Originally Posted by Fletcher
this dirt adds weight, so when you get "loud" the diaphragm will temporarily stick to the capsule's "backplate" causing the microphone to shut down for period of time.
While I do not have direct capsule cleaning / rebuilding skills, I was given the "being held by the backplate due to increased weight" information by Klaus Heyne. No, I don't have self generated empirical evidence, but when it comes to capsules I am more than willing to take Klaus's word on technical issues as Gospel.
I am sorry, Fletcher, there may have been a misunderstanding about what effect is caused by what action.
Let me quote you from my book's chapter on capsule contamination (shameless self-promotion for "The Vintage Microphone Handbook", to be published shortly by Hal Leonard):
The capsule should not get into contact with contaminants like dust, spit or dirt, for two reasons.
One, to keep the two types of capacitor surfaces - diaphragm(s) and back plate(s) - electrically well-insulated from each other: we are talking about a resistance of ten thousand-million ohms or more that is necessary to prevent the two capacitative sides to discharge against each other.
And two, to prevent these tiny foreign objects that have settled on the diaphragm to build up to a level where their cumulative weight would slow down the diaphragm’s ability to respond speedily and adequately to high frequencies.
Why does the mic's sound level drop and/or thundering discharge noises occur when I sing or speak into a contaminated capsule? Because the moisture emanating from my mouth will complete a conductive path formed by the accumulated contaminants. The moisture, with other words, will lower the high isolation resistance between the two capacitor plates enough to electrically shorten the capacitor (capsule) so that it no longer can hold its charge. This I refer to as ‘electrical collapse’.
The other form of collapse that usually exhibits slightly different audible symptoms than an electrical collapse is a mechanical collapse of the diaphragm.
It is most often encountered in AKG’s original CK12 capsules made from the 1950s to the 1970s: here, a large diaphragm surface (one inch or larger in diameter), which is entirely unsupported by any center attachment, unlike Neumann’s capsule designs, will with time and abuse eventually lose enough tension so that it can no longer resist the constant electrostatic suck created by the back plate’s polarization voltage. The diaphragm's center will then get sucked into and remain stuck against the back plate, either as a result of a hefty dose of ‘plosives, or sometimes just merely by the attraction of the back plate. The mic’s output drops significantly or, at minimum, the low end falls off so severely that the mic becomes useless.
In multi-pattern mics, a mechanically collapsed diaphragm can easily be confused with inadequate supply voltages- for example, a collapsed front diaphragm will show up as a healthy cardioid sound on the rear side of a mic that was switched to figure eight.