Sennheiser MD421-U still possible to repair ?

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Komputnik

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Hi,
although the thread is quite old, hopfully someone will give me a clue how to proceed in fixing the membrane of my newly acquired MD421-N NOS.
Since it has limited bass response, i opened it and found a way too much glued microphone.
Firstly, the cables were glued under the hood, so the length was very limited.
Secondly, the capsule was full of glue which i removed with isopropyl alclohol.
But then i opened the hum cancelling coil (don't use a screw driver, instead use your fingers and like opening a champagner bottle. Push slighty around the ring until it starts to loosen !) , which also was very gluey.
The membrane, IMHO, should be free floating as it's with all other dynami mics. (e.g. D20).
It doens't float free and to the half of the membrane the surface is gluey as well !
I think, that the glue also is under the membrane, but i really doubt , that it can be openend withouht killing it.
It's heavily glued as well !
Perhaps anyone can give me a direction how to get on !
Many thanks in advance
Franz
Attached, you'll find a pic and a direct link of the membrane and capsule housing.

https://dah.am/md421-repair/md421n-capsule.jpg
 

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Komputnik

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Hi,
i now managed to unsolder the noise cancelling coil.
What i forgot to mention is, that the DC resistant measures 199 Ohm.
When i got it, i measured 203 Ohm.
The membrane itself looks good, but the surface is gluey.
Could that be the origin of the lower bass response ?
Anhow, i try to clean the surface and will report back.
Rgds.
Franz
 

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Komputnik

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After softening the glue on the side (using propanol alc.) i loosened the membrane ring using my favorite spudger tool.
Then i removed the glue on the side and lifted the membrane to see inside.  No glue or anything else.
I found a scratch on the top of the membrane, but that only the surface but not going through the material.
I tested it, no hole at all.
I'm a little bit confused now. Since i repaired a lot of D20B and D12, there was always metal residue on the membrane, and after cleaning it worked perfectly.
Anyone a hint ?
Would be great to have been repaired the NOS MD421.
Many thanks in advance
Franz
 

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panman

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I don`t get it! Your pictures do not show any glue residue on the diaphragm. You say the diaphragm is not moving freely. What do you mean? Did you try, if it was moving freely? How did you try it? And you say too much glue ? From the pics I do not see that, but it of course could be as you say, because I can see the diaphragm has been swapped. That diaphragm is from a MD 21. Well, now you have taken the diaphragm loose and need to glue it back and at the same time tune it so that it has the correct sound. That is really not going to work unless you know exactly what and how to do it. You say: "Since i repaired a lot of D20B and D12". Well, if that is so, how come you really don`t seem to have much idea of dynamic mics? Or have I totally misunderstood what you wrote?
You did not mention, if you had checked out the bass roll-off filter before starting to take the capsule apart. It you bypass that, you`ll have the full bass-responce. Anyway, the loss of bass is most likely due to the fact, that the person, who swapped the diaphragm , was not able to tune it properly.
You said: "What i forgot to mention is, that the DC resistant measures 199 Ohm.
When i got it, i measured 203 Ohm." Think again! 203 Ohm is the resistance of the voice-coil and the humbucking-coil together in series.

 

Whoops

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Komputnik said:
The membrane itself looks good, but the surface is gluey.
Could that be the origin of the lower bass response ?

Hi Komputnik,
it's hard for me to see from the photo the "Gluey surface" you talk about.

As I wrote previously lack of bass response is unfortunately a common fault in 421 microphones, but theres a lot of different things that could do that. Hard to tell for me from the pics.

The first thing I always check in this mics is the Low Cut filter, like Panman said.
I always bypass that as I don't ever use it.
Although being honest, just in my personal experience , in all the 421 mics lacking bass that I tried to repair the problem was never from the filter, I was unfortunate because that would be an easy fix.

Also the mic has a bass chamber, and the hole in the body between the capsule chamber and the XLR connector chamber needs to closed. I think stock it has a kind of glue or Silicone in that hole. But I had to check in the mics I have because it's been some time since the last time I was working on those mics.

Panman is really experienced and knowledgeable and repaired a lot of dynamic mics check his advises.


 

Komputnik

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Hi Panman,
thanks for the reply.
Here are my anwsers:
I don`t get it! Your pictures do not show any glue residue on the diaphragm.
On one of my first pics of the membrane, you could see the shiny surface on the left bottom end of the membrane.
As i stated, by touching the membrane gently with the finger, i was able to detect the glue on the membrane.
Sennheiser seemed to have used spray glue, and the worker forgot to cover the membrane correctly.

