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Author Topic: Sudden awful noise on u67 - does this sound like it could be caused by moisture?  (Read 321 times)

arch801

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Hi all,

In the interest of brevity while I'm still doing some other testing, off hand does this sound like it could be caused by moisture, or more like perhaps a bad 7-pin or component/tube? This audio clip is the absolute worst of what I recorded, there are spurts where the signal is totally clean, but then immediately goes right back to this:

http://sndup.net/nr43

I never did bother to do the 'breath test' on this mic as I had never heard of that suggestion before today, despite reading up quite a bit about the mic in the years running up to purchase. It's a vintage U67 in amazing condition and sounds phenomenal, and has worked great for over 40 hours of recording over the last couple of months, then suddenly this. Any insight appreciated!

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afterlifestudios

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Does the noise persist if you remove the capsule from the amplifier/body?
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klaus

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Before you dismiss the breath test, consider this:
It's a simple, conclusive way to determine whether static in a condenser mic is generated by the capsule or by the mic amp electronics.

If you breathe gently up close onto the capsule, with as much moist breath you can summon, the noise (as monitored over headphones) will either change, i.e. noticeably get worse, or not.

If it does, you are most likely facing a capsule contamination problem. Here, the conductive bridge between the capsule's diaphragm and backplate formed by dust, dried spit and other contaminants that accumulated through the years is made even more conductive through the breath moisture. Conductivity between the two sides of the capsule (= two plates of a capacitor) that are usually isolated from each other by a resistance of around ten thousand million ohms then causes thunderous discharge sounds as the capacitive charge collapses.

If the breath test does not influence the noise in any way, the cause could be an intermittent connection in the high impedance (= highly sensitive) part of the circuit. I often find that a decent knock with my knuckle on the side of the mic will trigger a change in the noise behavior and then it's just a matter of detective work to hone in on the exact spot in the circuit responsible for the intermittent connection.
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Klaus Heyne
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arch801

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Hi Klaus, thank you so much for the reply!

Today the noise is MUCH less, but it's still there. I tried very lightly bonking the mic to see if the noise was affected, and it was not, so I went ahead and tried the breath test, and lo and behold the noise did get worse.

So it does indeed look like some capsule contamination or buildup. I guess I shouldn't be surprised considering how old the mic is, I was just shocked at how much noise suddenly appeared after weeks of it behaving totally fine.

As I'm writing this I'm still listening to the mic hot in my isolation booth and the noise seems to be getting even more and more sparse, so I'm very hopeful it's nothing too serious and can be cleaned by a pro and returned to its glory.

If you have a moment, would you be willing to suggest me a mic doctor that could help me get this cleaned somewhere near the west coast of the USA? I'm in the rocky mountain area.

Also, thank you so much again for this resource and your replies! It is very much appreciated!

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arch801

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Does the noise persist if you remove the capsule from the amplifier/body?

It looks like it is in fact the capsule, as it does not pass the breath test. If anyone knows of a good doctor in the western USA (or greater North America) that can help me get this cleaned, please let me know!

When I first purchased the mic I was planning on having it tested by a pro and I had a friend hook me up with someone in California who works on Neumann a lot but he never got back to me, so at this point I just need to find someone extremely trustworthy to send it to.
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arch801

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UPDATE:

After further investigation the mic cabinet I've been using is actually about 10 degrees colder than I had initially thought. The back of the cabinet touches a wall that gets fairly cold. That said, I don't think the mic was ever lower than 60 degrees Fahrenheit, but could it be possible that I simply turned the mic on too soon yesterday without letting it get to room temperature and it got some sudden condensation? I hadn't used the mic in about a week and it definitely has gotten colder outside in that time. In any case I'm definitely moving the mic cabinet somewhere that will stay a bit warmer.

I'm going to send this off to get cleaned either way, but I'm just wondering if this is what might have caused the sudden drastic increase in noise. The mic has been on for about 20 minutes now and the noise is now only about 25% as intense as it was yesterday, so I'm wondering if it's simply 'drying out' a bit. I want to make sure I'm not setting myself up for this to happen again in the future.
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klaus

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Condensation, for example caused by the sudden warning of a cold object, will exacerbate the problem, but without addressing the underlying issue, capsule contamination, you will run into the same problem again.
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

arch801

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Condensation, for example caused by the sudden warning of a cold object, will exacerbate the problem, but without addressing the underlying issue, capsule contamination, you will run into the same problem again.

Okay, thanks again very much for your input! Sending it to be assessed by a pro immediately, already found a few names on this forum that sound promising!
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klaus

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If you want to preserve the Neumann sound, make it clear to anyone you end up choosing that you will under no circumstances accept capsule replacement of rediaphragming!
 
A good service technician will have mastered the art of capsule restoration.
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com
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