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Re: [CQ-Contest] CM500 Headset mic element replacement

To: cq-contest@contesting.com
Subject: Re: [CQ-Contest] CM500 Headset mic element replacement
From: Jim Brown <k9yc@audiosystemsgroup.com>
Reply-to: k9yc@arrl.net
Date: Fri, 16 Jan 2015 12:59:51 -0800
List-post: <cq-contest@contesting.com">mailto:cq-contest@contesting.com>
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
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