15 inch with a gap a bit off kilter

[DavidF]

Perhaps for those who have seen a fair amount of rebuilds, I have a question. Have a core 2205 frame, an older green-grey ceramic build. Overall fair shape. BUT, the gap is slightly off center both visibly and in pulling a makeshift feeler gauge around the circumference. The magnet is firmly attached, no signs of distress anywhere. No idea of the history of the frame but the it must have worked at some time...I pulled a burned-coil cone assembly out of it.



Any chance this is not unusual for the JBL QC? Maybe -no- should be an obvious answer, but let me know anyway.



David F

[GordonW]

What's the tolerance... ie, what's the biggest and smallest dimension within the gap? I'd think there would be a small tolerance range, but if you can SEE the gap irregularity with the NAKED EYE readily, that it probably wouldn't pass.

Sounds possibly, like someone reglued the magnet sometime in the past, and didn't quite get it shimmed properly. Easy to do... I've actually had one driver (an Eminence, fortunately it had a BIG gap guage, so no clearance problems) in which the gap shim somehow got slightly cocked and mispositioned during re-gluing, so that there was a just-visible error. Driver still worked fine, but it was a visible difference. However, with good shims, and the patience sufficient to make sure you have the CORRECT shim "stack" thickness, it shouldn't ever be the case, though...

Regards,
Gordon.

[DavidF]

Sounds possibly, like someone reglued the magnet sometime in the past, and didn't quite get it shimmed properly. Easy to do... I've actually had one driver (an Eminence, fortunately it had a BIG gap guage, so no clearance problems) in which the gap shim somehow got slightly cocked and mispositioned during re-gluing, so that there was a just-visible error. Driver still worked fine, but it was a visible difference. However, with good shims, and the patience sufficient to make sure you have the CORRECT shim "stack" thickness, it shouldn't ever be the case, though...

Regards,
Gordon.

Thanks for the reply, Gordon. I can not measure the variance but what you mention sure sounds as though this is post-production issue. Frankly, I was not aware that the plates, magnets etc could be separated and realigned, at least not without some amount of damage risk to the assembly.

DavidF.

[Alex Lancaster]

Is the pole-piece glued on, or just held by magnetism like the Alnicos?

[GordonW]

Think all the woofers are glued. Only the horn drivers' magnet structures are held in place by magnetism, I believe...

[Alex Lancaster]

I remember centering Alnico 15" with a rubber hammer at JBL´s training course for dealers, like in '79, but don´t know about Ferrites.