You say the diaphragm is not moving freely. What do you mean? Did you try, if it was moving freely? How did you try it?
To test, that the membrane can move freely (up and down), press gently using a finger.
The membrane must feed back a little and should not get stuck in the gap between the coil, which it does before opening.
In fact, the membrane stuck in the coil gap and i removed glue too.
That diaphragm is from a MD 21. Well, now you have taken the diaphragm loose and need to glue it back and at the same time tune it so that it has the correct sound. That is really not going to work unless you know exactly what and how to do it. You say: "Since i repaired a lot of D20B and D12". Well, if that is so, how come you really don`t seem to have much idea of dynamic mics? Or have I totally misunderstood what you wrote?
I cannot see a similarity of the membrane of a MD21 -> MD421. Any picture i can find on the net about the MD21 shows a very different casule type comparing to the MD421.
Otherwise i followed the thread of user "Knuddi", his capsule (MD421-U) is very much the same as mine.
My MD421-N is NOS and  had never been opened until now though.
The AKGs, which i repaired (we have 4 D12 and 4 DB20b) suffered from debris coming off the magnet and therefore prevented the membrane from moving freely. I'm aware of tuning the membrane when assembling the mic.
Since the membrane of a D20B is screwed to the surface, of course the fine tuning is much easier to do.

You did not mention, if you had checked out the bass roll-off filter before starting to take the capsule apart. It you bypass that, you`ll have the full bass-responce. Anyway, the loss of bass is most likely due to the fact, that the person, who swapped the diaphragm , was not able to tune it properly.
You said: "What i forgot to mention is, that the DC resistant measures 199 Ohm.
When i got it, i measured 203 Ohm." Think again! 203 Ohm is the resistance of the voice-coil and the humbucking-coil together in series.
Hi, sorry, i forgot to mention that the brilliance switch didn't work and i wanted to mod it to XLR.
i have done this many times using the Sennheiser Option available from Thomann.
I measured the DC resistance w/o the hum cancelling coil, my fault, sorry.

Attached you'll find the gluey surface, which i'm ab about to remove right now.
Many thank for the hints.
Rgds.
Franz
 

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panman

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Komputnik said:
The AKGs, which i repaired (we have 4 D12 and 4 DB20b) suffered from debris coming off the magnet and therefore prevented the membrane from moving freely.
Attached you'll find the gluey surface, which i'm ab about to remove right now.

I do not see how 4 D12 and 4 D20 repaired is "alot".  Ok, a lot for your taste, but not mine. Besides, calling them *repaired", because just removing the debris and perhaps tuning the diaphragm is really overdone, but I have written enough of these things elsewhere and don`t want to go into that now. Nevertheless, it may be possible to get a usable enough D12/D20 just cleaning and tuning, but up to specs? Sorry! Mostly No.

I can see that glue now from the pic.  However I don`t believe the mic was never opened. I do not see how Sennheiser would ever have done such a lousy job with glue.  It is good, if you can remove it with isopropanol, but it may not be necessary after all, if the diaphragm moves correctly. Vinegar may work better than isopropanol depending on the glue. Tuning is more important and that it has no dents or bad scratches. You should always have a properly working one to compare, if you got the tuning right.  Once you`ve got the full bass, the rest is usually correct too. Use slowly drying glue. As for the diaphragm, both MD 21 and MD 421 have the same . That is technically the same, but for some reason the MD 21 from a certain period had a greenish dome and MD 421 has always had it transparent.  And I have seen a lot believe me or not. That`s why I think it was swapped. And " the cables were glued under the hood, so the length was very limited" does not fit to my idea of MD 421 and I don`t really understand what you mean. Stupid to say, but I could be wrong too. These mics were produced many decades after all. The very old MD 21:s had aluminium diaphragms and differend capsules alltogether. Not really sounding differend though?!
Just go ahead! I do not wish to discourage you, but on the contrary, I wish you good luck! And keep us posted!

 

Komputnik

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Hi Panman,
many thanks for the valuable  info you spent.
So, i'll contact the seller and will question him about his statment that the mic is NOS.
Best regards
Franz
P.S.: will keep on posting updates !
 

Komputnik

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Hi again,
i now managed to get a pile of MD421 spares. There's a broken capsule but the membrane was pristine.
So i loosened the membrane by using a daper feeler gauge (0.3mm) which didn't harm the membrane.
After measuring the winding impeance (208 ohm) i glued it on the workling capsule (using UHU max repair transparent. Stays flexible and worked out well on other mics), soldered the wires on the  humbucking coil. All worked out perfectly.
The sound is amazing.
Thanks for all the valueable input.
 

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Whoops

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Komputnik said:
Hi again,
i now managed to get a pile of MD421 spares. There's a broken capsule but the membrane was pristine.
So i loosened the membrane by using a daper feeler gauge (0.3mm) which didn't harm the membrane.
After measuring the winding impeance (208 ohm) i glued it on the workling capsule (using UHU max repair transparent. Stays flexible and worked out well on other mics), soldered the wires on the  humbucking coil. All worked out perfectly.
The sound is amazing.
Thanks for all the valueable input.

