Recoating Bozak Mids

Discussion in 'Speakers' started by Sandrion, Aug 12, 2017.

  1. Sandrion

    Sandrion AK Subscriber Subscriber

    Messages:
    124
    Location:
    Shelby, NC
    Is it possible to recoat Bozak mid drivers? What would I use? Is latex available in a form that could be sprayed on?
     
  2. Freddymac2

    Freddymac2 Super Member

    Messages:
    1,314
    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
     
    Retrovert likes this.
  3. Sandrion

    Sandrion AK Subscriber Subscriber

    Messages:
    124
    Location:
    Shelby, NC
    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.
     
  4. drbiggles

    drbiggles I like bacon Subscriber

    Messages:
    5,957
    Location:
    Richmond, CA
    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
     
  5. twiiii

    twiiii Super Member

    Messages:
    4,253
    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.
     
    Last edited: Aug 13, 2017
  6. gdmoore28

    gdmoore28 Super Member

    Messages:
    1,235
    Location:
    Central Arkansas
    If the aluminium cones look ok, I'd hit them with a light coat of aerosol clear lacquer of a good brand.

    GeeDeeEmm
     
  7. Sandrion

    Sandrion AK Subscriber Subscriber

    Messages:
    124
    Location:
    Shelby, NC
    If I end up getting them I'll be trying the paint method. That rubber in a can stuff is too hard to control.
     
  8. Retrovert

    Retrovert AK Subscriber Subscriber

    Messages:
    1,845
    Location:
    New York City, NY
    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.
     
    drbiggles likes this.
  9. Sandrion

    Sandrion AK Subscriber Subscriber

    Messages:
    124
    Location:
    Shelby, NC
    Wow. Okay, I'm planning on shooting these with a latex coating that Krylon makes. I'll get back to you after I'm finished.
     
  10. rvatfhunter

    rvatfhunter AK Subscriber Subscriber

    Messages:
    322
    Location:
    RVA
  11. Retrovert

    Retrovert AK Subscriber Subscriber

    Messages:
    1,845
    Location:
    New York City, NY
    He proposes, a latex or synthetic:
    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.
     
    drbiggles likes this.

Share This Page