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Thread: If a driver reads a good DCR, it is operational?

  1. #1
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    If a driver reads a good DCR, it is operational?

    Hi,

    I went to check some JBL drivers with a meter but didn't have a signal/amp source to drive them to make sound (idiotically forgot). The drivers' history suggests they may have suffered some kind of damage in the past but hard to tell obviously externally. They measured DCR's in the 6.5 to 6.9 range for 8 ohm drivers (I know some JBL drivers which purportedly are 16 ohm also measure DCR this low too).

    For a woofer, free movement and this measurement should mean an operational driver?

    For a mid comp driver or tweeter, same assumption? That would be much harder to tell I suppose.

    I suppose always could have a buckled or misshapened voice coil without it shorting or going open.

    So without a signal or sweep source, what are my chances of the drivers being ok, let alone good. This assumes whatever state they're in now, they're still at least 20 years old and may need diaphragm replacement to be perfect.

    Thanks for the reply.

    Regards,

    Bart

  2. #2
    Administrator Mr. Widget's Avatar
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    With a good DCR reading and a freely moving diaphragm, there is a very good chance the speaker will produce sound. Beyond that??? No guarantee of anything. The suspension could have softened, or stiffened. There could be rubbing that you couldn't feel. The magnet could be weak. These are a few things off the top of my head... ideally you'd fully test the driver. Unfortunately that is not possible for everyone, but a thorough listening test is certainly required before you know if you are dealing with a recone project or a nice piece of used gear.

    Widget

  3. #3
    Senior Member Flodstroem's Avatar
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    Hi mech986

    As Mr Widget pointed out:
    The suspension could have softened, or stiffened. There could be rubbing that you couldn't feel. The magnet could be weak.
    Also there is a possibillity that the speakers voice coil has been burned slightly so there is one or a few turns of voice coil winding shortened (due to rubbing when got hot). This will not show up when measuring with the multimeter, it only show up in the listening mode as low efficiency and /or distorted sound.

    Another possibillity of damage of voice coil, also due to overheating is that the coil, eg. part of the voice coil has loosened from the coil former. This will probably only show up in the listening mode as distortion, espescially at ceartain frequensis.

    Regards
    Flodstroem

  4. #4
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    The drivers' history suggests they may have suffered some kind of damage in the past but hard to tell obviously externally. They measured DCR's in the 6.5 to 6.9 range for 8 ohm drivers (I know some JBL drivers which purportedly are 16 ohm also measure DCR this low too).
    - DCR can tell you lots, unfortunately each JBL model has a different voice coil and therefore a different "Healthy" Range .

    - FYI ; That "dcr" health range for the le10h ( circa 1982 ) was 4.3 to 5.2 ohms . This for a driver that was labelled 8 ohms .

    - The 2420 ( le85 ) "healthy dcr range" was a DCR of 4.8 to 5.8 ohms ( circa 1982 ) .
    This was labelled as a 16 ohm driver .

    - The 2440 or 2441 ( 375 or 376 ) "healthy dcr range" was 6.4 to 7.8 ohms ( circa 1982 ) . Labelled as a 16 ohm driver .

    - If your "find" has older aluminum diaphragms that have been used extensively within the Sound Reinforcement industry, then it's likely they have started to "work harden". ( If one bends aluminum enough, it quickly becomes brittle and then snaps ). Obviously, metal fatigue in a compression drivers' diaphragm will change the response characteristics of the driver . A tone sweep would tell you something about a rubbing coil but it wouldn't really tell you ( the inexperienced listener ) what is mint & "off the shelf" vs fatiqued. It's a dilemma without having experienced this to create ones' own reference.


  5. #5
    Alex Lancaster
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    But if it reads "0" or infinite, You are sure it´s bad.

  6. #6
    Senior Member B&KMan's Avatar
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    Quote Originally Posted by mech986
    Hi,

    I went to check some JBL drivers with a meter but didn't have a signal/amp source to drive them to make sound (idiotically forgot). The drivers' history suggests they may have suffered some kind of damage in the past but hard to tell obviously externally. They measured DCR's in the 6.5 to 6.9 range for 8 ohm drivers (I know some JBL drivers which purportedly are 16 ohm also measure DCR this low too).

    For a woofer, free movement and this measurement should mean an operational driver?

    For a mid comp driver or tweeter, same assumption? That would be much harder to tell I suppose.

    I suppose always could have a buckled or misshapened voice coil without it shorting or going open.

    So without a signal or sweep source, what are my chances of the drivers being ok, let alone good. This assumes whatever state they're in now, they're still at least 20 years old and may need diaphragm replacement to be perfect.

    Thanks for the reply.

    Regards,

    Bart

    Well,

    For add at excellent remarks,

    play a sweep sinewave at relatively loud power and listend any rattle or weird reaction driver. this test is relatively easy with CD test or any generator.

    It is not perfect but is better confidence than play music...

    I have a old mid ( 2121) and I detect default driver only after play a special music ( paris texas where multi wind instrument play a controled " distortion " At this situation, my speaker have realy destroyed response...

    after sweep sinewave the problem is realy easy detect.

    Good Luck

    Jean.

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