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Below is how I have wired the MPET to be used for packet with the Mobilinkd TNC2 and my Arduino based TNC project.
This can serve as a foundation for those looking to use a Yaesu microphone with the MPET.
Changing the value of the resistor and adding in a volume control would be important to consider for voice transmission/reception.
In the case of the MPET for 1200 baud data, I am using R350 to give me a fixed audio level.