PDA

View Full Version : Coil Centering Questions



New At This
06-02-2005, 03:25 PM
I'm refoaming a set of 125A woofers from Decade 36's. Among the good information that Gordon W gave me in this regard was the following:

"...[use] the "30Hz method" (ie, play a 30 Hz test tone through the speaker while the glue is drying, and make sure it's centered (ie, not rubbing) correctly- it'll usually want to center itself, due to the influence of the test tone) to insure that the coil is centered. ..."

I'd like to do things this way, so that I don't have to cut the cap, etc.

Two questions:

1. When I try to press evenly on the cone of either woofer to make the coil (?) move in and out, it seems to rub almost every way I push it. (Rubby, skretchy sounds.) I tried this with some other woofers at a local recycling depot, and most of them seemed to have much more tolerance.

Is it possible that something has worn out inside the coil, and that there are particles of some kind impeding smoothness? (And, if so, are the woofers just shot; or, is there some counterpart of WD40 to smooth things out?) Or are these just adjusted to a very narrow tolerance?

2. And, come to think of it, why aren't the coils going to lose their centeredness when I stand them up, with the coil moving horizontally, rather than vertically?

Thanks for any information.

Robh3606
06-02-2005, 03:40 PM
What happens if you try a low level low frequency tone?? It should self center and not rub unless you drive it to hard and the coils tilts in the gap. Try not to move the cones by hand. JBL tends to have tighter gap tolerances than most and you can force it out of alignment. Do a search on refoaming on the forum. Bo has a good piece he wrote on the subject. Good Luck

Rob:)

New At This
06-03-2005, 12:20 AM
What happens if you try a low level low frequency tone?? It should self center and not rub unless you drive it to hard and the coils tilts in the gap. Try not to move the cones by hand. JBL tends to have tighter gap tolerances than most and you can force it out of alignment. Do a search on refoaming on the forum. Bo has a good piece he wrote on the subject. Good Luck

Rob:)

Thanks, Rob. I'm trying to make a 30 hz CD from a CD I have that alternates 30, 60, 90, and 120 hz every ten seconds. As soon as I figure out how to extricate the ten seconds of 30 hz (and figure out whether I can hook my receiver speaker cables right to the now-disconnected woofers, or have to reconnect the woofers to the cabinet circuitry to avoid harming anything), I'll give it a try.

By the way, just using my receiver's volume level (which, of course, you can't hear) as a guide, what might be a ballpark guide to having enough juice to move the coil, but not enough to drive it too hard?

Bo's piece is excellent -- extremely helpful!

Thanks again.

Tim Rinkerman
06-03-2005, 07:28 PM
The "spider" the accordion looking fabric circle under the cone is what really holds the voice coil centered in the magnetic gap. The only two things that make noise in the gap are if the speaker got dropped and the magnet shifted allowing it to rub,or a voice coil that got really hot and cooked the adhesive and got"unround" and rubbed. It's not un-common to find shifted magnets in almost all breeds of JBL 15 inch drivers. It only takes one hit or in advertent "oopps! I didn't think that thing was that heavy!" ( insert sound of speaker cabinet landing on something hard..)to do the damage.

johnaec
06-03-2005, 07:44 PM
The "spider" the accordion looking fabric circle under the cone is what really holds the voice coil centered in the magnetic gap.While the spider DOES center the VC near the top, if the surround is glued off-center it will bulge the spider and "lever" the far end of the VC out of center, causing a rub. This is where the problem arises...

John

New At This
06-04-2005, 01:04 AM
Well, I made a 30 hz CD, and tried it. (Neat: I can't hear a thing, but the cone is vibrating away.) Unfortunately, I couldn't find in the online Owner's Manual, etc., what black and green represented, and so went with negative on the post with "grabber clips," and positive on the post with none.

