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Help me repair my latest acquisitions from the junk bin!

Discussion in 'Microphones' started by tertiumsquid, Jun 13, 2015.

  1. tertiumsquid

    tertiumsquid Active Member

    Hi all, first post here, though I've lurked for quite a while now. I recently saved a handful of mics from the recycling bin, and need a wee bit of assistance with some of them.

    Before I begin, I should mention that I am a semi-professional electronics repair tech and I do a lot of work on amps, synths, and recording gear, but not a lot of microphones.

    First mic: Sennheiser MD421.
    md421 2.JPG md421 3.JPG md421 4.JPG md421 5.JPG md421 1.JPG

    I've already flattened out as many of the dents in the grille as I can. The mic seems to work fine - I have an XLR cable with alligator clips on one end, which gave me some good signal out of the connector at the bottom of the mic, but once I try and connect to the XLR connector (which I'm certain is not from the same model, as there were a few other smashed 421's in the bin) it buzzes and sounds terrible. The pins at the bottom of the mic correspond to XLR pinout, but the adapter is wired differently. - ground to pin 3.

    So I figured I'd re-wire the adapter. Easier said than done. Can't get the damn thing open! I managed to pop out the snap ring holding the XLR in, but there wasn't enough wire to pop it out. And I can't manage to unscrew the black metal ring from the rest (I assume the two slots are for unscrewing it...).

    Any suggestions?
  2. tertiumsquid

    tertiumsquid Active Member

    Second mic: Sennheiser ME64/K6P shotgun.

    k6 1.JPG k6 2.JPG

    Plugging in either the K6P power module alone or with the mic gave me only some clicking and popping. I managed to make the thing work properly already - there was a broken trace on the flimsy PCB-ribbon-whatever where the XLR solders in. The problem is that I don't have the metal tube that covers everything. If anyone has any suggestions where to get one, let me know!
  3. tertiumsquid

    tertiumsquid Active Member

    Third mic: Electrovoice RE20.

    re20 1.JPG re20 2.JPG

    Missing the XLR/mount from the bottom, the HPF switch, and all the foam from the inside. Second picture shows that the wire going up to the diaphragm is torn right next to the binding post. Doubt I'll be able to get this one working, but if you have a suggestion, please post!
  4. tertiumsquid

    tertiumsquid Active Member

    Fourth mic: RCA type 77-D.

    rca.JPG rca 2.JPG

    Sure looks beat up. The wiring at the bottom is pretty chewed up, but should be workable.

    I opened up the top of the mic and shone a flashlight through, and it looks like the ribbon is torn and dangling. It's unlikely I'll try and fix that myself.
  5. Kurt Foster

    Kurt Foster Distinguished Member

    send the 77 to Wes Dooley. he has original ribbon material. Contact ben@micdaddy.com for the RE 20. here's a link to sennheiser repair . they have repair information sheets available.
    DonnyThompson and kmetal like this.
  6. dvdhawk

    dvdhawk Well-Known Member

    << Cue the Sara McLachlan music >> won't you give these poor abused microphones a good home?

    A suggestion on the 421: The shield should end up on Pin #1, so rather than try to rework the mic - why not build a special mic cable with #1 and #3 reversed on the female XLR end? ( if you feel #2 is correctly wired to the hot ). In any case, it's got to be easier to re-work the cable rather than fighting with the mic.
    gitlvr likes this.
  7. tertiumsquid

    tertiumsquid Active Member

    Thanks for the contacts, I'll save them for later - I'm a broke student at the moment. Same with the Sennheisers, I'm trying to fix them on the cheap, hence why I'm trying to crack open the 421's connector. I'll try and contact Senny for the tube on the shotgun, but I don't think it's likely they'll sell me that part directly. I wonder if I can contact

    Hah, I do hope to be a great home for these poor abused mics!

    On the 421: I had that thought, but it's a transformer coupled mic, and the transformer inputs would still be going to pin 2 and ground on the capsule end. I'm also not certain whether the HPF connects before or after the transformer - if before, it might not behave properly.
  8. DonnyThompson

    DonnyThompson Distinguished Member

    Do you know which 421 you have? ( there are several different models of the 421, each with different signal and ground pins...)
  9. tertiumsquid

    tertiumsquid Active Member

    I'm not certain, as I have zero experience with the 421's, but I'm pretty sure it's a 421-2. I can't find any markings on it, inside or out, so I'm not certain if the guy who gave it to me peeled the stickers off, or even if it's a real Sennheiser.
  10. DonnyThompson

    DonnyThompson Distinguished Member

  11. dvdhawk

    dvdhawk Well-Known Member

    You can get a good signal with the alligator clip cable, just replicate that pinout.

    If you have an oscilloscope on your bench you should be able to determine which pin produces the positive impulse from the transformer.
    DonnyThompson likes this.
  12. tertiumsquid

    tertiumsquid Active Member

    The transformer is actually inside the XLR plug section (not sure what to call it), right above the HPF ring. I can create any adapter I want from the XLR end, but the mic isn't feeding into the transformer properly. I have to fix it before it gets to the XLR.

    And I do have a scope here. I'll test polarity later.

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