AKG C12 / Siemens SM204 help needed

Well spotted Rossi! I too assumed  it was uf too! Like you said that chainges things a bit!  ;D

In my Ck12 experiments
I have found that the shallow capsules can still be setup to sound as bright or dark as the deeper ck12, the difference is more the the shallower version is a bit more mid forward or less scooped depending on how  you  look at it.

What about (membrane) tension?

The diaphragm tension on a Ck12 is a critical factor too.

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The size of the internal chaimber doesn't  affect  what diaphragm  tensions can be used it's just  small mechanical difference that happens to  change  the mids and the output.

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So here are the promised pictures of the 3rd SM204. This is a later one (Nr.280). I was asked to check the capsule´s health and overall originality. As far as I can see in the amp - all original. Interesting though: the Siemens MP is dated to mid 1954, wereas the one in SM204 Nr.25 is dated 1955.


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Here is the caspule. It seems here are some CK12 experts present. This may be interesting for you.

The backplates are very early ones. But the diaphragm rings are from the late 50´s. That´s a bit unusual!
Diaphragms are the 9µm that were used until early 1960´s. Although I didn´t measure them that was obvious. Those have a surface like no other film used in "Brass" CK12.  But moreover the sound is different from any of the various 6µm films that were used later. It has a certain richness to it and more velvety top end.

Luckily diaphragm tension was prefectly fine. Sound is very balanced and smooth. Pretty linear as it should be for this version.

However this capsule´s backside has a massive structural problem! It´s visible already in the middle pic.


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Finally this should ring alarm bells! Can you spot it?


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MS Vienna said:

Finally this should ring alarm bells! Can you spot it?

Click to expand...

Sorry, but i don't see that... Don't know what should i look for

Here's a problem

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MSVienna is referring to the fact that the acrylic is cracked in multiple places. It's very likely that the capsule was exposed to alcohol. Enjoy the capsule as long as it keeps working. There are people around who can restore this.

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I have and thinner also. They have their virtues but there is a reason most manufacturers settled on 6 micron.
That topic has been discussed to death.
One of the greatest achievements of AKG which is being overlooked here isn't the small details of construction but the fact that we are discussing a working 60 year old microphone. It's easy to make small numbers of capsules but this capsule is fraught with problems and manufacturing large numbers compounds that.
We are way off topic in helping the OP repair his mic.

Tim Campbell said:

MSVienna is referring to the fact that the acrylic is cracked in multiple places. It's very likely that the capsule was exposed to alcohol.

Click to expand...

Exactly!

Tim Campbell said:

Enjoy the capsule as long as it keeps working. There are people around who can restore this.

Click to expand...

That´s what I told him.

I offered the possibility to make a precise duplicate of the damaged backplate, except that I would use the improved all-screw-joint construction as used from the mid 60´s that offers inherently stressfree joints and long life.  A somewhat expensive restoration though but you won´t see or hear  a difference to the old backplate and retain the high value of the mic.

BTW all CK12 backplates were exposed to alcohol! Alcohol was used to clean the components before assembly (even the backsides of diaphragms!).
Of course the material tensions introduced by machining were taken care of in the manufacturing process (this one might have slipped through...). Remember this was one of the most expensive microphones at the time and made to the very strict quality requirements of the ORF. With parts that need to be precise in the micron range you certainly don´t want stress in the material that unloads itself unpredictably. Even after 60 years of unknown usage/storage conditions those backplates are usually still quite acceptable.

These backplates use the earlier press-fit construction, a bit problematic tension-wise but quicklier to manufacture. However this backplate shows exeptional damage. It´s likely to disintegrate once the caspule is disassembled. Obviously a QC issue.

If you´re checking a CK12 for health look out for such cracks. The capsule still might sound well but if cracks are contaminated with debris this can cause insulation problems that are beyond cleaning. 



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Tim Campbell said:

I have and thinner also. They have their virtues but there is a reason most manufacturers settled on 6 micron.
That topic has been discussed to death.
One of the greatest achievements of AKG which is being overlooked here isn't the small details of construction but the fact that we are discussing a working 60 year old microphone. It's easy to make small numbers of capsules but the this capsule is fraught with problems and manufacturing large numbers compounds that.
We are way off topic in helping the OP repair his mic.

Click to expand...

I know I'm drifting, but it seems I'm not the only one discussing capsules and such. 
The OP never mentioned the capsule in his (unless I missed something; it happens).
He did like the overall conversation, from what I understand.
And he said to report back when he was done.
But in hindsight, yes, perhaps I should have started a separate thread after all.

Anyway, carry on.

IMHO talking about capsule acoustics is a bit like dancing about architecture especially in a very complex design like a CK12.
In the end it´s all about long practical experience. If you don´t know by experience how each of the (many!) parameters affects sound and interact with each other - by actually having heard that all it in person - you can only speculate with quite mild success. E.g. there are alone 3different kinds of damping involved (friction-, elastic-, and mass damping) that make the system work and there are many aspects and little details that are  easily overseen.  In an attempt to just talk about it you could go on and on and on...

Plus there is so much technical data in the microphone meta that answers most, if not all capsule questions. I think Jakob or I posted things  there like the AIP Microphone Handbook 12 or 15 years ago which goes a long way to explain the mechanisms at work.