MD421 Loss of Low end Fix?

abechap024

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Hi!

So I have an MD421 that has no low end. I hear this is a common failure with these mics (when they fail) I opened it up, and figure it must be a voice coil rub, but anyone have any tips on how to fix it? Returning it to sennhiser isn't an option as It is really beat up, but would be nice to use it on stuff besides hi-hatt :)

Thanks!!
 

abechap024

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Okay, well learning a lot about what to do and what not to do  :eek: If I get it working I will let ya'll know. I see that the "ring" that the voice coil sits in is lop sided just a bit.
 

poctop

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Hi abechap , I have having experience when tthat symptom occur as I. Have tweaked. A lot of akg d12 , basically it is nearly impossible to twweak the diaphrgame on these because it is glued but from my experience when that symptom occur on 421 it is the outer edge of the membrane that is actually ripped of slightly on the outter edge and then the bass response goes away as a part of the output as well. Have a look with a magnifier on the outer edge to find out
Hope this helps
Dan,


 

MicDaddy

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If the coil starts to fall apart, as they are self bonding, it's pretty much toast or new coil time.

Check for ferrous or other debris pinching the diaphragm preventing from full excursion as this will hurt the low end.

As poctop mentioned, any air-leaks around the diaphragm will cause the response to go wonky.

I see that the "ring" that the voice coil sits in is lop sided just a bit.
  Can you snap a macro photo of this ring? 

If the coil has gone oblong, or is pinched in the gap it may be tough to bring back.  Unless you care to completely disassemble the motor without destroying anything, re-seat everything, reassemble and seal it as it is supposed to be.  The biggest obstacle is not destroying the coil/diaphragm, as well as not disturbing any restrictive air paths. 

You can very gently touch the dome in the center of the diaphragm and observe if it moves symmetrically.  Otherwise you can take a very small DC source like 1.5V and 'hit' the coil leads near the diaphragm (be sure the lead that goes to pin2 sees the + of the DC)  this will cause the diaphragm to 'bounce' up.  You can observe if any area of the coil is pinched or inhibited.  You can also do this with a signal generator and apply a veeeery low freq <10Hz and observe the movement of the diaphragm.
 

abechap024

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Cool, thanks for those detailed responses guys. I ended up messing with it some more, but (stupidly) got impatient and broke one of the hair thin wires on the diaphragm. Also sent a 100hz signal to the mic and it was buzzing. So somwthing wasn't right. Anyway, looking now to replace the capsule...maybe turn it into a md421 condesner ::) Anyway looking forward to trying to fix another one sometime.
Thanks,
Abe
 

abechap024

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MicDaddy said:
Did you rip up the diaphragm or is it still intact?  Care to donate the motor to science?  8)

I'm going to figure out some way to mount another capsule in there, If I can do it without using the original motor as support (I'll try) then I'll gladly send it along to you. And ya the original diaphragm is pretty broken..
 
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