any sign of breakage or damage.

I've read on this forum that reconing kits don't exist, so I'm hoping someone has some tricks for further investigation or repair of the existing one. I've got nothing to lose by trying!

Time for E-Bay.

Rob:)

Rats. Well, maybe someone will be selling parts on Boston's craiglist soon so I won't have to take my chances with e-bay and with shipping.

Since seeing this thread, I contacted Harman / JBL because I also have damaged surrounds on my 708g-1s and we can't all buy those parted out drivers. They supplied me with part # 73543. Said it is a spec'ed replacement for the 708g-1 and required no modifications to use. $102.17 each. Anybody gone this route yet?

Thanks for the info, Rusty. Is it a complete replacement driver or just a recone kit? Did they give any data on it, like weight, freq. response, or where else it is used?

They gave me no info other than it has been spec'ed as a direct replacement for the 708g-1. I was hoping some of the super knowledgeable folks around here might know more about it's compatibility. It is a complete replacement driver.

I actually got the whole cone and coil assembly of my open circuit 708G-1 removed in tact. I very carefully inserted the small blade of a small screwdriver just underneath the outer edge of the spider and then slowly worked it around the entire perimeter where the spider is glued to the frame. That completely released the spider without any damage. Then I applied heat from a hair dryer and did the same screw driver thing to the outer lip of the surround which completely released it without damage. (I'm not sure the heat did any good). Then I could take the whole cone assembly out and see the problem causing the open circuit (see photo).

The good news is that there is continuity between one side of the break in the coil wire and one of the braided wires that go to the connection terminals, and there is continuity between the other side of the break and the other braided wire. So theoretically, I if could reconnect the broken wire (and realign the two out of place wires so they lie flat), everything should be OK. The bad news is, how the heck do you do that?

Has anyone ever done this successfully? Better yet, is there a way to remove the two out of place wire sections and then reconnect things? Even if it meant shorting out (bypassing) a small portion of the voice coil, I think it would be worth it.

All ideas are welcome. Thanks.

I know you've been asked why not buy a used replacement driver, and one comment was to the effect of swapping used for slightly less used, but here's the first brand new 708G (pair at that) I've seen for sale

$120 with shipping....yes, I know you can buy a whole L7 for that but if this is indeed NIB, that's a pretty rare find, no affiliation

Thanks for the link but I'm crazy enough to want to try to fix the broken VC wire first. Can't make things worse than they are!

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Originally Posted by jbl-ahhh

Thanks for the link but I'm crazy enough to want to try to fix the broken VC wire first. Can't make things worse than they are!

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There is one slight problem with repairs like what you want to do! THEY WON'T HOLD UP UNDER USE! Even IF you do it perfectly, it always lets go, EVEN if you arent driving the hell out of your speakers!

GET THE NIB PAIR of 708,s and BE DONE WITH IT!

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Can't make things worse than they are!

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Nearly true, but you -can- waste a lot of time :)
(you've probably already learned what you can by opening it up
and poking at it ... that wasn't wasted time)
... and a failed repair could pop your amp or possibly damage
the crossover.

Thanks for taking the time to post these pics . They really are quite illuminating .

http://www.audioheritage.org/vbullet...1&d=1276457835

- This coil looks to be wound with round wire , in a double layer winding .
- That's a cheaper ( & effective ) way to raise the BL value ( motor strength ) .
- It's in contrast to JBLs' tradtional method of winding a voice-coil using the "squared-off" edge-wound wire ( which offers a higher density pack when compared to a winding using round wire of equal length ) .

The "injury" to this voice-coil appears to have been caused by the outer winding contacting the gap wall / then rolling off the inner winding & away from the former / eventually binding witnin the gap / which subsequently pulled the stressed wire apart .

Good luck on splicing the ends together in a way that it all fits back into the gap with out binding .

Perhaps flattening the mating ends / then adding tiny bit of solder / will allow the splice to fit back into the gap without rubbing . Add a bit of varnish to the repair to reseal the outer layer .

If you are successful, then fuse this speaker / starting with a low value ( like 1.0 amps ) / then increasing the value if it blows due as normal levels / until you balance safety with functionality .

<. cheers :)

ps: Some of the smaller "Array" series speakers used woofers with the same voice-coil winding approach .

http://www.audioheritage.org/vbullet...1&d=1156686149
http://www.audioheritage.org/vbullet...1&d=1156649479

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Good luck on splicing the ends together in a way that it all fits back into the gap with out binding .

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:yes:

... and that doesn't cause a local hot spot (potentially melting the solder) due to
higher resistivity in the joint.

... and un-stretching that bent-over section (leave a small s-curve?).

The soldering job is doable, depending on one's skills and copious free time.

Sounds like your hell-bent on doing this so do be patient, meticulous, and cautious.

I appreciate all the input. I got a chuckle from Grumpy's comment about "hell-bent on doing this..." Maybe there is some hell-bentness, I don't know. It just feels good to try to fix it, and it will be great if I actually manage it.

I understand completely. :cheers: