Solved: Diagnosing popping/level changes AKG c414 B-ULS

[szegedin]

Hello, if anyone can help diagnose a problematic c414 b-uls, would be grateful.

The mic powers on and makes some loud popping (clips), similar to when you switch polar patterns or a drop out in phantom power.
The popping starts in just about 30 seconds after starting up or changing polar pattern and goes for a minute or so, and then generally settles down and doesn't reappear.  It seems to 'warm up.'
At the same time, After 30 seconds, the level of the front capsule (or 0 degree axis, anyway) drops a good 10db or so. So that registers as a drop in level in cardioid/hypercardioid, and a drop in level from the front of the mic in omni and especially fig-8.

Does this sound like a problem with the capsule itself, or a problem in how the preamp circuit is applying voltage to the capsule(s)? How to go about figuring out, without swapping out both?

Any help appreciated. I have a clip of the sound but not sure if I can post to this forum.
EDIT: I did already put in a new polar pattern switch, with no effect.

 

[gyraf]

capsule collapsing? You should be able to see this, if the light is right.

Jakob E.

 

[szegedin]

Okay thank you -- what does a collapsing capsule look like specifically? Is this sound a symptom of that? I can see some imperfections in the capsule  (it's 30 years old) but don't want to prejudice the diagnosis.

 

[Tubetec]

My guess is moisture , the old 414s I used use years ago often had similar troubles .
Place the mic somewhere warm and dry for a day and see if the problem still occurs .
When the capsule makes contact with the back plate ,you would normally be able to tell because the surface isnt flat .While the capsule itself maybe contaminated with dust ,especially in the 414 as the mesh is very light , any wrinkles or gold worn off could signal worse trouble ,so easy to damage if your not carefull cleaning it.

 

[szegedin]

Thanks for the reply.
The mic passes the 'breath test' of exhaling gently onto the capsule; it has no effect and does not cause a loss of signal or any static noise.
As for the condition of the diaphragm, there is some small amount of rippling around the edge visible from a certain angle, and clear marks where there is a loss of gold. But I can't tell that it is touching the rear element.
Again, this problem starts more or less exactly 30 seconds after it's turned on.
FURTHER EDIT: it seems to not exhibit the popping at all and not exhibit the difference between front and rear capsules when the 20db pad is engaged.

 

[szegedin]

The gentleman from Land Audio says it is likely something in the preamp circuit. I'm sure he can fix it, but the mic is not worth enough to justify 3-4+ hours on the bench. Can't see any obvious bulged capacitors.

It certainly sounds great for the first 30 seconds or so, then the level drops -- what part of the circuit in effect "warms up"? Or accumulates? Also is fine with -10db pad; any help identifying the components of the pad circuit in the schematic?

If someone has a junk b-uls with a bad capsule and wants to sell me pcb boards, pls message me.

Otherwise, next step, will probably recycle the capsule into some diy project.

 

[radardoug]

FURTHER EDIT: it seems to not exhibit the popping at all and not exhibit the difference between front and rear capsules when the 20db pad is engaged.

Theres your clue right there! Study the circuit now, and you can probably find the problem. Those mikes are hard to work on.

 

[Gus]

Do a web search for the schematic and manual you can find it on the web.
Note what changes when you engage the pad(Trace the switching in the capsule voltage section).

Pay attention to  page 9 of the manual