R/E/P > Klaus Heyne's Mic Lab

Sudden awful noise on u67 - does this sound like it could be caused by moisture?

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arch801:
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!

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

klaus:
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.

arch801:
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!

arch801:

--- Quote from: afterlifestudios on December 20, 2021, 06:01:14 PM ---Does the noise persist if you remove the capsule from the amplifier/body?

--- End quote ---

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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