[CQ-Contest] CM500 Headset mic element replacement
k9yc at audiosystemsgroup.com
Fri Jan 16 15:59:51 EST 2015
See comments interspersed.
On Fri,1/16/2015 7:47 AM, Dave Hachadorian wrote:
> The microphone in my Yamaha CM500 Headset has failed. DC measurements
> at the plug show:
> tip and ring shorted
That's normal -- it's wired that way, I think, for compatibility with a
wide range of computer stuff.
> 1270 ohms tip to shell with negative ohmmeter lead on shell
> 810 ohms tip to shell with positive ohmmeter lead on shell
The output of the mic is a FET-follower that gets the very high Z of the
mic element to an impedance that can drive our circuits. That bias is
applied through R on the order of 5K to the tip, and the 5K resistor
serves as the load. As I recall, Elecraft is using 5.6K. So the FET is
what you're reading with the Ohmmeter.
> The above measurements lead me to believe that the problem is not at
> the connector end, but rather in the element. I'd like to take a crack
> at repairing the unit. Mouser has a wide selection of electret
> elements, starting at $0.58.
I've been using CM500s for nearly 5 years now. I've had two with broken
cables, both of which I have retired and replaced with a new CM500.
> Any ideas as to how would I go about disassembling the mic enclosure,
> and can someone recommend a specific electret element?
Electret capsules are pretty generic, so almost anything would likely
work if it fits. No ideas about getting it apart. However -- is it
possible that the bias circuit or connector adapter has failed?
I really like the CM500 for radio, but the difficulty of repair is a
negative. The way I look at it is that I can buy several of these for
the cost of other stuff that is less comfortable and doesn't sound as
73, Jim K9YC
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