Congrats Komputnik,
I'm really happy for you. Thank you so much for sharing your fix.

I was never as triumphant as you, as I always broke the thin coil wires when I tried to solder them

Please keep sharing your repairs

Thank you so much

 

niva

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Hi there!

Reviving this old thread.

I recently got 3 x MD421 that has the same problem as discussed here: low bass response due to the diaphragm/red ring being partially unglued from the base.

I succeed removing the black protective cover of the 1st "aspirant" as it was already broken... Will buy some UHU Max repair glue as I will use it for SM57 diaphragm having the same problem. Will apply lightly with a toothpick only to the area where it is unglued. Notice that the red ring + diaphragm are unglued next to the solder tabs.

Anyone can send me infos/pics on how to detach the protective black cover from the base to get full access to the diapghragm? I fear ripping the diaphragm with this manoeuvre. Thinking about dissolving a bit of the glue from the black cover with isopropyl to help removing it, but I don't want it to unglue further the diaphragm from the base... Is it a good idea?

Thanks for your help!
 

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panman

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Sorry to say, but there is no way to fix that diaphragm. The diaphragm has to be totally faultless and you will not be able to glue that back and get the perfect form back. It needs to be replaced with a proper diaphragm/voice-coil and that`s the work of an expert, if you want it to function properly. However, if you want to go through the DIY-experience, you yet have two more and can see what you find under the hood there. First solder off the humbucking-coil wires from the lugs. Then carefully prize up the "hood"little by little. I use a small Swiss-Army knife to do that at first(goes into the gap) and go on with a screw-driver. Usually not a big deal, but sometimes there was too much glue used and then it just was getting into wrong places making it impossible to remove the hood without damaging the diaphragm, but it is rare. Good luck!
 

niva

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OK thanks Panman! You seem to be THE reference for that type of work! ;)

That one is the one that has the worst condition. That's why I began with him.

I think one has "only" a dirty diaphragm.

Is it OK to clean it carefully with 99% isopropyl and a Q-tip? Pure or diluted into distilled water?

One last question: what type of glue do you use to reglue the diaphragm?

Thanks for your help!

Guillaume
 

panman

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Isopropyl and Q-tips are not needed or doing any good with MD421s, unless there really is some dirt under the "hood". I cannot imagine what that could be, but there may be iron-dust or iron-particles around the dome jamming it and they need to be removed one by one picking them(very carefully to not let the magnet pull the the tool and denting or punching through the diaphragm) with a tiny-tip magnetic screwdriver. But let us first see what is under the hoods!
To reglue the diaphragm you can use slow-drying glue, because you need to "tune" the diaphragm before it fixes. UHU max would do just fine.
 

Whoops

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Anytime you want to test a xfmr, particularly line input and output types. The only ones you don't need above 50k are the high-Z mic input types.
Also many power amps have a very high -3dB point.
For a 1st-order (one dominant pole) circuit, for achieveing -0.1dB at 20kHz, the -3dB BW must be 130kHz.

OK thanks Panman! You seem to be THE reference for that type of work! ;)

I can vouch for that Esa does and amazing work on dynamic microphones, it’s an art that probably there’s no one in the world that has his level of expertise.

here is my D12 fixed by Esa which is getting much praise:
 

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niva

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I finally removed the hood of my two other "aspirants".

First one is "kaput" i guess (first two pics).

But I guess there is some hope for the second one (third and fourth pics).

Then, what would be the safest tool (screwdriver) to remove the iron-dust particles a very small sharp or flat one?

I have a vast array of tinny flat head screwdriver to choose from.

Thanks!
 

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Whoops

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The first one you could probably fix it by replacing the complete Diaphgram with a donor from a cheaper mic that has the same size
 

panman

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Use a very small flat-tip one. Do not let the screwdriver slip, because the magnet is going to pull it and destroy the diaphragm. Support your hand firmly while picking the iron-particles away! After removing the debris, the diaphragm should move freely up and down. Check if there is any coil-rub! If there is any, the mic will not work properly.
 
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niva

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Ok!

I cleaned up the diaphragm of the first one. It was pretty much straightforward with the right tools.

I managed to clean it with a small flat screwdriver, and a very soft paint brush.

But still, there is some residue, kind of a glue all around the capsule and there is a debris close to the voice coil wire (see the photo), and a red dot(?)


I tested the capsule (without the humbucking coil) straight to a preamp and the sound is low and not very detailed. Is it normal? Would it sound as it should only when reassembled (with humbucking coil, body, grills. etc)?

The voice coil still measures 208 ohms and moves freely, at least form my perception.
 

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