On Fri, 11 Jul 2008 06:34:55 -0500, "Lloyd E. Sponenburgh"
<lloydspinsidemindspring.com> wrote:

>They never tell you what they "did" (or didn't).

Did you ask?

>I'm competent and equipped to do everything you suggested, but there
>isn't enough time in my whole life to go trouble-shooting crap hardware
>just to avoid buying more crap hardware. sigh...

Your decision. I have the same problem but tend to do it anyway, just
to satisfy my own curiosity. Such reverse engineering doens't make me
any money, but I find it interesting.

>You're right about one thing. We are in Florida. It's HOT and humid
>here, and the phones go in and out of AC all the time.

Bingo. The humidity also causes some circuit boards to expand and
contract. It also gets inside the mic elements. Ask around about
cell phone failures.

>Nonetheless, I'm banking on a mechanical failure of the PCB or an SMT
>part.

The first step to solving a problem is to assign the blame. In this
case, that's a fair guess, but lacking in one respect. You have no
direct evidence.

>We change batteries in the phones a lot, and it takes a great deal
>of squeezing and hard pulling to open the case; Ten or fifteen pounds of
>force. It's likely that stress on the hard-mounted board that is busting
>something. They didn't grommet the mounting points; so the board twists
>with the case.

Yech. Also look for cracked components and surface mount solder
joints. Why do you need to juggle batteries?

>The mics in the Durafons use a system I haven't encountered, and will
>have to study more to understand. The mic looks like a standard electret
>element, but has no electrical connection to the board. It's
>mechanically pressed against an insulating (checked it) silicone pad
>which is pressed against two coaxial rings of pcb land. The interface
>appears to be made by varying the capacitance between the rings.

That's commonly used on cordless phones and cell phones. The
concentric rings are the contacts. The ones with external DC bias
have 3 rings. The ones without an internal amplifier have two rings.
The mic fits in a molded hole in the plastic front panel. There's
usually a foam or rubber ring to provide tension. The assembly is
compressed by the board mounting system. The problem with this method
is that condensation from the speakers voice tends to make the
contacts rot. Gold plating is common. Mounting the mic element
directly on the board isn't very useful as the mic element will not
survive reflow soldering.

The electret mic elements aren't totally hermetically sealed and will
also condense water inside, causing the capacitor to discharge. If
you have any old Motorola flip phones floating around, each phone will
have TWO electret elements you can borrow.

>I've worked on a lot of things, but somehow I've missed that method up
>til now.
>
>LLoyd


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Jeff Liebermann jeffl@cruzio.com
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