[CQ-Contest] CM500 Headset mic element replacement

[Jim Brown]

Hi Dave,

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 
good. :)

73, Jim K9YC