Troubleshooting a dynamic microphone

Discussion in 'The Projects Forum' started by roty34, Dec 30, 2011.

  1. roty34

    Thread Starter New Member

    Dec 30, 2011
    3
    0
    Hi guys!

    My first post here so my problem will probably sound very basic but I'm just starting out: I got a Shure 55SH Series II dynamic microphone that generates no output.

    Here are a few pics of the open mic:

    CAPSULE

    [​IMG]

    SWITCH

    [​IMG]

    XLR PLUG


    [​IMG]

    FWIW, a schematic is available from Shure's website here: http://www.shure.com/idc/groups/tech_pubs/@global_managed/documents/webcontent/us_pro_55sh_series2_ug.pdf

    A few questions: what does each of these wires coming out of the capsule do? The blue wire goes to pin 1 of the XLR plug, the black to 2 and the yellow to 3. Black and yellow also pass through the switch. Am I right to assume these last two are positive and negative and the blue some sort of ground wire?

    So far I have tested continuity and resistance for each path. The weird thing is if I put one prob on the black wire next to the capsule and the other probe on pin 3 of the XLR plug (yellow wire), there is continuity. Is this normal or does it mean there's a point of contact between black and yellow wire somewhere in the circuit?

    Finally, how would I test whether the capsule itself is working or not? Would I want to speak into it and measure if it generates any current? If so, how would I do this with the multimeter?

    Thanks in advance!
     
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    Here is a link to a datasheet for the microphone: a wi=ring diagram is shown on page 10. http://www.shure.com/idc/groups/tec...cuments/webcontent/us_pro_55sh_series2_ug.pdf

    This is a balanced output microphone, the blue terminal is the centre tap. There is a shorting switch across the two ends of the capsule (black to yellow): the capsule is shorted to turn it off.

    Ensure that the short is not present: with an ohm meter, you should be able to measure a lowish resistance between black and yellow with the switch in the "on" position, maybe not exactly the 270 ohm AC impedance. With the switch in the "off" position the resistance should be very low. If the resistance is very low in both switch positions, you might disconnect one of the wires at the switch in case it has failed.

    The output from this type of microphone will be much too low to be measured with an ordinary multimeter. You must use a reasonably sensitive amplifier to test it. Possibly you might get a weak response from the microphone input of a PC, but the level may not be sufficient.
     
  3. roty34

    Thread Starter New Member

    Dec 30, 2011
    3
    0
    Adjuster, thank you for taking the time to help out!

    When the switch is in the "on" position, I get nothing. In the "off" position however, I get an unstable reading from about nothing to 150Ω. Should this indicate the shorting switch may be the problem?

    I need to get some desoldering braid and I'll disconnect one of the wires to bypass the switch as you suggested and report back.

    I have a usb recording interface, with two preamps (specs here: http://www.focusrite.com/products/audio_interfaces/scarlett_2i2/specifications/). Is there any way I could use it for this purpose?
     
    Last edited: Dec 30, 2011
  4. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    There may also be a short elsewhere, especially in the microphone cable, if any, and especially at or near plugs or sockets.
    Flexible cables are notorious for failing, more usually open but shorts can happen too.
     
  5. roty34

    Thread Starter New Member

    Dec 30, 2011
    3
    0
    I checked the microphone cable I used for testing with another microphone and it performed just fine.

    However, after reading your last post, I took a closer look with a magnifier and one of the wires inside this "broken" dynamic microphone seemed to have a cut in the plastic casing (insulation) showing bare metal. Another possible problematic area...

    Thanks again, this helps a lot!
     
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