small microphone element -- non-electret?

[Percival P. Cassidy]

My mother-in-law spilled something on the handset of her CrystalTone
amplified telephone, and the outgoing volume level is now pathetic;
i.e., the people she calls can hardly hear her, even with the "boost"
button pressed.

The mic element looks pretty much the same as the electret ones sold by
Radio Shack, but I cannot detect a sufficient polarizing voltage for an
electret -- just 0.7V.

The only two mic elements RS lists are electret, and I have been unable
to track down any mic elements of a similar size that are not electret.

Any ideas?

Perce

 

[Michael A. Terrell]

"Percival P. Cassidy" wrote:

My mother-in-law spilled something on the handset of her CrystalTone
amplified telephone, and the outgoing volume level is now pathetic;
i.e., the people she calls can hardly hear her, even with the "boost"
button pressed.

The mic element looks pretty much the same as the electret ones sold by
Radio Shack, but I cannot detect a sufficient polarizing voltage for an
electret -- just 0.7V.

The only two mic elements RS lists are electret, and I have been unable
to track down any mic elements of a similar size that are not electret.

Any ideas?

Disconnect the old microphone and measure it again. All it takes is a
slight tracking across the electret, and the current load will go up and
pull the DC supply low. The preamp is a FET and will work at a lower
voltage than a bipolar amp.


BTW, this type of question belongs on news:sci.electronics.repair


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On Sep 14, 2:22 pm, "Percival P. Cassidy" <nob...@notmyISP.net> wrote:

My mother-in-law spilled something on the handset....

The first thing that must be done when this happens is shut the power
off immediately! If left on and the liquid is conductive, things start
to corrode quickly and it won't take long to chew up tiny conductors
(bigger ones take longer). Examine things closely (magnifying glass),
looking for corrosion. If you find it, clean it off (soap and water
and brush), dry things off and whatever physical damage that has
occurred must be repaired by soldering in bypassing wires, etc.
Assuming all this is done, and physically at least, you are back to
square one, it is likely that the mic is an electret and whatever
polarizing voltage was being supplied is no longer there due to
electrical damage. You could remove the mic. and try treating it as an
electret in a separate mockup and see if it works.