Is this 3a woofer toast?

[TimmyTonga]

Hi everyone,

 

First off to say kudos to those who complied the 'Restoring an AR-3a' document; it is absolutely crucial for those attempting it for the first time. Without that detailed knowledge, I just wouldn't have attempted it at all.

 

But I wonder whether anyone would like to give me a prognosis on this 3a woofer.

 

As you see, it's riding pretty low in the saddle, lower than its twin, and has a bit of a droop on the RHS, making it a kind of ellipse shape.

 

Resistance reads as it should, about 2.6. You can pull it up quite a long way, and push it down a bit, but it's difficult to do either without hearing the VC scrape.

 

Other opinion has offered me the possibility that it has spent too long on its back, and that I might want to take off the dustcap and stick in some shims to perk it up a bit.

Trouble is, I don't want to replace the dustcap with a new one, so would have to take it off in one piece and then glue it back on. This is, by all accounts, not for the faint-hearted.

 

Would anyone just refoam it, and see what happens?

 

Thanks for any input. I have enjoyed reading many posts on this site, and found them really helpful and informative, but this is my first, and I send you best wishes from the UK.

 

T

[Aadams]

Timmy  

The perspective of the photo gives an off-center look to the edge where the previous surround was glued, so assuming it is just a perspective issue you are better off shimming IMO.  I have used both the 30hz signal method and shimming to install surrounds and I prefer the freedom and flexibility afforded by shimming .  Shimming, to my knowledge, is the only way to set the height of the cone and insure its concentricity in relation to the voice coil when installing a new surround.  The dust cover can be opened without removing it by leaving a small section of its base uncut to allow the cover to be turned up as though on a hinge.  It  can be glued back in place after the surrounds are installed.   You just have to get over the anxiety of making the cut.

Adams

[TimmyTonga]

Thank you A. It was really difficult to get a shot which adequately showed the distortion of the cone; it is definitely not the same height all the way round, but I'm happy to accept that shimming Is best way to resolve it. If I could impose further, I would ask you - a) how do shims help you determine the height, and b - what do people use for shims. There doesn't seem to be anywhere to source them in the UK.

Also, do you favour softening the glue round the dustcap with a solvent before removing?

many thanks 

t

[owlsplace]

Welcome T,

I can understand your apprehension as it took me quite a few refoams before it became old-hat. Stay away from the solvent glues and make sure you get the correct foams. Sometimes the original dust caps will lift off without cutting. I would avoid solvents on the cone but you could try some controlled heat from a paint stripping gun or similar or even warming in the sun.

I sometime use aluminum shims cut from printing plates or I think most people use the clear acetate document covers cut into strips of varying widths. Shims determine the height by allowing you to position the VC at various points held in place by friction.

Finding some less expensive woofers to practice on may give you some confidence.

Roger

[TimmyTonga]

Thank you Roger, your advice and info equally appreciated. I've refoamed a pair of AR18s and AR7s without disaster, but cutting the dustcap off is in a different league because of the possibility of damaging the cone. I will find some cheap speakers and practice on them as you suggest.

hooray for CSP

T

[lARrybody]

I vote for putting new surrounds on these woofers. Make sure you get the correct foams. The ones with the 5/8 in. rolls are preferable over the 1/2 in. roll, but both will work. The more compliant the surrounds the better. 

As far as to shim or use the 30Hz test tone method there are arguments pro and con on both methods. The last 3a woofers I did was with the tone alignment method. I have probably done fifty sets this way and never had a problem when the voice coil and spider were in good shape.  Actually cleaning the old foam and glue off is the part I dislike. I stuff folded up sheets of cloth between the cone and frame to elevate the cone and glue the  inside of the new foam centered  onto the cone. Aleene's Tacky is my glue of choice. When gluing the outside of the surrounds apply a thin even cote to the foam surround and woofer frame and remove the cloths with tweezers or needle nose pliers. Put the tone through and keep adjusting till you are satisfied and the glue is drying. The first few times were tedious, but now I know what to expect.

I have also used the shim method. When you are removing and replacing the dust caps its not that bad. To remove the dust caps I make a cross cut, paying attention to the voice coil wires and carefully remove in four sections with sharp xacto knife and sometimes solvent and heat. I have a mini butane powered heat gun I use. I use a insulin syringe filled with MEK (Methyl Ethyl Ketone) and use it sparingly. I depends on the type of glue that was originally used. You can get a dust cap slightly larger to cover the original glue lines. Make sure the dust cap is made out of the same material as the original.  Again taking your time and paying attention to the voice coil wires are very important.

