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[afterlifestudios]

I recently acquired a pair of b&k 4006 omnis.  They have a beautiful, detailed sound. (As long as the details you’re recording are beautiful!)
However, suddenly one of them now intermittently outputs a constant “storm” of noise. 
I’ve isolated it to the mic (swapped pres, cables, converter channels, everything in the chain. And since I have two 4006’s, I’ve swapped the mics out on the floor and the problem follows the mic.  I’ve 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 414eb’s 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.  It’s been working great ever since.

Before I go attempt something equally foolish on this one, does anyone know of any common faults in these 4006’s 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 it’s the amplifier or not.  But I’ve never opened up a 4006.  I’m comfortable inside my u67 and 87’s and schoeps m221’s etc, but if any of you have any tips on disassembly etc would be appreciated.

Thanks for listening to me thinking out loud...

[afterlifestudios]

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.

[Kai]

However, suddenly one of them now intermittently outputs a constant “storm” of noise...
...The wind-like noise is similar to when one of my pair of brass capsule ck12 414eb’s 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.  It’s been working great ever since.

Before I go attempt something equally foolish on this one, does anyone know of any common faults in these 4006’s 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 it’s the amplifier or not.  But I’ve 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.

[afterlifestudios]

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

[Jim Williams]

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.

[afterlifestudios]

Thanks Jim.

[afterlifestudios]

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

[afterlifestudios]

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

[afterlifestudios]

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!

[Kai]

 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, 105°C, lo ESR.
I am not the right person to comment on the sound of parts, that's Klaus's speciality.

[afterlifestudios]

Just an update:
I replaced the two 47uf 25v electrolytic (coupling?) caps.  So far so good!  The noise is gone.
I used the only caps I could find that were physically small enough, which were Vishay MAL203036479E3  and only 85c rated.
Do these appear to be ok?
https://www.digikey.ca/product-detail/en/vishay-bc-components/MAL203036479E3/BC5237CT-ND/8557366

All tighter spec'd and higher temp caps I could find had too large a diameter to fit.  (I know for sure because I tried some...)

The other thing is that the measured capacitance on these caps is higher than rated, (about 62uf each), but I guess that's not a bad thing in this application?

I

[Kai]

The 25V might be a bit low, I suggest you measure the voltage across the caps in action to be sure they're not overloaded.
Else I don't see any problem, so it really were the caps causing the noise - the easiest possible fix

[afterlifestudios]

Whoops.  Spoke too soon.  It’s back...

[Kai]

Maybe 25V was too low, now the caps are toast.
Measure the voltage.

[afterlifestudios]

Update: 
Both amplifiers work perfectly with "capsule A".
The problem follows "Capsule B" with either amplifier.

The capsules are seemingly impossible to remove from the housing/body itself.  But at least I can rule out the amplifiers now.  (Should have done that as first step.)

The surface of Capsule A diaphragm actually looks much more contaminated than capsule B, so I don't think it's that...

Any other thoughts about this?
Thanks,
John