Cross-overs Most Likely Component To Fail

Discussion in 'Home Cinema Speakers' started by keicar, Mar 2, 2013.

  1. keicar

    keicar
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    Hi, new to this to - be patient!

    One of my Celestion F30's has lost mid-range, by substitution I have worked out that the problem lies in the cross-over, which has just 5 components, 3 coils and two caps, a 9uf 50v and a 10uf 100v which bridges the MF output.

    My question is would the most likely failure be one of the caps? and could I replace the 9uf with a 10uf?

    I'm guessing the coils would be almost impossible to replace (.85mh & 6mh) as Celestion don't seem to exist as far as domestic speakers are concerned.

    Thanks in advance for any help.
     
  2. KelvinS1965

    KelvinS1965
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    I think the coils would be more unlikely to fail but you can check this with a meter since if they have failed then there will be no DC resistance. Capacitors can tend to dry out (though usually it's electrolytic ones that do this). If you fit a different value then it will effect the crossover frequency so not recommended. Having said that the tolerance on some capacitors can be 10 or even 15% so 10uF in place of 9uF is almost within this tolerance at the high end if you got a 5% tolerance replacement. Who knows what the original 9uF actually measured when new anyway?
     
  3. keicar

    keicar
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    Kevin, thanks for the prompt reply.

    Not completely au-fait with testing the coils, they all seem to be zero ohms? Maybe its just me!

    Would I be right in assuming that the caps are bi-polar?

    Thanks again

    Keith
     
  4. KelvinS1965

    KelvinS1965
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    That they measure 0 ohms sounds like they're good since they are effectively just a length of wire wrapped round a coil, so that shows they are intact.

    Bi polar probably wouldn't be a good choice in a crossover since the AC signal would reverse the voltage across the capacitor and bi polar ones can be damaged by this action. However, that doesn't mean that they are never used in crossovers.

    See also this link:

    non polar/bipolar capacitors - Audio-Technology - Audio
     
  5. D.D.D.

    D.D.D.
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    Post the photo of crossover, then it will be easier to diagnose.

    Have you checked it for cold solder joints?
     
    Last edited: Mar 3, 2013
  6. BlueWizard

    BlueWizard
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    Here is a link to a NON-Polar Electrolytic Capacitor -

    Non-polarized Electrolytic Capacitors in the Speaker Components Department at Parts Express | 293

    100uF 100V Non-Polarized Capacitor 027-360

    Notice in the close up, the grooves or rings in each end of the capacitor. Electrolytics are dirt cheap, but they don't last long and they aren't very stable or accurate.

    Here is an example of a Film Capacitor, these are more expensive, though not excessively so, and they are far more accurate and stable, as well as durable -

    Film and Foil Capacitors in the Speaker Components Department at Parts Express | 295

    Note the color doesn't matter, they can be any color -

    Price can be from moderate to extensive -

    Dayton Audio DFFC-0.33 0.33uF 400V By-Pass Capacitor 027-456

    Audiocap PPT Theta 0.47uF 600V Film/Foil Capacitor 027-718

    Metalized Film Capacitors are the most expensive and probably the best, but overkill for all but the most expensive speakers -

    Metalized Polypropylene Capacitors in the Speaker Components Department at Parts Express | 294

    Jantzen 15uF 800V Z-Superior Capacitor 027-498

    Definitely some type of Film Capacitors is the first choice for replacement.

    However, some Crossover Networks can have fuses or circuit breakers (thermal fuses).

    Coils are just that, a coil of wire. Not a whole lot that can go wrong. Though the enamel coating the acts as insulation can break down and short all or part of the coil. Extremely rare but, due to excess heat, the coil can burn open, but that takes a massive amount of heat to accomplish. Very rare and unlikely.

    Solder joints can break down for a variety or reasons. Interconnecting wires or traces of the circuit components can come lose or break down.

    Connection between the crossover circuit board and the driver components can come loose.

    Also, are you sure the driver component (woofer, mid-range, tweeter) has not failed. Usually tweeters are the first do go as they are the weakest link. Capacitors are usual rated at voltages far higher than they will ever be exposed to.

    A simply 1.5volt pen light battery (AA or AAA) across the speaker terminals should cause you to hear a "Click" when you connect and disconnect the battery. Don't keep the battery connected, just touch and release the connecting wire and you should hear the driver 'click'.