At any rate, the following happened:

1. When I have the woofer so that the cone (and so, the sound) is pointing straight up, no rubbing in either woofer;

2. When I have the woofer so that the cone is pointing horizontally -- as it would be in the cabinet -- and rotate it around its axis 360 degrees, I get rubbing on about 1/3 to 1/2 the possible placements, on each woofer.

So....I wonder whether: A. I've got a much bigger problem than a refoam (in which case I'm pretty much out of business); B. I could just place each woofer into the cabinet in one of the 1/2 to 2/3's of positions in which there was no rubbing, or; C. Neither A nor B really is quite the case.

As always, any guidance is really appreciated.

Zilch
06-04-2005, 01:31 AM
It's unclear whether you have installed the new foams yet or not. What are the conditions of the surrounds under which you're doing these tests? None, old, or new glued in?

[Ain't science FUN? :bouncy: ]

johnaec
06-04-2005, 06:42 AM
When I have the woofer so that the cone is pointing horizontally -- as it would be in the cabinet -- and rotate it around its axis 360 degrees, I get rubbing on about 1/3 to 1/2 the possible placements, on each woofer.'Poor Man's solution - mount it in the cabinet in the same orientation that results in no rubbing... :D

John

New At This
06-04-2005, 02:16 PM
It's unclear whether you have installed the new foams yet or not. What are the conditions of the surrounds under which you're doing these tests? None, old, or new glued in?

[Ain't science FUN? :bouncy: ]

Sorry not to have been clearer -- there are no surrounds at this time; I've cleaned the remains of the old ones off of the cones and the frames.

Zilch
06-04-2005, 02:34 PM
there are no surrounds at this time.You can't expect them to stay centered playing horizontally with no surrounds.

I'm bettin' once you install the surrounds and let them self-center using the 30 Hz technique while drying everything is gonna be PERFECT! :D

edgewound
06-12-2005, 12:45 AM
The spider is NOT the only thing that really centers the voice coil. In a single spider arrangement such as this 125A, and any other single spider transducer, the cone surround is the OTHER means of centering. If the cone has no support, the voice coil will rock and rub in the gap. Anywhere from 2 to 10 volts input signal, starting at 2 volts and increasing...(you'll need a volt/ohm meter to do it right)is sufficient to move the voice coil for centering a new surround. I like to start at 60Hz for the initial signal to dynamically center the coil and work the wet glue down to the frame and adjust as you go...and then work down to 20Hz and lower (less voltage input for smaller voice coils/magnets, up to ten volts say for a 2235H, 136A,etc.) till you have no rubs when the glue starts to grab. A rubber and gasket cement applied to one surface only works best for this, because it give you some open work time to get the cone and coil in the proper rub-free alignment. If the voice coil former is bubbled due to overheating/burning or damaged from over excursion(when the coil is driven out of the gap) it's most likely going to rub anyway and need to be rebuilt anew. And BTW, if the magnet pole piece shifts, it clamps the voice in place and wont move. They can be realigned, but that's a whole 'nother matter. Sometimes you can get away with injecting a little Krytox grease in the gap to lubricate a slight rub away from the pole piece. Ohm Acoustics did this years ago with a yellow dielectric grease...also provides some coil damping. Geez...giving away valuable trade secrets here that I found along the way. Anyway, hope this helps the DIYer's.

boputnam
06-12-2005, 01:03 PM
edgewound...

Nice! Thanks for chiming in.

The guidance on voltage drive is very good - do not overdrive the transducer when it is not in an acoustic suspension (cabinet). All you want is some flutter motion of the cone - listen closely for rubbing which is different from the air flutter. If you don't have to listen closely, you've got bigger issues! :p

As we've posted elsewhere, slower setting glues are desirable because they give you time to get things centered - however, that means you must stay with it during the drying.

New At This - you're on the right track, but since the spider's still in place, don't worry about "centering" until the surround is being glued. Do one at a time. Keep them on their motor (cone up). Go slow, and you'll get a feel for the gentle movement you might need (if any...) to get the voicecoil centered.