semi on-topic (but not really:) Help repairing an old hi fi speaker of strange design

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by brad347, May 19, 2008.

  1. brad347

    brad347 Member

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    There's such a large pool of knowledge here I figured I'd take a chance and see what you guys know...

    I got for almost-free a set of old hi fi flat panel speakers.

    The speakers are Bertagni SM300 flat panel speakers. They pretty much rule, they're 4-way speakers made of huge polystyrene panels and they sound great, but the HF driver in each is a little piezo(?) tweeter, and one of them is not working. I know what some of you are thinking and yes, it is worth it. I know piezo tweeters are not a hallmark of quality speakers but these are very nice flat panel speakers and pretty expensive when 100%, the piezo design was a necessity because of its location and how its mounted, and truth be told sounds pretty good (in the other, 100% working speaker :) ). It's also crossed over very, very high (like above 10k) so it's just adding some "air" up there. To tell the truth, it took me a minute to even figure out that something was amiss.

    Anyway, I hooked up another tweeter (of a different design) in parallel briefly and got sound, so it's gotta be the tweeter itself, nothing is wrong in the crossover etc.

    Any idea where/how I would source something like this? Any chance I could try to fix the actual tweeter myself if I can't source a part, and if so how would I do this? bear in mind the new one has to be able to be glued to the styrene LF driver (see pix).

    Weird, I know.

    [​IMG]
    [​IMG]

    any slight lead or direction anyone can steer me in would be great, tests to perform, measurements to take, etc. There seems to be continuity and very low resistance between the two leads on both the working and non-working transducers. Poking around with my meter, I can't find anything obviously different between the working and non-working tweeters. Of course I did not disconnect them from the circuit so my measurements are probably meaningless.

    It seems almost like there's a little piezo transducer attached to the cone some kind of way. I'd happily replace that little piezo element if (a) I knew what to replace it with, exactly and (b) how to get it off.

    Oh, here's a pic of the speaks in case yr interested. :D

    [​IMG]
     
  2. donnyjaguar

    donnyjaguar Member

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    Are you sure its not just a case of carefully gluing the crystal back onto the cone apex? I specialize in this kind of microsurgery but if you don't you could easily shag that devil. This design looks very similar to Astatic crystal microphones and phono cartridges of yesteryear. If the crystal is shattered there's no hope. Check also for series resistor that is open-circuit, although admittedly this wouldn't explain why you can measure resistance across the leads.

    FWIW, the theory is that the crystal becomes mechanically deformed when a voltage it applied to it. The opposite is also true, when you deform it mechanically it creates a current. The voltage can be very high, indeed. Think click-style cigarette and BBQ lighters here. This will also explain why both of the aforementioned have rather short lifespans. The speakers on the other hand should last a while. :)


    I remember similar speakers about 22 years ago that used styrofoam for the cone and were as flat as a plate of piss. They sounded surprisingly good.
     
  3. brad347

    brad347 Member

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    The crystal appears attached to the cone apex very well and, like I said, another tweeter hooked up to the terminals works fine.

    So I think it's a case of the piezo element itself being bad.

    To that end, I've bought an old Motorola piezo tweeter driver that looks like this:

    [​IMG]

    When it comes in the mail, I'll carefully try to get the piezo element and cone out of the plastic housing and just replace the whole shebang. It looks very similar and frequency response is ±3db up to 22k, which is the exact spec for the tweeters in these speakers in the factory brochure, and I know Motorola/CTS has made the best piezo tweeters for a long time, so it's possible/probable that this was the correct OEM part. Certainly looks similar.

    I'll let you know how it goes. Thanks for the input!

    Oh yeah, sounds like what you saw were very similar in design to these (Bertagni/BES is now Sound Advance and have always been at the pioneering stages of flat-panel speakers).

    Here's a pic of the speakers with the back covers off:

    [​IMG]

    They sound excellent. ±3db from 30Hz up to 22k
     
  4. brad347

    brad347 Member

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    Here's the reply I got from Sound Advance, in case anyone in interweb-land needs the info in the future. It's as I expected, I got the oem part. Awesome!

     
  5. donnyjaguar

    donnyjaguar Member

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    Excellent on finding the appropriate replacement part. Thanks for the feedback too. These speakers look better than the one I was talking about years ago, which I believe was made/marketed by Pioneer or JVC.
     

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