sennheiser low end loss (voice coil rub?)

Seditionary

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hey -- we have a vintage 421 that has no low end.  :-\  it just went away.

i read that this could be from voice coil rub on the capsule.  i know sennheiser doesn't fix these and they don't sell the old capsules any more.  do any of you guys know how to operate on a capsule that has voice coil rub?  or is it foolish to mess with it?

thanks :)
 

martthie_08

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me too :)

seriously, I have script logo 421 with no low end collecting dust, if anyone has some advice on this please do chime in!

it was definately one of my favourite mics back when it was still working...

cheers, Marten
 

pasarski

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My understanding is that these things die, and symptom of dying is low end loss, no cure. Or maybe there is?
 

Seditionary

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yeah -- checked the bass roll off switch first (which is working).

the low end loss is coming from the capsule... i read that this problem comes from voice coil rub.  if there is anyone out there who knows how to repair the capsule on dynamic microphones, please chime in !

unfortunately, i heard it is very difficult... but if we can build op amps on this forum, we can fix voice coil rub... right?  ???
 

pucho812

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does it rattle? I had a new one on the bench recently that got hit so hard to broke a capacitor that was soldered inside, part of the 5 position switch. nyway was a real pain to solder to get that cap back in but I did it.
 

Seditionary

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no, it doesn't rattle.  it wasn't dropped or anything.  maybe the capsule just got some loud spl though.  we had it on a snare drum... the drummer was playing loud, but he didn't hit it or anything...

the electronics inside, i believe are fine... i think it must have to do with the capsule.  here is a picture with the outer screen off.  i guess i need to get inside of that mesh to investigate?  i've never worked on a mic before... especially not a capsule, so i'm wandering in the dark.  thanks for maybe bringing some light :) 

P1010290.jpg

P1010289.jpg

P1010291.jpg
 

Seditionary

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i had read that this is the humbucking coil (pictured above) and not the voice coil

i read the d12 link that sredna posted... it says you can pick the capsule up a bit to maybe fix the problem (for the d112)?  but i heard the voice coil is glued down in the 421... so am i out of luck? 

would i have to unsolder the tiny wires (from the humbucking coil?) to get to the voice coil and see if i could move it? 

thanks for helping with my first attempt at mic repair. 
 

Sredna

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Ok, now I get the BIG picture!  ;)

It's defenitely very tight in there and if you think that the wires that you see (humbucking coil)
are thin, wait til you see the voice coil!

There is a good chance that you'll damage the capsule but if you are willing to take the risk you could try following:

Carefully cut off the protecting "tissue", now you can see the diaphragm.
See if you can find any spots around the rim where the diafraghm got loose.

You can try to use the method in the link I gave you earlier, connect your tonegenerator to the two outer terminals on the capsule. With a softer tool, try to move the rim just a little bit.

As said: this would be a last resort if nothing else works and if it really is the voice coil.

Good luck!
 

Michael Tibes

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I have some old MD21 which show the same behaviour. I concluded that the membrame material might have lost it's flexibility over the years, but I never verified the theory.

Michael
 

Seditionary

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thanks for your time, sredna! ! 

Sredna said:
Ok, now I get the BIG picture!  ;)

ha ha -- sorry for such large photos!  :D

Sredna said:
Carefully cut off the protecting "tissue", now you can see the diaphragm.
See if you can find any spots around the rim where the diafraghm got loose.

when you say, the protecting tissue... do you mean the tissue all the way around the mic, which is pictured (with the frayed edges)?  or do you mean just the protecting tissue right in between the plastic of the humbucking coil?

Sredna said:
You can try to use the method in the link I gave you earlier, connect your tonegenerator to the two outer terminals on the capsule. With a softer tool, try to move the rim just a little bit.]

by the two outer terminals on the capsule, you mean where the blue and orange wires are attached the the humbucking coil?  or will i see more terminals when i remove the tissue? 

for a softer tool -- would a toothpick work well?

Sredna said:
As said: this would be a last resort if nothing else works and if it really is the voice coil.

i don't know what else the problem could be.  i know the problem isn't in the m/s switch at the bottom and i'm pretty sure it isn't anything on the circuit board. 

thanks again for your time, sredna!
 

squib

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check the surface of the diaphram that no metallic dust has been attracted and held there. I've seen this a couple of times on dynamics. The dust like metallic particles pin the diaphram down against the magnet causing loss of low freq response.
 

Sredna

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when you say, the protecting tissue... do you mean the tissue all the way around the mic, which is pictured (with the frayed edges)?  or do you mean just the protecting tissue right in between the plastic of the humbucking coil?

Oh sorry, I was not clear there! I meant the tissue all the way around with the frayed edges.
Underneath you'll see the diaphragm and you could access the rim.

you mean where the blue and orange wires are attached the the humbucking coil?

I meant the the two outer terminals of the three that you can see on your pictures.
One have the thicker red wire and the other have a thin copper wire soldered to it,
dont use the middle (blue) terminal (solder lug)

I used the back end of a toothbrush wich had a kind of "rubbery" handle.
Dont use metal and a tooth pick might be to weak...

Squibs idea about checking for metallic dust is good, I've often found that in my MD421s

Best,

Anders
 

pucho812

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wow those pictures are huge :eek:, check and make sure all the solder joints are in working order. does the 5 position switch effect the sound at all, you should have better low end response when in M mode and the start of the rolloff the moment you move it out of m and all the way down to S. M for music, S for speech and a few position inbetween.
 

Whoops

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This Issue is frequent on a number of Dynamic mics over the years, and its a well know problem in the Sen 421.

Sennheiser doesn't fix the capsule, but I'm sure that someone could fix it, if the capsule were built they can be fixed.

I have two Sen 421 microphones with the same issue
 

DerEber

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Ok I have one with the low end problem.

I opened it up and removed the humbucking coil and the black screen so I can inspect the capsule.
I found with an mignification galss one very small crack in the diaphragm it is on the side near the rim.
So what now???  :'(
Anyone is simply gluing little cracks? It should be not too nice to add some more wight to the diaphragm.
But it is on the very edge. So maybe it doesn't harm too much.

Too bad Sennheiser is not making spare parts for this anymore.

best,
Stephan
 

Whoops

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DerEber said:
Ok I have one with the low end problem.

I opened it up and removed the humbucking coil and the black screen so I can inspect the capsule.
I found with an mignification galss one very small crack in the diaphragm it is on the side near the rim.
So what now???  :'(
Anyone is simply gluing little cracks? It should be not too nice to add some more wight to the diaphragm.
But it is on the very edge. So maybe it doesn't harm too much.

Too bad Sennheiser is not making spare parts for this anymore.

best,
Stephan

I think you can buy a capsule for the 421 from Sennheiser But the cost is almost as much as buying a new microphone.

I have two 421 with this problems, also some AKG old Mics like the D12 with the same thing.

It's probably a really hard thing to fix, but if someone could do it it would be amazing tall the great microphones that could be brought to life again.
 

Seditionary

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Hey guys -- just finally getting to this project a year later...  I have determined it is voice coil rub as I sent it into Sennheiser who gave it that diagnoses and said it was not serviceable by them... so have to do it DIY with the help of you microphone gurus :)

Ok -- I took off the humbucking coil.  There is a photo below.

The diapghram looks ok.  It seems to be sealed against the kind of pink outer piece that you see pictured (which was glued to the humbucking coil).  How exactly do you lift it up?  There is no way I am getting a big rubber part of a toothbrush in there.  Should I try to unglue the pink part on the outside? 

Thank you all so much!

 
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