AKG C414EB P48 repair and restoration little help needed!!

[ricardo]

I think there are only couple of people in the world that can do it properly.

Do you think that they possesed their magical abilities from their gods?

No.  But this is what they (should) do.

• First, diaphragm pairs are matched for resonance.
• They are tried on the metalwork.  Front & back capacitance is checked & adjusted with shims.
• Response front & back is measured in Fig-8 and should be identical.
• If not, some small adjustment with the shims is possible.

The Debenham, Robinson & Stebbings article on a DIY C12 gives a flavour of what needs to be done.

BTW, this is from Bernhard Wiengartner who designed C12 for AKG and says good things about DRS.  Anyone (?) can re-skin a capsule but Bernhard would be upset if you didn't do the above for his design.

 

[ln76d]

Hi Ricardo,

thanks again

did you mean this article? http://www.sdiy.org/oid/ldc/Debenham-StereoCondenser.pdf

This topic is going in wrong way

Still is about brass capsule.

I don't want to push theory in this topic about something completely different.

I've only want to say, that those people who do that, they didn't get their knowledge and skills from cosmos.
Everyone can learn do things like this and i didn't mean manual dexterity only.
Ofcourse it's impossible without a lot of  knowledge and measurement procedures,
but this is all about - to learn something.
I don't say that's it's simple - it is a challenge
Teflon CK12 is little bit simpler in design with marginal interference from outside.

I'll try to measure capacitance of capsule.

 

[ricardo]

I don't want to push theory in this topic about something completely different.

Err.rrh!  Theory and practice for 414, C12 & DRS are exactly the same.  In particular what you need to do to match diaphragms.

Like I said, anyone can re-skin a capsule.  If you are happy to do this without taking the steps I've outlined, go ahead.

I'm not sure why you are asking these questions if you have such firm opinions about the right way to do things.  I'll just shut up.

 

[ln76d]

Ricardo -  don't be angry

It's misunderstanding

I didn't understand you correctly

I know english from movies only, so it's sometimes happens

I was thinking that you write about brass CK12, and this "theory" post was about tension regulations on it etc. - i thought that you mean this.
I didn't realize that's the similar shims regulation.
I thought that's there is two types of these shims only.
Plastic and foam type.
On few diaphragms from teflon ck12, which i have, there are both types used with one shim for one diaphragm.
I will try to measure capacitance later and read what to do if they will be different.

I really appreciate your advices!

I hope you are not angry

Really thanks!

 

[ln76d]

How to measure the capacitance of the capsule?
Simple readings with capacitance meter or should i apply some bias voltage?

In case of simple measurement with meter on my CK12 is diference +/-6pF.
I checked rode nt2 capsule, overall capacitance is lower about 30pf from ck12, but the difference on both sides it's +/-4pf.
On second CK12 difference is +/-5pF

What do you think about those results?
What can be maximal difference in matching?

 

[ln76d]

I screwed a little tighter diaphragm module, now the difference is 5.5pF.
All incisions in the module are in the same position relative to the diameter of the capsule vertically and horizontally.
Do this difference in capacitance could affect on symptoms which i described earlier?
In fact, on omnidirectional polar pattern microphone sounds best.
When switch to another polar pattern, microphone sounds different.

 

[ricardo]

The capacitance tells you the spacing between diaphragm & backplate.  This affects the sensitivity of that side as well as the damping of the resonance.

The spacing is adjusted by using different thickness precision shims.  These should be hard plastic or Aluminium.  I suspect the foam shim is not from AKG.

Instead of measuring the resonance, you can measure the diaphragm tension by seeing how much the capacitance changes when you apply a voltage.

If you are swapping diaphragms from another mike, swap both because they are a matched pair.

But the aim is to get the front & back responses of the fig-8 the same.  If they sound the same, you are probably as close as you can get without measuring gear.  You do this on fig-8 instead of omni because omni is much less critical.

