Neumann M367 polarization mystery

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mad.ax

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I'm currently working on a pair of these beauties. For those who are not familiar with the M367, it's an ORTF (French broadcast organisation) version of the U67. Main difference is that it uses an AC701 in lieu of the EF86.

Both mics work and sound well but level seems a bit shy. Measuring capsule polarization voltage reads 54V instead of the expected 60.
Thing is 120V reads 120V. Both R12 & R13 measure 2M2. C4 is good and shows several MΩ of ESR....

But I still get only 54V! What is it I am missing?

Axel

367 schemo.png
 

dmp

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The 2.2M resistors form a voltage divider, so you should expect the voltage to be 60v,
V = 120*R12/(R13+R12)

HOWEVER, when you measure it with your multimeter, the impedance of the multimeter can affect the measurement. Say the multimeter has an impedance of Rm, that resistance is also going to ground, in parallel with R12.
Then, V = 120 * (R12||Rm)/(R13+R12||Rm)
If your multimeter has an input impedance of 10M, you do that calculation, the expected measured Voltage would be 54V
 

dmp

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First, determine how much the level is 'shy' in dB by comparing to another mic. (set up both mics side by side through the same preamp at the same level seeing the same noise source and compare the level)
If you do have a problem, sweep the calibration following the instructions in the U67 manual to see that the negative feedback is correct. You can use software like RMAA to do this.
Since these are extremely valuable and historic mics, I would kindly recommend you not desolder or work on them without a LOT of experience working with electronics, including mics with their sensitive high impedance section.
 

mad.ax

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Thank you both! Now I feel like an idiot🥵
I thought that my Brymen 867s had an higher input impedance, but it is indeed 10MΩ... Silly me!
Just checked with the Keithley 616, and spot on: 60V...

Axel
 

mad.ax

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Our 'reference in that respect' happens to be a M269C wich is almost the same mic.
More investigations to come...

Axel
 

abbey road d enfer

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Our 'reference in that respect' happens to be a M269C wich is almost the same mic.
More investigations to come...

Axel
You could swap capsules, that would be a first step, but I doubt the two capsules in the 367 have identical defects.
Are you equipped for gain calibration (injecting signal in the "calibration input" and measuring output level)?
 

mad.ax

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I'm not at my lab, but at the studio, so no full equipment here to measure... But I will find a way...
At the moment the difference is pretty obvious by year, so obviously >3dB.
Both M367 capsules sounds identical. The 269 capsule is tricky to swap because the head assembly is wired diferently (KK269 vs KK67)
KK269 works on the 367 (and has the same level than KK67) but not the other way around.

Axel
 

dmp

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Just put up the M367 and 269 next to each other through the same preamp gain and play some music into them to see how many dB different the levels are
 

Gus

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How is the transformer wired?
I assumed when I made my first post in this thread they were both wired the same 50ohm or 200ohm. 50 ohm has more of a step down so a lower output voltage
 

mad.ax

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Hi Gus,
367 and 269C are both cathode biased.
All the mics are straped for 200Ω operation.

Axel
 

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