Friday, November 13, 2015

Headset Mic/Phones to my FT897 adapter

Over the weekend, I know I had something to do that is long overdue..

I have an old Telex headset mic given by a friends a few years ago. My initial plan was to use it with my FT-897 transceiver and couple it with a foot switch to trigger the PTT. I don't know what shall I name it since it is triggered by stepping onto the foot switch.. shall we call it a STT (Step to Talk) then?

I actually measured and compared the DC resistance of the dynamic mic from the YAESU MH-31 and the Telex dynamic mic and it does gave me the same value.. which is about 190 ohm. I did this because i have no information what was the Telex mic impedance. My impressions is that the Yaesu mic has between 150-200 ohms impedance as stated in the transceiver manual. I could be wrong.

I also measured the DC resistance because I have no other way to accurately measure the impedance in a more appropriate way without having a complete test bench.

My first test was not really good. On air test received a bad report which has a high pitch noise in the background. After some thought.. I actually found the mistake..

1. I did not replicate the actual MH31 circuit. There is a 680R resistor in parallel with the mic.
2. Ground loop! Mic Ground MUST be isolated from PTT Gnd.

The circuit above is just an example of how I did mine. You can do yours according to your preference and you may also choose to use electret microphone. BUT you need to add some circuit for the electret mic as it needs voltage to operate. The output level of the electret mic is also higher and you may need to turn down the mic gain. You can find a lot of such circuit over the internet on how to modify the MH31 to use electret mic.

Thank you for reading.. de 9W2DTR

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