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Author Topic: KM56c problem  (Read 1178 times)

Kai

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Re: KM56c problem
« Reply #15 on: December 30, 2016, 05:28:12 am »

I would ask you to redo the measurements  after making your self familiar with your meters:
First identify the pins, according to the schematic above.
Let's name the pins 1 to 3 from top to bottom.
Now you need to measure capacitance and resistance of each pair.
1-2 C= 45pF, R= >10GOhm
2-3 C= 45pF , R= >10GOhm
1-3 C= 1025pF (+/- 10%), R= >5GOhm
As your meters probably cannot measure higher than several hundred MOhm, simply check if they read "no conductance" in the highest  resistance range.
If there is an Ohm reading value on any pin combination your capsule mount is faulty.
As this does not necessarily mean the capsules themselves are broken, you should consult a very skilled person to fix it.
I would strongly suggest not to mess around by yourself, as these precious diamonds are not repairable, it's very likely you finally break them if you try.
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afterlifestudios

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Re: KM56c problem
« Reply #16 on: December 31, 2016, 06:13:56 pm »

Thank you, Kai.  Here are the results of my capsule measurements according to your pin out nomenclature.

pin 1-2 C = 1079pf R = no conductance 
pin 2-3 C = 0pf, R = 3.8 kOhm
pin 1-3 C = 1079 pf, R = no conductance

Clearly there is a problem.  I would love to send it to a "very skilled person" to fix it.  As far as I know from comments made earlier in this thread by Klaus, Thiersch only reskins diaphragms for these capsules.  Based on the photos of my diaphragms and the readings I'm getting on the capsule pins, does  it really seem like a diaphragm problem?  If so, great, I'll send it to Thiersh.  If not, does anyone have a recommendation for a "very skilled person" willing work on it?
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uwe ret

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Re: KM56c problem
« Reply #17 on: January 01, 2017, 02:36:12 pm »

It looks as if your capsule head has an internal problem between the back plate and the rear diaphragm. Kai was close with the expected measurement values. The actual values between either diaphragm and back plate should be 70 pF (35 pF in parallel with the series combination of 1000 pF and 35 pF), and the capacitance between the two diaphragms should be 1018 pF (1000 pF in parallel with the series capacity of the two 35 pF capsule capacitance). Resistance should be infinite for measurements between any of the 3 terminals. As is the capsule could still be used in cardioid only mode.
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afterlifestudios

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Re: KM56c problem
« Reply #18 on: January 01, 2017, 09:46:51 pm »

Thank you, Ewe.  Indeed, that's what it seems.  But I don't have a healthy cardioid "as is".  Perhaps if I completely lifted the rear diaphragm from the circuit, then the front side would be normal cardioid?  (I'm not ACTUALLY going to do that!)  But right now it definitely is cardioid only, but very low level.  Handling or tapping mechanically on the mic housing gives very full, loud clear results.  Just not from the capsule. 

Could the fact that the rear side is shorted to the back plate be causing that problem with the front side?

John

 
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afterlifestudios

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Re: KM56c problem
« Reply #19 on: April 07, 2017, 07:45:33 pm »

Just a happy follow up here...

Once it had been determined that the problem was capsule related, not amplifier, I sent the capsule to forum member "Panman".  He found the short between capsule and backplate and corrected it.


I got the capsule back from him today and it sounds marvellous in all 3 patterns. Output level in cardioid is generally on par with my km84's.  Noise floor is about 5db higher than the 84's.

Most importantly, it sounds great!  I thought she was a goner...

Thank you, Panman!

John
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