All double diaphragm mike sound different at different patterns.  Just look at the maker's response curves.

There is a pdf book on the Neumann website that explains some of this stuff.  It is the only useful and accurate book on mike design I've come across.

 

[ln76d]

Thank you Ricardo!!!

The foam shim is genuine akg part. I was mistaken, because both shims (plastic and foam) are used with one diaphragm.
On saturn sound is really good explanation of construction ptfe CK12

http://www.saturn-sound.com/Curio%27s/akg%27s%202072%20z%200005%20capsule.htm

http://www.saturn-sound.com/Curio%27s/akg%27s%202072%20z%200009%20capsule.htm

There's no hard plastic or aluminium shims.

Plastic shim is very thin and there's no option of regulation as i think before, because when add second shim should be worse sensitivity.

From saturn sound:
"Very thin insulated "Distance Rings" are used to determine the exact distance, air gap, between the rear surface of the diaphragm and front of the backplate. The distance ring rests on the machined reference, around the outer edge of the backplate.

As with the original "Brass" CK12, these "Distance Rings" effecting the sensitivity of the capsule e.g. thinner the ring, the greater the sensitivity. The rings also having an influence on the frequency response and polar pattern of the final assembled capsule."

I do the picture where are all of the parts in order, of one diaphragm module.
There's no diaphragm - ofcourse it should be on the first ring from left.
I think the result of capacitance measuring is not worst.
I can bet that many of those old c414, used in studios on whole world, can have even worse. Especially those microphones, which are used for vocals.
Dust and saliva sticked to the diaphragm with aging of material can increase capacitance between both sides.
In fact, that both diaphragms which i use are from used capsules and the aging factor probably is very important here. I think the result is good and all those defects what are now, i think are related with the circuit.
I belive that can be difference between the shims on different revisions of this capsules, but these which i'm using are from the same rev.
Maybe i could change the plastic shim from the first capsule, but it's to risky.
In fact i really don't need ideal capsule, because probably i will not have it at all.
Most polar pattern which i'll be using will be probably cardiod.

Maybe lets we focus on circuit for now. I can always come back to the capsule issue, but if there's probability of defective circuit it can be the main clue.

I know that there's difference between polar patterns, but those are too big i think.
There's big difference on output level, and frequency response have big difference on sound also. I have no time to do the samples, i'll try today.

First big problem is disapearing sound when i switch between omni and cardiod.
It sounds like electrolytic capacitor is charging slow (or recharging).

 

[gyraf]

First big problem is disapearing sound when i switch between omni and cardiod.
It sounds like electrolytic capacitor is charging slow (or recharging).

Are you sure it's not expected to do this? The G7 does this also, and  I don't consider it an error.

However, if the model 414 in question is not expected to do this, my guess is that it is capsule collapsing. This can be seen in the right light, at the right angle.

The CK12, nylon or not, is an extremely complicated device with several interacting parameters. What frequency did you e.g. tune the membrane to?

Jakob E.

 

[ln76d]

Thank you Jakob!

Are you sure it's not expected to do this?

That's good question
I really don't know, last time when i use c414, it was good few years ago so i don't remember.

If there's someone on forum, who can check switching in his C414, i'll be appreciate for any info!!

This can be seen in the right light, at the right angle.

Really, i don't understand what you mean?

What frequency did you e.g. tune the membrane to?

I didn't tune it at all, i really don't know how to do that with no option for any adjustment. Changing those plastic shims?
For first look those plastic shims looks identical. Ofcourse this can be micron difference.
Maybe i'm wrong, but i always thought, that this design of capsule was dictated by economic factor. Lower quality but cheaper, with simplified servicing, reducing most of all hard to made adjustments.
Looking on developement production of dynamic microphones capsules, i think it can be most deciding factor.
Just compare akg d12 capsule with capsule in d112 and what's more important compare the sound of both.

