dead M7 capsule?

Hi all

Have some problem with my dale M7 capsules, both capsule are in use with MK47 pcb.
diaphragm are sucked to the back plate, only on the cardio position, front side
microphone still producing sound
I need advice, can i recovery this?
is that heat pb? polarisation pb? recording? (I always use anti-pop when micing vocal)

i will perform test and comparison by reversing the capsule wiring, can't do this before next week

suggestion welcome

regards

Zam

second one...

You should check your polarization voltage to confirm that it's 60V or a little less. Confirm the resistor values in the divider portion of the circuit as well as your voltages in that particular path.

Some capsules may be more sensitive to this than others but you generally don't want more than 60V between back-plate and diaphragm since that greater difference in potential increases tension and in cases which are closer towards the extremes can literally make it suck.

Hope that helps.

Thx,
jb

Well that's not good! Is this one of chuck d's b stock or a production? The capacitance could be high as well.  Is permanently stuck or only when plugged in?

Eric

tks JB

I read aprox 56V after divider it's ok
unfortunately i can't measure hi impedance circuit over 200mV or 2V cal with my 8068a
after 100M and backplate i read 5.7v with 10M multimeter load

to be honest I suspect bad capsule as my first order were returned time ago when i purchase it, 3 of the 4 capsules side were totally sucked to the backplate!!! look like tensioning diaphragm problem, or mylar?

is there a way to unglue myla from backplate?
In fact it is difficult to see if the diaphragm are already sucked or just got the backplate "print" on it from a short contact

At the moment my only option is to reverse the capsule to use other diphragme (bye bye omni) and see what hapend with the time

tskguy said:

Well that's not good! Is this one of chuck d's b stock or a production? The capacitance could be high as well.  Is permanently stuck or only when plugged in?

Eric

Click to expand...

As say previously it's difficult to say. But when i look with right incidence LED light, it looks more permanently. PSU On or Off !
It's a Chuck standard production not a B stock...

At this point the Response will never be normal , the capacitance is one factor but it will not tell you much under polarisation condition. a higher capacitance at rest will most likely increase the chance of this phenomena 72pf beeing ideal, in your case the capsule has lost tension on the diamter with the glue beeing affected, this is a typical Failure often seen even in Neumann capsule.

your best Bet is the no Omni Mode or a Reskin By Dr. TSKGUY,  Or a New Capsule.

Best,

PS: I seen in the past some humidity in the capsule wich are punch as the K87 that putting it near a hot Stove for sometime Cleared the problem Away.
Caution

Zam, 56V between the middle of the divider node and the 100M should be fine. Measuring between the 100M and backplate will provide inaccurate results with the typical 10M meter like you explained so that looks to be on the up and up.

If you have perhaps a cheap k67 style capsule harvested from a doner project you could use that to test the circuit too or to prove to yourself that a physically different capsule doesn't have this problem or behavior.

In any case I would let ChuckD know the details. He should be able to service or replace it if the capsule was defective or otherwise damaged.

Simply re-glu'ing to the backplate isn't necessarily optimal since I think it needs to be held at a specific tension against the backplate during that fastening-procedure. Any moisture/dust/debris that get's in there can make it not work reliably so if there's a hole you could try the breath-test too.

My approach usually is that if it's already broken.... No loss and maybe I can fix it or learn something helpful.... I would get in touch with ChuckD first though with some details before declaring it a loss and taking pliers to it.

Cheers,
jb

I try to contact Chuck, i get no reply
I don't think he will made a second replacement 1 year after.

don't have any other capsule available around to perform test  :-\

zamproject said:

I try to contact Chuck, i get no reply
I don't think he will made a second replacement 1 year after.

don't have any other capsule available around to perform test  :-\

Click to expand...

are both side appeared problematic at the same time ?
D

poctop said:

are both side appeared problematic at the same time ?
D

Click to expand...

I don't know, I do studio maintenance check last week, desk, outboard etc... and see the capsule
The mics produce audio, and i really don't ear any radical change at one precise time in the past
I can say i find the mic sounding more midrange than begining, maybe some low drop, as it appear to be a smooth change thought the time i don't care about it (thinking about tube etc...). I also notice some strange phase things in the mid with a particular vocal on sibilance.

