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Author Topic: Neumann M147 needs multiple "boot ups" + What is the 'Breath Test'?  (Read 6780 times)

allemande

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Here is a question about an unusual behavior of my M147:
The mic, when first turned on and left on for 10-20 minutes to warm up, invariably cuts out. I have to turn it off and back on, sometimes twice, in order to get audio to pass through. This has been happening for years, possibly even since I bought it new. Even if I leave it on for days (which I often do) I sometimes have to "re-boot it" to get the audio to come back. Seems like a dysfunctional condition, no?

Thanks for your help and suggestions.,

Hans Christian
Allemande Music
Studio 330
Sturgeon Bay, Wisconsin

klaus

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Re: Neumann M147 needs multiple "boot ups"
« Reply #1 on: February 14, 2013, 07:43:23 pm »

Because there's so much SMT chipping in these mics, component-level troubleshooting and repairs are quite limited, usually to the level of examining intermittent connections between component groups/sub-assemblies.

Try this:
When the mic cuts completely out, hit its body hard with your knuckle, as hard as you can stand it. If it makes a difference in the audio output, and the mic suddenly wakes up, you've got a lead: testing and reinserting boards, connectors, wiring.

There is one other condition that behaves this way: a bad tube. Again, the sudden shock of your knocking will give answers.

By the way: I assume you have made the usual capsule breath test, to positively exclude a contaminated, moisture-prone capsule?
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

radiovinhet

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Re: Neumann M147 needs multiple "boot ups"
« Reply #2 on: February 15, 2013, 10:28:29 am »

Beware... if you wanna try to swap out the tube, you can damage the PCB. Let an expert make this job. Why? The tube is soldered.
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allemande

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Re: Neumann M147 needs multiple "boot ups"
« Reply #3 on: February 27, 2013, 08:03:55 pm »

Hi Klaus,
Thanks for your reply.

what is the "usual capsule breath test"? The mic sounds good when it works. And no worries, I would never disassemble this beauty. Let Klaus or Neumann do it.... ;)

klaus

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Re: Neumann M147 needs multiple "boot ups"
« Reply #4 on: February 27, 2013, 08:22:39 pm »

The breath test can help diagnose whether a capsule's surfaces are contaminated with dirt, spit, dust and other foreign objects without having to take the mic apart.

A little background, for better understanding:
The operating principle of a capacitor (condenser) capsule requires that the two closely spaced plates of the capacitor remain extremely well insulated from each other, to hold the electrical charge that is applied (polarization voltage). If the capacitor discharges, due to an electrical path formed through dirt and moisture between the plates, thundering sounds and other odd noises can be heard. 

In most condenser mics the isolation resistance measured between the plates is in excess of 10000000000 ohms (10 gigΩ). Imagine now that the surfaces of an abused (or incorrectly stored) capsule are covered with dried spit, dust, etc. All it takes is a little bit of moisture or excess humidity to form a conductive path between the plates- a path prepared by the layer of contamination.

To assess whether a capsule is contaminated, the breath test, then, will give you some clues of how clean or dirty the capsule is: Breathe closely into the mic's capsule area, as if trying to fog up a mirror, and listen what happens next:

The audio from a clean, dry capsule will disappear momentarily (due to the breath humidity bridging the two plates): for a brief second the capacitor electrically collapses (shorts out), and its two plates lose the isolation resistance in-between. As soon as the excess moisture has dissipated- typically within a second or two- the capsule's performance returns to normal.

Depending on the severity of capsule contamination, the breath test will trigger thunderous discharges, whistling, and other odd sounds, in addition to often lengthy disappearance of audio. If these discharge sounds continue for several seconds or longer, capsule restoration through removal of contamination from all high impedance components and wiring in the capsule area or, in severe cases, capsule replacement is required to return the mic to its full functionality.
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com
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