Recoating Bozak Mids
- Thread starter Sandrion
- Start date
Freddymac2
Super Member
The Bozak mids that I had are aluminum. The very old ones were paper. If you have pock marks of corrosion I would not do anything because bonding to aluminum is difficult when it is clean. What ever you put on it will flake off in time. You also don't want to add any mass to the cone
Sandrion
Active Member
Let me clarify. These are aluminum b209Bc drivers. Not the older paper cones. The latex coating that was originally on these drivers is coming off and is almost completely gone. I was thinking of recoating because they are supposed to have that coating with the aluminum cone.
drbiggles
I like to pull weeds.
The word from lifelong Bozak fans is to use a black, semi-gloss enamel from an aerosol can. It'd be better to use a latex, but haven't found it in a spray can. This is of course, not factory correct. But it's been done by people who know the proper sound of the driver. It works just fine and is far better than tossing the drivers. Simply, ever so lightly, cover the cone in paint.
Biggles
Biggles
twiiii
Lunatic Member
If I were adventurous I'd want to try some of that spray on rubber that is advertised to seal anything on TV. Figuring out how to apply a thin enough coat that would stick would be the issue. I'd sure want to practice my applications of some Reynolds Wrap aluminum foil. RV'rs are always looking for a flexible product that is easy to apply to stop rubber roof, slide out, and window leaks. Maybe some of those rubber roof repair adhesives would work?. Boat and Marine supply houses might have something. My 209 B and Bc are looking just fine. Its the tweeters that are the issue. Join the Bozak owners group or check with Face book, Ken and Judy are the experts. The coating, Bozak, Altec, JBL used on the accordion cloth speaker surrounds might work with thin applications. We know it will stick and is flexible.
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Some thoughts on this, which I've discussed and debated.
Cone Mass
The aluminum cone was used for the B-209 and other drivers because it is stiff and low mass. HF reproduction requires a stiff, low-mass cone. LF reproduction requires a stiff, high-mass cone. Adding mass increases LF response at the expense of HF response. This is why subwoofer cones allow additional mass to be added. The HF response of a subwoofer is obviously not an issue. The change in the HF performance of the B-209, however, is an issue and would need to be tested before and after.
Surround Mass
The surround damps out resonance. Stiffening a surround raises Fs. This is, of course, very significant in woofers. Does it matter for a midrange? Not for Fs, of course, but possibly for resonance. (The midrange is above the passband where Fs would matter for, say, a woofer.) If the surround is stiffened, however, it may reflect the waves instead of damping them. That was the key change between the B-209A and B-209B. Again, performance would need to be tested before and after, with particular emphasis upon effects near resonance.
Coating Materials
The surround repair for AR speakers is butylene rubber dissolved in toluene. Butylene is very flexible and very sticky. It takes a decades to dry. This is the same material originally used on a cotton-fabric surround to seal it, a crucial issue for acoustic suspension as the trapped air forms a restoring force. Leaks change the performance.
Only the surround in a cloth-rubber surround speaker is to be repainted, never the cone, because that change the characteristics of the cone and would add mass.
Adding a sticky substance to the B-209 would likely attract dirt. It also has unknown properties for damping a cone. Testing would be required, but this looks, from the start, to be sub-optimal.
It is not clear how long a replacement coating will last. As the coating drys it shrinks. This will place additional strain on any existing coating which is already de-adhering to the surface. Latex paint may pull an underlying oil coat off the wall in this fashion.
Proposal
Hypothesis 2 To Be Tested: I have proposed that someone (on an donor driver) remove the loose or loosening old latex with a light brush and solvent wash, and then recoat using an airgun loaded with an aqueous latex solution and some carbon black for color. I would not spray more glop onto on as a topcoat for the above reasons.
I do not believe it would be harmful to add a very thin coating of latex, because the change in mass is likely equal to the lost mass from the flaking coating. It isn't clear how much cone loading this would cause. Again, investigation is needed.
Hypothesis 2 To Be Tested: A hard acrylic paint, does not acts as a damping material to lower the Q. It will, instead, vibrate and permit propagation of the resonant wave. This should be tested if one claims it works. I am dubious for the above reasons.
Cone Mass
The aluminum cone was used for the B-209 and other drivers because it is stiff and low mass. HF reproduction requires a stiff, low-mass cone. LF reproduction requires a stiff, high-mass cone. Adding mass increases LF response at the expense of HF response. This is why subwoofer cones allow additional mass to be added. The HF response of a subwoofer is obviously not an issue. The change in the HF performance of the B-209, however, is an issue and would need to be tested before and after.
