Beyer MC734 capsule repair & tuning / grille modification

Hi everybody,

my Beyer mic had a problem: low output along with very strange sound.
So I took it apart and found quite a mess. The MC734 is a stage vocal mic (condenser). The capsule was just wrapped in foam and stuck into the grillecap. By now this foam wrapping has deteriorated pretty generously. So I carefully disassembled the capsule to check contamination. What I found was a surprisingly clean, yet broken SDC element.

Beyer uses an interesting way to clamp diaphragms in place, similar to a drum head. On my capsule the ring that clamps down the diaphragm was broken probably due to material shrinkage.


So I made a new ring. I used Polycarbonate since the material properties are pretty much perfect for this application: flexible, very hard to break, suited to precision machining and very heat resistant (important for the stabilisation process of the newly tensioned diaphragm).



I machined the ring´s inner diameter 0,01mm smaller than the brass rim it´s going to sit on. The diaphragm  is 3µm thick (Polyester), so the ring is 16µm too small for a straight fit.



Polycarbonate is quite tough stuff so a stretch by this amount is enough to hold the delicate diaphragm securely in place.
Needless to say that I did need more than one try to get the diaphragm´s tension nice and even to a preliminary tension. After the stabilisation process the diaphragm is now settled to it´s final tension (and it´s gonna hold it for a long time). I´m sure I didn´t hit the original value but it´s in the ballpark and near enough (speaking from the resulting sound of the capsule).





BTW: The nice thing about this construction is thatI could re-use the original diaphragm - so this repair is quite close to a restoration
Looking through the clear new plastic ring you can see that this diaphragm  originally must have been trimmed before tensioning (really interesting IMHO).

Here are all the components of the MC734 capsule




The next step ist to replace the deteriorated foam insert with a fine wiremesh layer and to make a capsule mount.

To be continued...
 

Great work!
Waiting for next steps

Great. Thanks for the pics.

I´m not going to wrap the capsule in foam again so I chose to add a little wind protection by using an extra layer of wire mesh.

The original mesh is very open.


I wanted the extra layer on the frontside to have a little distance to the existing grille, similar to a Neumann KMS140. To lock it in place I found a brass ring in a scrap box that was a perfect fit in diameter.


I machined away most of the material to avoid unwanted acoustical effects. Then glued on the mesh.

 
For the sides I first laid out the needed pattern on paper.


Inner mesh inserted. Note the gap between the side mesh on the front half and the brass ring. That will hold a thin piece of open cell foam I removed from inside an AKG C4000B.

I additionally sandwiched a very fine fabric weave that I once found at a wedding (table decoration stuff). Not sure if it does anything though.

A bit better now

Nice!

So for the new capsule mount...
I´m struggling a bit on this one and haven´t had an idea yet that really satisfies me.
I looked for some materials I could use for a temporary solution and found this:


Looks weird, is weird. That´s the bottom and the lid of a coffee box and some rubber cord. At least three different materials with different sound transmission properties.

I didn´t use a Swiss Army knife but could not help to feel like Mac Gyver.


It works somehow but isolation isn´t great. I will try a different rubber mixture if I find one.

So how does that thing actually sound? Without some capsule modification pretty unusable IMHO.

Since the capsule was designed to sound good when wrapped in thick foam it doesn´t sound as nice without the foam - not a big surprise.  A very scooped sound (but then I owned a MC740 that sounded scooped too).
Here´s a sample:
https://www.dropbox.com/s/lzqppnhl7se3lh5/734%20stock%20voicing.WAV?dl=0

Here´s how a Schoeps MK4+CMC6 sounds in comparison. I always like to use a Schoeps as reference for excellent, well balanced response (very linear):
https://www.dropbox.com/s/ij5dujeddkbtlju/Schoeps%20CMC6-MK4.WAV?dl=0


Clearly something had to be done about the Beyer´s capsule sound with the new grille situation!
So I modified the capsule´s acoustics to archieve a more suitable voicing. I never did that to a german capsule. I admit the construction of this one is very clever indeed!
Here´s the result:
https://www.dropbox.com/s/uqtrk9trxwjjtia/New%20capsule%20voicing%20MC734.WAV?dl=0

I want this mic for open air use too so I went for a voicing that still sounds good when a Rycote fur Windjammer is added. That´s the sound with the Rycote:
https://www.dropbox.com/s/t6wg69vmb8khua3/New%20capsule%20voicing%20MC734%20%2B%20Rycote%20Windjammer.WAV?dl=0


I´ve done some more reference samples with other proven standard mics, just to avoid fooling myself...
Neumann KMS105:
https://www.dropbox.com/s/wb7e3xyl5ujpd7v/Neumann%20KMS105.WAV?dl=0
AKG C451EB/CK1:
https://www.dropbox.com/s/ma09cypyw1939zd/AKG%20C451EB.WAV?dl=0


I am happy with it.  It feels right and pleasant in the cans while playing. That indicates to me that the transients are processed in the right way.
For the ones who cared to listen: Sorry for the boring, expressionless playing. I just use this little strumming for testing capsules/mics since many years and just don´t care for an exciting performance...

Just realized that the Rycote thing might be a bit vague. So here´s a picture.


Basically it´s a hairy sock. As you can hear in the sound file above it does change the mic´s sound but not in a wild way.

I have used it for years when playing open air gigs with a Neumann KMS105 to pick up my acoustic guitar. Works great. No need to worry about wind noise. When in use on a vocal mic it might feel a bit funny though.

Somehow reminds me of the Muppet Show.  ;D

On Saturday I did a gig and tested the mic live. I´m used to Neumann (indoors usually a KM185) but I´m really sold to this Beyer now!
I used it with the 200Hz rolloff engaged. I normally tend to dislike the sound of onboard filters on mics but this one does not produce weird artefacts. It provides a very effective and natural sounding compensation of proximity effect. But above all no EQ was necessary on the mixer! It just gave a very natural and immediate sounding representation of the instrument (btw a better guitar than I used for the sample files). Never enjoyed my live sound as much before! 
As a side note: we played at a historical building here in Vienna. A carved stone tablet on the wall stated that Mozart, Beethoven and Schubert personally performed at the venue some centuries ago...made me feel humble!

Attached is the data sheet and schematic of the mic.

I think those old Beyer mics are quite underrated. They can usually be had for a bargain today. I really can recommed the MC 734. Just make sure that it sounds right and the foam insert is intact! (you probably don´t want to do a repair like this)