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Author Topic: How To Test Vintage AKG C414EB?  (Read 2397 times)

rstl99

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How To Test Vintage AKG C414EB?
« on: April 25, 2015, 09:42:00 am »

Hello all,
I will soon be acquiring an AKG C414EB with original capsule, from its original owner.  It's a long-distance deal and I am told the microphone works well.  I will be testing all the switches and polar patterns as well as I can (I have never owned or worked with a C414).

I have read on this forum about the breath test to check the condition of the diaphragm (tension, contamination), so will do that.

Are there any other things I could check on it to assess condition?  I read somewhere else to plug it into a good preamp and listen to it with the gain on high, for any extraneous noises.

I expect it's not too difficult to remove the grille and visually inspect the capsule.

Thank you.
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klaus

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Re: Testing AKG C414EB
« Reply #1 on: April 25, 2015, 01:00:00 pm »

If you have the time and skill, indeed remove the two half-grills, after removing the four slot screws at the bottom of the mic, and strip off the silver housing. With the capsule and its two diaphragms now in plain sight, plug in the mic (no audio!) switch to figure-eight and put a couple of nice, hard explosives from your mouth ("pops") onto the front, and later, back, of the capsule.

With a strong light pointing right at the gold sputtering of the diaphragm, observe whether the diaphragm, after forced inwards, against the backplate from the strong air blow, returns back out, to its resting position, right way. If it does, and if the outline of the backplate's pockmarked, irregular surface is only visible through the thin membrane for a fraction of a second, the tension of the diaphragm is good, and you can assume it's healthy. If the diaphragm stays stuck against the backplate for a second or more, and cannot overcome the electrostatic suck of the backplate, the capsule, specifically its diaphragm, is shot.

Repeat on the rear, and you will have as good a confirmation that the capsule still works with its diaphragms at acceptable tension as a lay person can get.

A second issue afflicting old CK12 (and for that matter every condenser mic capsule), but one that can be often repaired, fortunately: dirt accumulation. Here is a sticky post on that subject: http://repforums.prosoundweb.com/index.php/topic,36764.0.html

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Klaus Heyne
German Masterworks®
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rstl99

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Re: How To Test Vintage AKG C414EB?
« Reply #2 on: April 25, 2015, 10:23:03 pm »

Thank you Klaus, that sounds like something I might be able to do, once I carefully take the covers off.  I suppose that's the primary concern with those older mics, but obviously there may be other issues (failing electronic component or switch) that may result in noise, so I'll do my best to listen for any anomaly when running it through the various patterns, and other switch settings.

By the way, I've never owned an LDC mic like this, with diaphragms on both front and rear of the capsule.  I gather that's necessary for figure 8 use, but how do the two sides work for other modes like cardioid or omni?  Is there a "main" side (the silver one)?  Or can either side be used indiscriminantly?  Sorry for the most basic question...

  --Robert
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klaus

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Re: How To Test Vintage AKG C414EB?
« Reply #3 on: April 25, 2015, 10:31:11 pm »

Dual-membrane capsules have been around for many decades to facilitate patterns in addition to the front-access-only cardioid, usually ranging from extra wide to extra narrow in focus.

The way the three main patterns of dual-membrane capsules (cardioid, omni, figure eight) and their derivatives are achieved is by way of applying different levels of polarization voltage to the rear diaphragm, and thereby charging the rear side in relation to the fixed polarization voltage applied to the central electrode (the so-called back-plate). The front side in most cases does not get any voltage.

If, for example, you apply 60VDC to the center, and 60 VDC to the rear, the voltage differential between center and rear is zero, and between center and front is 60VDC. This results in a cardioid pattern.

If you apply no voltage to the rear, then you get omni, and if you apply either a negative 60VDC to the rear (your C414) or, in case of most vintage tube mics, 120VDC, you then obtain a positive voltage difference of 60VDC between front and center, and a negative voltage difference of 60 between center and rear. This results in a figure eight pattern, with the rear's signal being out of polarity with the front.
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Klaus Heyne
German Masterworks®
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rstl99

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Re: How To Test Vintage AKG C414EB?
« Reply #4 on: April 26, 2015, 11:00:52 am »

Thank you Klaus for taking the time to explain the dual membrane capsule operation.  Much appreciated.
  --Robert
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