It's there any existing source of information about adjustments on teflon CK12?

In case disappearing sound.
Why only on switching from omnidirectional to cardiod?
This shouldn't be also on changing from hyper (or super) cardiod to figure eight?

Work on different types of polar pattern don't charges different values of current?
Maybe one of transistors is on way to die?

I'll try with second, unfortunately used, capsule, which looks like originaly no touched, serviced etc.

My only problem is juggling on those capsules, because i don't want to mechanically damage them.

 

[ricardo]

Are you sure it's not expected to do this?

I expect most dual diaphragm mikes to cut out when you move between certain patterns.  You are changing the polarizing voltage by as much as 60V and this is fed directly to the FET i/p.  The time constant is the capsule capacitance(s) and the 1G or bigger resistors.

What frequency did you e.g. tune the membrane to?

I didn't tune it at all, i really don't know how to do that with no option for any adjustment. Changing those plastic shims?
For first look those plastic shims looks identical. Of course this can be micron difference.

As I said earlier, these are 2 separate adjustments.

The resonance can be checked directly or by seeing the change in capacitance when voltage applied.  At Calrec, we used the 2nd method.

The shims adjust the spacing which affects damping and sensitivity.  Yes the shims will be different by just microns.  These are what I call "hard" plastic shims.

 

[ln76d]

Thanks again and again

Now the things are more clear

The resonance can be checked directly or by seeing the change in capacitance when voltage applied.  At Calrec, we used the 2nd method.

How to check it directly?
In second method, should i use regulated voltage supply and measure capacitance with different voltage ratings or can i measure capacitance in circuit with phantom supply?

The shims adjust the spacing which affects damping and sensitivity.  Yes the shims will be different by just microns.  These are what I call "hard" plastic shims.

Genuine shim from this capsule has very little scratch/hole. Could this affect?

 

[ricardo]

The resonance can be checked directly or by seeing the change in capacitance when voltage applied.

How to check it directly?
In second method, should i use regulated voltage supply and measure capacitance with different voltage ratings or can i measure capacitance in circuit with phantom supply?

To measure directly, place the diaphragm mounted in its ring in the soundfield of a speaker and look at it with a microscope while sweeping the frequency.

At Calrec we used just 2 voltages.  48V and then a higher voltage to make sure the diaphragm doesn't collapse onto the backplate.  The capacitance has to be very accurate.  We used a high impedance Wheatstone bridge with a FET differential amplifier as detector.

But what you want is simply equal responses front/back on fig-8.  You may want to get a system to measure response of microphones instead.  This is useful for other stuff.  Try ARTA.

Without a lot more equipment & know how, the best you can do is to get equal sensitivity front/back.

 

[ln76d]

I apologize for the lack of response!

Thank you Ricardo and Jakob for help!

I'm done with c414.
Now sounds as it should be - i think.
Except disappearing sound (which is back now little faster) on switching from omni to cardiod, everything works excellent. No hum, no buzz, sounds really good on all patterns.
What i did!
I change electrode wirings, which are now in proper order to diaphragm electrical contacts.
I've tried to measure capacitance with aplied 48V but my capacitance meter reads the jumping uneven results.
I have no oportunity to check with ARTA, but i will try soon and post the results on forum.

Tommorow probably i will use it to record drums, so if everything will works fine i'll put sample here.

Thank you Guys!

 

[ln76d]

Two short samples of drums ambient.
Recorded in very small, with low ceiling, rehearsal room.
Unfortunately i forgot to make few captures on different patterns, so here's only cardiod.
In a days i'll be recording bass and electric guitars, then i'll try to capture different polar patterns sound, with double sides of figure eight.
Distance from drums +/-3m, microphone directed slightly upwards.
Yes, those cymbals, in this rooms, sounds crappy

http://soundcloud.com/diy76/ambient-14-1/s-FPbo0
http://soundcloud.com/diy76/ambient-22/s-SGS22