    To put that in perspective, if we assume 1.5v across an 8 ohms speaker equals about 0.3 watts. However, while the battery is connected you have a continuous flow of DC current. The typical DC resistance of an 8 ohms speakers is about 6 ohms, which equals about 0.4 watts. Not really dangerous, but don't take any chances, especially with the tweeter. So, just touch and release, and you should hear something.

    Steve/bluewizard
     
    Last edited: Mar 3, 2013
  7. keicar

    keicar
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    Thanks again for the replies,

    Will resolder the board today, if still unsuccessful post a picture.

    I have read that using two electrolytics back to back (minus to minus) you can use in place of a bi-polar cap? but twice the capacitance and half the voltage.

    ie to replace a 10uf 100v bi-polar the nearest would be 2 x 22uf 50v radials back to back, is this correct, if only for test purposes?

    By substitution, I have confirmed all the drivers are working.
     
  8. D.D.D.

    D.D.D.
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    Just for test purposes it is OK.
    In mids crossover you probably have 2 electrolytics - one is parallel to driver, another is on signal path.

    You can locate problem by shorting electrolytic which is on signal path. If it is this one - when shorted you will hear you mid working. Do not short another one, you can damage your amplifier. If parallel to driver cap is "open" then mid driver will play anyway.

    Just follow from "+" from input to driver and check everything on signal path.

    If you want repair you speaker - it is better to do changes in both crossovers - your speakers must be same.

    Non-polars: take a look on Mundorf: it is not expensive and it is specially for crossovers:
    Mundorf ECap - Fidelity Components Shop

    9uF: you can use 6.8+2.2 in parallel.
    If this capacitor is in signal path: replace it to MCap250:
    Mundorf MCap - Fidelity Components Shop
     
  9. Kei86

    Kei86
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    I wouldn't go shorting anything out in the crossover as you run serious risk of blowing stuff up. (taking them out of the circuit and measuring their value with an LCR meter is the best method) In this case, i'd say you are best off removing and replacing the capacitors as a matter of course as they aren't particularly expensive. I had to do my celestion ditton 25's last year but they are getting on for nearly 40 years old. Replace like for like capacitance where possible and use parallel addition in places where it's not possible to get a direct replacement. (voltage ratings do not matter too much, so long as you match or exceed the existing rating) You may have trouble with space if you replace bipolar electrolytic with plastic film as they are much larger.

    Solen MKP capacitors are a good choice from Falcon acoustics. The advantage of film is that it almost never needs replacing where as electrolytic have a limited lifespan.
     
  10. BlueWizard

    BlueWizard
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    Don't screw around with Back-to-Back Electrolytic when common Audio Crossover Electrolytic are so cheap. Also, you can't really predict the final value of polar electrolytic. One will be properly polarized, meaning current flowing into the (+) lead, and will have its proper value. However, the other will be REVERSE polarity with current flowing into the (-) lead which will effect the value of the capacitor.

    Polar capacitors are typically used to block DC voltages and prevent them from passing from one stage of the amp to the next (coupling capacitors). The values on these are not as critical as Audio Crossover capacitors.

    To get a proper value, it is best to put NON-Polar in parallel. In parallel the capacitance values ADD together.

    For example, if you need 8uF (Micro-Farads), then put a 4.7uF in parallel with a 3.3uF. It is best to keep the capacitors of a similar size; 4.7uF and 3.3uF make more sense than 7uF plus 1uF.

    To achieve 9uF with standard values, you could try 6.8uF plus 2.2uF for precisely 9uF. Or you could try 4.7uF plus 4.7uF for a total of 9.4uF. I have seen 5.6uF, which could be combined with 3.3uF for a total of 8.9uF.

    The actual selection of standard values depends on the rated tolerance of the Capacitor. 10% will have a gap of 10% between values. 5% will have a smaller gap and therefore more values of capacitors to select from. 1% are the most expensive, but give you the greatest selection of values.

    CORRECTION:

    In my previous post, I said Metalized Capacitors were the most expense, they are not. Metalized Polypropylene are very inexpensive and at the same time far more stable that non-polar electrolytic.

    Steve/bluewizard
     

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