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Author Topic: B&K 4006 noise problem  (Read 1222 times)

afterlifestudios

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B&K 4006 noise problem
« on: September 19, 2018, 09:53:32 pm »

I recently acquired a pair of b&k 4006 omnis.  They have a beautiful, detailed sound. (As long as the details youíre recording are beautiful!)
However, suddenly one of them now intermittently outputs a constant ďstormĒ of noise. 
Iíve isolated it to the mic (swapped pres, cables, converter channels, everything in the chain. And since I have two 4006ís, Iíve swapped the mics out on the floor and the problem follows the mic.  Iíve cleaned the xlr pins and checked that the mic pres are outputting a solid 48v phantom. 

The wind-like noise is similar to when one of my pair of brass capsule ck12 414ebís was acting up a few years ago.  Against all better judgment, I I eventually EXTREMELY gently cleaned some debris from the edges of that capsule with distilled water and a camel hair brush working with magnifying glasses and the mic above me so as to have gravity in my favour.  Itís been working great ever since.

Before I go attempt something equally foolish on this one, does anyone know of any common faults in these 4006ís as they age other than capsule contamination?  I feel like I used up all my luck on the ck12. 

I suppose I better at least transplant the other capsule into it to confirm if itís the amplifier or not.  But Iíve never opened up a 4006.  Iím comfortable inside my u67 and 87ís and schoeps m221ís etc, but if any of you have any tips on disassembly etc would be appreciated.

Thanks for listening to me thinking out loud... ;)

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afterlifestudios

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Re: B&K 4006 noise problem
« Reply #1 on: September 20, 2018, 12:48:34 am »

OK  Investigating further...  From the DPA literature:

* The microphone cartridge is tightly secured to the main body housing,
and no attempt should be made to remove it. If a replacement cartridge
is required, contact your local DPA Microphones representative.

and,

* Over time a visible dust layer can build up on the
diaphragm. Since the mass added is extremely small and the influence
on the frequency response is negligible, the dust will not change the
characteristics of the microphone. Therefore cleaning the diaphragm is
not necessary and must not be done.

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Kai

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Re: B&K 4006 noise problem
« Reply #2 on: September 20, 2018, 06:01:05 pm »

However, suddenly one of them now intermittently outputs a constant ďstormĒ of noise...
...The wind-like noise is similar to when one of my pair of brass capsule ck12 414ebís was acting up a few years ago.  Against all better judgment, I I eventually EXTREMELY gently cleaned some debris from the edges of that capsule with distilled water and a camel hair brush working with magnifying glasses and the mic above me so as to have gravity in my favour.  Itís been working great ever since.

Before I go attempt something equally foolish on this one, does anyone know of any common faults in these 4006ís as they age other than capsule contamination?  I feel like I used up all my luck on the ck12. 

I suppose I better at least transplant the other capsule into it to confirm if itís the amplifier or not.  But Iíve never opened up a 4006. 
Don't ever mess with the capsule, the microphone will never be the same again. Anyway it's a back electret type that does not need strong isolation or such on the accessible parts of the construction, the diaphrag is connected to ground level.

What you have is a common fault of a capacitor inside the amplifier.  I had to change several of those already.
Accessing the microphone amplifier is simple, there is a securing ring on the XLR connector insert, if you remove the ring you can pull out the amplifier.

I am not in my studio right now so I cannot look inside one of those, but as I remember it's quite obvious which cap is the candidate for being replaced.
 If I remember correctly it is an electrolytic or tantal type.

Be careful not to touch the top part of the amplifier with your fingers, remains will kill the very high isolation that is needed in this area of the pc-board.
Wearing disposable medical rubber gloves is a good idea for this job, they cost a penny.

Unlike B&K's measurement equipment no schematic seems to be published, at least I could not find any.
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afterlifestudios

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Re: B&K 4006 noise problem
« Reply #3 on: September 20, 2018, 08:47:25 pm »

Thank you Kai.  I will be careful.  I've never dealt with a "permanent" pre polarized capsule before.  And I'm pretty sure I could make it less permanent rather easily... 
I'm not having any luck finding drawings of the amplifier circuit out there.  Do they exist?
Thanks again,
John

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Jim Williams

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Re: B&K 4006 noise problem
« Reply #4 on: September 21, 2018, 11:03:44 am »

I have 4003 prints here. Those mics are similar except the 4006 has an output transformer, the 4003 does not, it's a single ended output into their external preamp box.

You have a 300 pf glass capacitor feeding a old National NF5102 jfet. That is fed current through a PNP U13106 and a NPN 2N5550. Another PNP U13106 is a current booster. If any of those parts are bad you will hear noise. One can determine that by disconnecting the capsule and shorting the glass capacitor to ground. There is also a 5.6 gig leak resistor to ground, if that part is off it will generate noise.

Yes, there are improved jfets and bipolar transistors that can lower the self noise. The LS170 jfet and 2SC3329 NPN and 2SA1084 PNP are good choices.
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afterlifestudios

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Re: B&K 4006 noise problem
« Reply #5 on: September 21, 2018, 12:04:41 pm »

Thanks Jim.

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afterlifestudios

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Re: B&K 4006 noise problem
« Reply #6 on: September 21, 2018, 06:10:03 pm »

Unfortunately, my amplifiers appear to be different revisions.  The bad one, does have two 47uf electrolytics in parallel.  They read ok (90uf) on my capacitance meter.  Should I replace them regardless?  (Any recommendations here?  (panasonic FM series is what I have around...)
John
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afterlifestudios

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Re: B&K 4006 noise problem
« Reply #7 on: September 21, 2018, 09:10:50 pm »

Here are the two revisions.   The one with the big blue cap is the one making the noise.
https://www.dropbox.com/s/r4llzfs3hh066uy/B%20and%20K%204006%20amplifiers.png?dl=0

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afterlifestudios

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Re: B&K 4006 noise problem
« Reply #8 on: September 26, 2018, 05:43:20 pm »

Hi Kai.  Here's a closer pic of the trouble board.  There's two of those big 47uf caps, and then the smaller one which I'm assuming is the 300pf glass cap Jim mentioned.  Did you use anything special for replacing the 47uf caps on yours?  Just about to order a few parts and wouldn't mind a cap suggestion from someone who's been here before... 
Thank you!
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Kai

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Re: B&K 4006 noise problem
« Reply #9 on: September 29, 2018, 07:55:47 pm »

 Sorry, I am still not in my studio to have a look.
Of course any of the transistors can have gone wrong too.
To find out which one you can short the interconnects to ground one by one using a small film cap of about 1uF, until the noise disappears.
Always connect this cap before applying voltage or you can kill a transistor.
But giving the elko a shot doesn't hurt, for testing put in anything you have lying around, even the value does not need to be exact, it just has to meet the voltage specs.
From my remembrance it wasn't the electrolytics but the styroflex that was broken, but I am a bit unsure, I thought it wasn't styroflex but something different.  I am sure it was not the glass C!
Might have something to do with the fact that I repaired the transformersless versions, which uses a higher voltage of 120V instead of 48V, so some parts may be different.

For ordering the caps: something decent like Panasonic, 105įC, lo ESR.
I am not the right person to comment on the sound of parts, that's Klaus's speciality.
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