warped voice coil

[CraigB]

Hey, everybody, Happy New Years, I hope 2010 treats you all really good!

I've got an old alnico 12" speaker that sounds really nice, but has a warped voice coil, so it only sounds nice until it gets moving.  I've read a process Ted Weber details on his Speaker Resources Q&A and wondered if any of you have tried it: 

The noise is definitely a rub, either from, as you suggested, a warped (from overheating) voice coil, or flakes of paper or other material stuck in the gap between the voice coil and pole or front plate hole. There is a way to correct that if it isn't too severe. I'll detail it here, then you can make the decision whether to try it or not. The result is that you correct the problem without reconing the speaker, thus preserving the value of the original speaker. First, since you will be performing this operation without demagnetizing the magnet, make sure your work area is very clean and you have plenty of light. Lay the speaker on its back with the cone facing up and with a scalpel, carefully cut out the dustcap, leaving about 1/16" of dustcap where it is glued to the cone. This is important because the voice coil wires pass through this point and you want to make sure you don't cut them. Next, use a vacuum cleaner or clean, dry pressurized air to suck or blow the dust and other debris out of the gap. If you hold the speaker upside down with the cone facing downward it will probably help getting the dust and debris out. Next, take a 3x5 index card and cut it into a strip that is the correct length so that you can form it into a circle and stick it down into the gap between the inside of the voice coil and the outside of the pole. This will help form the voice coil back into a circle. Next, lay the speaker back down on its back. Take a Q-tip or small paint brush and dip it into a bottle of acetone (finger nail polish remover). Spread a small amount of this acetone on a couple of the rings of the spider, which is the brownish/yellow corrugated disk attached to the backside of the cone at the base of the basket. Next, place a jar lid or other disk on the cone where the dustcap was and let the speaker set overnight. The lid or disk will prevent dust from getting into the gap overnight, and the acetone causes the spider to relax and reposition slightly, thus repositioning the voice coil. The next day, remove the lid and the index card strip and see if you still have a rub. If you do, try the acetone again, same procedure. If, after a couple of tries, it seems hopeless, then professional reconing is the only resolve. I think it's worth trying though, to preserve the value of the original speaker. If it works, contact me with the size of the dustcap and I'll send you one to replace the one you cut out and instructions on how to replace it.

Thanks - Craig

[PRR]

The right way would be to unglue the cone and spider, shim the gap, and reglue.

I would not try that on any speaker over 20 years old. The paper is too brittle.

Ted's method is the obvious handyman trick. Clean, shim, soften, hope for the best.

The "acetone" raises issues. Nail polish remover is available non-acetone, and I dunno if the alternative solvents will soften speaker shellac. And then there are more-modern phenolic melamine spiders and epoxy cements which won't soften in any solvent.

[CraigB]

Thanks PRR.  The speaker is from the late 60's for sure, so the "more-modern" may apply to this one.  I think I'll look into sending it off to someone who can do it right.  Craig

[Frankenamp]

That trick is pretty much common sense and has a good chance of being successful if the VC is a bit off centered due to the former or glue getting deformed due to heat or age. Weber's way is the quickie way. I would use a similar method removing the dust cap, (careful with that vaccuum)  and shimming the gap. I'd cut a piece of card stock about 1/4" by 3" and slide it into and around the gap to feel for debris. (which can be blown with canned air or vaccuumed) If it is 'sticky' or tight in one area (out of round VC) you can try using a slightly thicker card stock (a slice off a manila envelope- if it will fit without damaging the former) about a quarter to a half inch wide to move the tight spot away from the pole piece. leave it there for a few days and see if it does the trick with just pressure first before applying solvent. If you are worried about overdoing the solvent, put a little in a foil cupcake or muffin insert and turn the speaker upside down over the tin of acetone, cover it with plastic (trash bag?) and leave it for a day or two. Do this outside or in a well ventillated place NOT next to your water heater please! winter might not be the best time for this since acetone is not nice to your central nervous system and other things, and is highly flammable. I'd be tempted to try the cheap and easy solution first, before investing in a total recone job- especially if the surround and cone are in pretty good condition.