Surround Mass
The surround damps out resonance. Stiffening a surround raises Fs. This is, of course, very significant in woofers. Does it matter for a midrange? Not for Fs, of course, but possibly for resonance. (The midrange is above the passband where Fs would matter for, say, a woofer.) If the surround is stiffened, however, it may reflect the waves instead of damping them. That was the key change between the B-209A and B-209B. Again, performance would need to be tested before and after, with particular emphasis upon effects near resonance.
Coating Materials
The surround repair for AR speakers is butylene rubber dissolved in toluene. Butylene is very flexible and very sticky. It takes a decades to dry. This is the same material originally used on a cotton-fabric surround to seal it, a crucial issue for acoustic suspension as the trapped air forms a restoring force. Leaks change the performance.
Only the surround in a cloth-rubber surround speaker is to be repainted, never the cone, because that change the characteristics of the cone and would add mass.
Adding a sticky substance to the B-209 would likely attract dirt. It also has unknown properties for damping a cone. Testing would be required, but this looks, from the start, to be sub-optimal.
It is not clear how long a replacement coating will last. As the coating drys it shrinks. This will place additional strain on any existing coating which is already de-adhering to the surface. Latex paint may pull an underlying oil coat off the wall in this fashion.
Proposal
Hypothesis 2 To Be Tested: I have proposed that someone (on an donor driver) remove the loose or loosening old latex with a light brush and solvent wash, and then recoat using an airgun loaded with an aqueous latex solution and some carbon black for color. I would not spray more glop onto on as a topcoat for the above reasons.
I do not believe it would be harmful to add a very thin coating of latex, because the change in mass is likely equal to the lost mass from the flaking coating. It isn't clear how much cone loading this would cause. Again, investigation is needed.
Hypothesis 2 To Be Tested: A hard acrylic paint, does not acts as a damping material to lower the Q. It will, instead, vibrate and permit propagation of the resonant wave. This should be tested if one claims it works. I am dubious for the above reasons.
rvatfhunter
Active Member
I found this interesting. http://www.google.com.gi/patents/US3093207
He proposes, a latex or synthetic:
He also weighs in on the thickness:
Which confirms that this must be carefully applied to avoid altering the frequency response.
But, again, given that the film has already been applied adding more is going to be tricky. Someone needs to do a before/after experiment on this to see what happens to the frequency response.
The material which gives excellent results as a damping film on the diaphragm is a natural or synthetic rubber latex applied in liquid form. For example, neoprene latex and other synthetic rubbers such as styrene-butadiene and =acrylonitrile-butadiene copolymers, polybutadiene, polyisoprene and polyisobutylene, dissolved or suspended in suitable solvents, may be employed. These materials may be applied either by brushing or spraying. I prefer to spray the latex on the diaphragm as this gives the thinnest possible film."
He also weighs in on the thickness:
It is important that the film of damping material be as thin as possible so that the weight of the diaphragm will not be objectionably increased. In this connection I have found that for a diaphragm thickness within the range specified hereinabove the thickness of the film of damping material should not exceed the thickness of the diaphragm itself. In the case of the preferred form of structure where a film of damping material is applied to both sides of the diaphragm the combined thickness of both films should not be greater than about twice the thickness of the diaphragm itself. When the film thickness is kept below this limit the increased weight of the diaphragm due to the film of damping material does not substantially interfere with the response characteristics of the diaphragm.
Which confirms that this must be carefully applied to avoid altering the frequency response.
But, again, given that the film has already been applied adding more is going to be tricky. Someone needs to do a before/after experiment on this to see what happens to the frequency response.
Steven Ver
Active Member
Consensus seems to be leave the cones alone until someone sacrifices drivers as guinea pigs...alnico drivers from the 60's aren't getting any cheaper...I assume that any loose coating should be gently removed so the cone surface is smooth. Patent materials relay how critical a smooth light cone surface is.
Ramseybella
AK Member
The photos of this cone flaking show a tight squeeze on the backside of the cone.
And the coating looks extremely thin.
And is a surround involved?
I'm thinking air brush thin coating, getting Latex thinned down enough to pass through an Air Brush could be impossible..
And the coating looks extremely thin.
And is a surround involved?
I'm thinking air brush thin coating, getting Latex thinned down enough to pass through an Air Brush could be impossible..