Saving the original dust cap can also be done. You just have to be very diligent and take your time. Not for the impatient or faint of heart. 

Shimming insures that the voice coil is truly properly aligned. Especially good for those voice coils with narrow gaps. Some woofers voice coils (like Advents) you can drive a truck through.  Also shimming allows you to adjust the cone height making it much easier to glue the surrounds on. Just make sure to clean the voice coil out with a vacuum to keep any foreign material out.  Its really not near as involved as it sounds.

[owlsplace]

13 hours ago, TimmyTonga said:

Thank you Roger, your advice and info equally appreciated. I've refoamed a pair of AR18s and AR7s without disaster, but cutting the dustcap off is in a different league because of the possibility of damaging the cone. I will find some cheap speakers and practice on them as you suggest.

hooray for CSP

T

De nada, you should be cutting away from the cone ... hopefully someone will post a photo of one that has been done to show what it looks like.

Roger

[tysontom]

16 hours ago, Aadams said:

Timmy  

The perspective of the photo gives an off-center look to the edge where the previous surround was glued, so assuming it is just a perspective issue you are better off shimming IMO.  I have used both the 30hz signal method and shimming to install surrounds and I prefer the freedom and flexibility afforded by shimming .  Shimming, to my knowledge, is the only way to set the height of the cone and insure its concentricity in relation to the voice coil when installing a new surround.  The dust cover can be opened without removing it by leaving a small section of its base uncut to allow the cover to be turned up as though on a hinge.  It  can be glued back in place after the surrounds are installed.   You just have to get over the anxiety of making the cut.

Adams

With the surround gone, and spider (inner suspension) will cause the cone to collapse down below the "center" position, and frequently it will be tilted and will rub.  This woofer (in question) is the original-type 200003-0 AR-3a version, second iteration short-wire version, likely 1971-1973 or so, and it is actually one of the more desirable versions of this superb woofer.  This woofer is extremely compliant, and you must shim this woofer when you re-foam it to be sure of proper voice-coil alignment.  Do not cut the dust cap all the way off (see my pictures below), just cut it very carefully with a razor blade or similarly sharp knife blade around a circle (watch carefully for the lead-in wires to the voice coil assembly under the dome) and leave perhaps 10-20 degrees out of 360 degrees (perhaps 10mm) enough to fold the dust cap back away, exposing the voice coil assemble and cone apex. Never replace the dust cap with a new plastic or paper version!  This dust cap is slightly porous, and it lets air from under the dust cap escape very slightly.  You risk raising the free-air resonance of the woofer if you replace the dust cap; leave the original in place, but you must shim the voice coil.  When you use a proper 5/8-inch foam surround (check with RoyC and others to get an idea of the best surrounds available today) and shim it properly,  you should end up with a free-air resonance ("fs") of 17-19 Hz, and the speaker will have the proper system resonance of 41-43 Hz.  Distortion will be extremely low with this woofer.

It is possible that someone over-drove the woofer and caused the voice coil to slam against the back plate and damage the coil assembly; it's also possible that the woofer has been damaged by a defective or inadequately powered amplifier (most properly working amplifiers will do no harm to this woofer under normal music and speech playback) or extreme over-drive or clipping by an amplifier.  Some amplifiers go bad and produce excessive dc offset in the output, and this will damage the woofer.

AR9Lsi 12-inch woofer, similar to the 200003-0 AR-3a/AR-LST-type woofer.

AR9Lsi woofer

AR-12-inch voice coil and dust cap shimming.

--Tom Tyson

[TimmyTonga]

Thanks to everyone for their contributions, especially Tom for the photos. For me, just doing the surrounds without shimming is a bit of a gamble, because if simply putting on new surrounds doesn't correct the orientation of the cone, I have to take off the surround and start again, this time with shims.

It was my understanding, though I'm happy to be corrected, that all 3a surrounds in the entire world come from the same former, whatever country you buy them in. What do we think? Is this an urban myth?

[lARrybody]

In your case Timmy shimming may be the way to go. Centering the voice coil with a 30 Hz tone seams to work fine also. I have done both. 

All surrounds are not the same. Here are two different AR3a surrounds from two different suppliers. Not only are there size differences, but the density of the two foams are totally different.