To be precise it's not the both side of the same mic, but the both side of the cardio side of my pair
I use cardio over omni most of the time (75/25 maybe)

I perform a quick 20-20K sweep mic 30 cm from speaker, no big anomalies in the curve, but it's absolutely not a scientific measurement... i do it trought my cheap tannoy... i will do it again in front of my PSI A21
i also puch hard low frequencies in opposite diaphragm to see if this will unsuck the front one, don't work :

not to mention I take care of all my stuf

Zamproject,

I would be happy to reskin your capsules if chuck isn't willing to do it for you.
Pm me and I can give the details if your interested.
Tskguy

Hi All.

Ok here goes;

There are a few possibilities that can be causing your issues though at a guess, i'd say that the capacitance of the capsule is high but that is not your problem.  To me it looks like there has been excess cutting compound that has not been removed before skinning the said capsule.  Cutting Fluid is hard to remove, if you use hot water it can be enough but more often than not, other cleaning methode need to be used.  This is not a problem for mics that utilise internal spacers as the capsule can be lapped before skinning which is a great way to remove this excess without chemicals and works very well but with an M7, this can not be done so the capsule must be cleaned many times before it is skinned to ensure that there is no residue left.

Usually when a capsule draws down, it is an even drawing that will leave an imprint whilst drawn down of the metal but with compound excess this looks different, your looks like compound excess.  I hope that helps.

cheers

tks all for advise

I leave for a week of work, i will try things when i'm back...

If i remove the central screw, diaphragm have a hole right ?
Maybe i can try to unglue the diaphragm?

@tskguy, tks for your offer but i'm in EU, i think it's more simple to do this here
As i'm little disappointed about this M7 (i got same problem with the first i ordered) I think i will go for a Thiersch as soon as i have the money for it

I keep you informed of evolution.

Zam

I think Thiersch can re-skin the capsule if you're in the EU... maybe it's not necessary to buy a new one?

mics said:

To me it looks like there has been excess cutting compound that has not been removed before skinning the said capsule.  Cutting Fluid is hard to remove, if you use hot water it can be enough but more often than not, other cleaning methode need to be used.  This is not a problem for mics that utilise internal spacers as the capsule can be lapped before skinning which is a great way to remove this excess without chemicals and works very well but with an M7, this can not be done so the capsule must be cleaned many times before it is skinned to ensure that there is no residue left.

Usually when a capsule draws down, it is an even drawing that will leave an imprint whilst drawn down of the metal but with compound excess this looks different, your looks like compound excess.

Click to expand...

I agree with Ben that it doesn't look like excess Vp or the wrong spacing.

With those, the diaphragm collapses to show a clear imprint of the backplate.  Yours seems to have a lot of crud under the diaphragm.

Chuck finaly contacted me
He send me one new capsule, received today
I will perform comparison.

hello
Yesterday i perform test, comparison and swap between the two defect capsules and the new one i receive days ago.

what i can say at the moment is that i have huge difference, no one sound the same and with large different curve response especially in the hi with more than 5dB boost with defectives capsules (relative to good one).
Swapping same capsules between mic electronic don't change the sound and response curve, capsules seems to make 95% of the different sound characteristic.

of course i reverse the two defect capsules to have good remaining diaphragm on front, and only test with cardio pattern
reversing defect capsule give me back the aprox 10dB drop i get with sucked diaphragm

so my question is...
what are the influence in cardio mode coming from back diaphragm, can a sucked back diaphragm mechanically interfere to the mobility and moving specification of the front diaphragme and/or alter the backplate polarisation?

to be continued 

Zam

That would depend on the switching setup used. A simple cardioid/omni switch on the mic (like the original U-47) would have no interference on the sound because the back diaphragm is switched out of the circuit in cardioid mode. Multipattern switching setups will vary the voltage to the back diaphragm so it may suck in and seal the backplates holes to some extent. What might it do to the sound? I'll leave that to someone more knowledgable than myself.

-James-

zamproject said:

what are the influence in cardio mode coming from back diaphragm, can a sucked back diaphragm mechanically interfere to the mobility and moving specification of the front diaphragme and/or alter the backplate polarisation?

Click to expand...

The back diaphragm is part of the cardioid acoustic back access for the front diaphragm.

If it collapses early, it means the tension is too slack. Cardioid response & pattern will change.  This happens even if the back diaphragm is not polarized so it has not collapsed.  The slack diaphragm is enough.