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Adding mass to a cone changes its characteristics. Subwoofer tuning does this, albeit with a much greater mass. It is not recommended as a general practice.
Consider the physics. It is well accepted by physicists, if not by audiophiles, that F = m x A. (F = Force, m = Mass, A = Acceleration.) Given F is constant (motor power from voice coil and magnet are unchanged) then if M increases so A must correspondingly decrease to maintain a constant product. The tweeter may not be able to properly reproduce high-frequency sound with a reduced cone acceleration. MMS is the relevant T/S characteristic, and it affects all sorts of parameters.
Now, having written that the argument is that the amount of latex (or paint) is small. While true, the question becomes how much paint and how by what percentage is the cone weight increased over that of the uncoated cone or the factory-coated cone. Such questions are currently mysteries. We do not, for example, have any before/after measurements for the re-coated drivers. So at this point the effects of re-coating are unkown, and acoustic memory is so unreliable nobody could tell. The effect may not show up until the B-200Y has reproduction problems anyway, as it starts to die above about 9,000 to 10,000 Hz, so the issue of reduced high-frequency reproduction may be moot.
I had discussed removing the latex and then replacing it, but I need to locate some sacrificial drivers upon which to experiment. I believe this can be safely done without deforming the cone or detaching the surround, but experiments are required.
Consider the physics. It is well accepted by physicists, if not by audiophiles, that F = m x A. (F = Force, m = Mass, A = Acceleration.) Given F is constant (motor power from voice coil and magnet are unchanged) then if M increases so A must correspondingly decrease to maintain a constant product. The tweeter may not be able to properly reproduce high-frequency sound with a reduced cone acceleration. MMS is the relevant T/S characteristic, and it affects all sorts of parameters.
Now, having written that the argument is that the amount of latex (or paint) is small. While true, the question becomes how much paint and how by what percentage is the cone weight increased over that of the uncoated cone or the factory-coated cone. Such questions are currently mysteries. We do not, for example, have any before/after measurements for the re-coated drivers. So at this point the effects of re-coating are unkown, and acoustic memory is so unreliable nobody could tell. The effect may not show up until the B-200Y has reproduction problems anyway, as it starts to die above about 9,000 to 10,000 Hz, so the issue of reduced high-frequency reproduction may be moot.
I had discussed removing the latex and then replacing it, but I need to locate some sacrificial drivers upon which to experiment. I believe this can be safely done without deforming the cone or detaching the surround, but experiments are required.
Ramseybella
AK Member
Let us know if the results are positive or negative.
Problem is that the results must include actual measurements performed using instruments before and after the recoating, including changes to the T/S parameters and frequency response. Ringing behavior may be worsened if the cone is stiffened. That's the problem with rigid cones made from aluminum or other metals. The "well, it sounded ok" isn't going to reveal the whole story. This is why the experiment is tricky to perform.
Steven Ver
Active Member
On that auction site there are several aluminum tweeters that are wrecks...too bad they're still expensive even as beaters...that spray job looks good...still holding back on anything that irreversible though. I agree with Retrovert that real testing would benefit Bozak nuts for years to come...all the good aluminum drivers are crumbling slowly but surely it seems
Ramseybella
AK Member
To bad most of us can't hear all mid to high frequencies like a bat or the lowest like a blue Whale 2000 miles away.
What I'm getting at is can you catch the most subtle differences on how your speaker sounds from one day to the next?
From one artist to the next?
Not trying to be sarcastic with all this but how does the speaker sound now after years of the coating ever so slowly coming off, has it changed so drastic?
I don't think anyone can answer that other than a machine.
The part i would like to know first is how do you get the stuff off the cone and with what solvent.
What I'm getting at is can you catch the most subtle differences on how your speaker sounds from one day to the next?
From one artist to the next?
Not trying to be sarcastic with all this but how does the speaker sound now after years of the coating ever so slowly coming off, has it changed so drastic?
I don't think anyone can answer that other than a machine.
The part i would like to know first is how do you get the stuff off the cone and with what solvent.
Steven Ver
Active Member
Just got these B209b drivers....had significant bubbling but all the flakes came off when I gently flicked the rough areas with a soft plastic packaging corner...getting a feel for how aggressive to be, and it is nice and smooth now with no damage to the cone. I guess about 98 % of the original coating remains. If they were really bad I might have considered something else. Cleaned the dirt and dust off gently with alcohol/distilled water mix I use to clean records. Hope the photos post. Hope this helps someone else