[CraigB]

I will follow that procedure, since I got a response from Orange Co. Speaker that the only way to resolve is a total recone/rebuild.  What could it hurt?  This thing is in beautiful shape, the paper looks good.  There's two side covers that were glued into the "horsehoe" that goes over/around the magnet.  I pulled those off and can see the bottom of the voice coil and there is a small area in the circumference of it that touches the pole (if that's the right term).  I think I may be able to shim from this side rather than going in and cutting out the dust cap.

So, Frankenamp, basically the idea is to fume it, rather than actually put the acetone on the spider and let it wick up into where the voice coil is glued on?  I like it.  Sounds like it is worth a try.  Thanks for the info.

PRR, how would one go about ungluing the cone and surround?  Just curious how that is done.

Regards,
Craig

[Frankenamp]

Yes, 'fuming' is a good term for it. (although that brings back memories of my ol' man's personality- fuming and fulminating come to mind) If you can work off the side covers that would give you a pretty good view of whats going on underneath, but not inside. Maybe some canned air and test tones at 5-6 volts might disloge any dirt or loosened glue particles that may have found their way into the gap. The only worry is ferrous debris which will tend to get stuck in the gap via the magnetic flux. As far as loosening the cone, soaking it in acetone is usually the only way-  and that is almost always fatal to the cone. PE and MCM have dust caps as does OC. getting one slightly oversized and setting it over the sliced & diced dust cap may help to cover a multitude of sins. Another helpful trick is to get one of those plastic round flat 'turntable' bearings Tap plastic has them pretty cheap- big help to move the speaker smoothly.

[CraigB]

Well, I cut the dustcap off and got a piece of card stock worked into the gap so that it sticks out slightly on the other end.  The gap is very tight, so that's all that will fit.  I've got a plastic container that I'll use as a hood to put over the speaker and tin w/acetone.  How much acetone would you use if you were doing this, like 1/4 cup, something like that?  Also, I am putting this out in my tool shed which is away from the house and has no electrical or appliances, just a shed.  It is vented, but I planned on closing the door.  Seems like this would be safe, what do you think?

Regards,
Craig

[Frankenamp]

Were you able to slide the card all the way around the pole piece? If you can, great- you want to be able to 'feel' if there is any place that is tighter than the rest. the tight or scratchy place is likely the problem area. you will then keep the card stock slip stuck in that area when you set the speaker over the cup of acetone. The shed is a great place to keep the project away from kids, pets and other dangers. I assume it's a concrete floor, put a trash can liner on the floor between the acetone and speaker, then you can put the container over everything (better yet put it all in a can liner).  start out with a couple tablespoons (little less than a quarter cup) and see how that goes. Depending on the temperature, it will take somewhere between a few hours on a hot day- to several days in near freezing temps. I'd put it in in the morning before work, and check it when I got home to see the progress.

[CraigB]

Yes it is a concrete floor.  I planned to lay the plastic storage container lid on the floor, place the acetone (in a cut-down soda can) on it, then place the speaker face down as close to center as possible over the soda can, then place the container onto the lid.  Certainly not airtight, but should keep the fumes contained.  Sound OK?  Then once I get it "fuming" for a couple days, should I also wait a day before pulling the shim out so the softened glue has a chance to re-harden?  Thanks again for the suggestions.

Regards,
Craig

[Frankenamp]

yep, you're on the right track!

[CraigB]

Hey guys, it worked - very nice souding speaker and no more rub!

Now, what do I use for glue to put the dust cap back on?  Can I use the original dust cap?  Thanks,

Regards,
Craig

[Frankenamp]

PE has a speaker glue

http://www.parts-express.com/pe/showdetl.cfm?Partnumber=340-076%20%20%20%20%20%20%20%20&FTR=speaker%20glue&CFID=136462&CFTOKEN=72109027

I think they also sell dust caps, as does MCM.