Here is a set in the final stages of glue drying. The little tick marks were to help center the surround onto the cone. They both preform flawlessly. I guess there are more than one way to replace foams. 

[bassment142]

30 hz test tones are for lazy people.  do it right.  shim!

[TimmyTonga]

So, to be clear, from Tom's pictures it looks like you don't mess with the flange of the dustcap that is glued to the cone. You cut at the junction of the flange and the dome, is that right? So, in effect you are not unglueing anything?

From other posts I have seen elsewhere, it seems there may not a consensus on this.......

T

 

[owlsplace]

9 hours ago, TimmyTonga said:

So, to be clear, from Tom's pictures it looks like you don't mess with the flange of the dustcap that is glued to the cone. You cut at the junction of the flange and the dome, is that right? So, in effect you are not unglueing anything?

From other posts I have seen elsewhere, it seems there may not a consensus on this.......

T

 

Unless the original cement is failing, yes you would be cutting above it. Here are a few more photos that may be of some help. A brand new #11 X-acto blade works wonders.

Roger

[bassment142]

whoa.. who poured caramel into the voice coil area????

[TimmyTonga]

OK, that's interesting - in the top one you took off the whole cap including the glued flange, and in the bottom one you cut above it. Thanks Roger.

[owlsplace]

7 hours ago, bassment142 said:

whoa.. who poured caramel into the voice coil area????

That was Kent's question. These photos are from the archives. That is actually a thermal grease.

7 hours ago, TimmyTonga said:

OK, that's interesting - in the top one you took off the whole cap including the glued flange, and in the bottom one you cut above it. Thanks Roger.

The whole cap isn't visible. It should be hinged around the voice coil wires. The flange is still visible as it was cut high.

Roger

[TimmyTonga]

3 hours ago, owlsplace said:

That was Kent's question. These photos are from the archives. That is actually a thermal grease.

The whole cap isn't visible. It should be hinged around the voice coil wires. The flange is still visible as it was cut high.

Roger

Oh OK, I see it now. Sorry, my brain with interpreting the visual data incorrectly. WelL, that looks a lot easier that trying to get under the flange, especially as all mine are covered with butyl rubber.

And I was going to ask about the wires but you just answered that. As long as they are not too far apart it makes sense to use that section as the hinge.

 

[ra.ra]

Roger, I'm curious about the second pic you show for the AR-3 type woofer. With a cloth surround fully intact on this woofer, what was the reason for cutting the cap and shimming? Did this involve a spider repair?

[owlsplace]

12 minutes ago, ra.ra said:

Roger, I'm curious about the second pic you show for the AR-3 type woofer. With a cloth surround fully intact on this woofer, what was the reason for cutting the cap and shimming? Did this involve a spider repair?

That appears to be the case. Here is the post:

Carl mentions Orange County Speakers as an adhesive source but I believe they are out of speaker repair and have a name change.

Roger

[tysontom]

20 hours ago, bassment142 said:

whoa.. who poured caramel into the voice coil area????

WTF!  Never, ever put anything like that on the inside of the voice coil!  To add grease to that woofer (one not designed for anything like this) would seriously degrade the performance of the woofer and cause it to be very over-damped.  The grease will always be somewhat in contact with the pole piece and coil, and it will drag it down a small amount, just enough to ruin the normally ruler-flat bass response of this woofer.  Bass response would seriously suffer and the roll-off in deep bass would be very audible. 

Someone had the erroneous notion that putting grease in between the voice coil and the pole piece would create a path for heat dissipation and give the woofer greater power-handling capability, much like the addition of Ferrofluid to a tweeter voice coil.  It won't work this way with a woofer with such great excursion (greater than one-half inch, peak-to-peak, or more).

--Tom Tyson

[owlsplace]

Tom, I think that grease is an AR original ... I'd have to look up the post again.

Roger

[RoyC]

1 hour ago, owlsplace said:

Tom, I think that grease is an AR original ... I'd have to look up the post again.

Roger

I have never seen anything like that in any AR woofer. I have seen a few Ohm woofers with a light grease of some type in the gap, but nothing as gunky as that.

Roy

[owlsplace]

4 hours ago, RoyC said:

I have never seen anything like that in any AR woofer. I have seen a few Ohm woofers with a light grease of some type in the gap, but nothing as gunky as that.

Roy

Oops! It was from Kent's Ohm model L.

Roger