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Re: Schematics > mailed
Author: Uwe Sattler/NeumannUSA
Date: 03/03/2014
Hello Jean-Michel,
the bias voltages you measured are correct, and indicate that your meter has a standard input resistance of around 10 MΩ. Together with the 68 MΩ series resistance the meter's input resistance does constitute a (68 + 10) / 10 (roughly 8 / 1) voltage divider, resulting in the readings of between 6 and 8 V. The proper DC-DC converter operation also strongly indicates the correct function of the balanced output drivers T5 and T6, which may suffer from exposure to excessive phantom power of reverse polarity, which may happen when this type microphone is connected to the common +48 V phantom power standard instead of the required -9V.

In order to determine whether the capsule or the FET impedance converter are defective, simply connect an audio tone generator to the calibration input. Expect unity gain and relatively flat response between 40 Hz and 16 kHz (and beyond) for signal up to nearly 400 mV. Should this test fail, suspect the FET and measure the voltages at its drain (around 10 V) and source (around 3 V).

Lastly, perform the 'breath test' and inspect the capsule for contamination and other defects, like rippling of the diaphragm, peeling gold electrode or holes. Remember to observe the customary caution when attempting to disassemble the microphone head! If you are not comfortable or experienced with these operations, I suggest to contact Sennheiser France and their experienced technicians:
Sennheiser France SARL
Parc d'Activités Mure 128 bis, Av. Jean Jaures
F-94851 Ivry-sur-Seine Cedex
France
Tel.: +33 (1) 498 70 317
Fax.: +33 (1) 498 70 318
E-mail: compta@sennheiser.fr
Good luck, and best regards from Connecticut,
Uwe Sattler  
 
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