Re: Yaesu FT290
There is a precedent for this in the form of the old K40 speech processor / compressor microphone which was originally designed for for the USA CB market, ie, for AM and SSB.
Since few CB transceivers had + supply available on the mike socket the solution was as outlined in some of the suggestions above - a small amount of power was drawn from the PTT rail during RX mode to charge a capacitor which provided the compressor IC with a power supply during TX.
I actually often considered connecting an original K40 microphone (which I still have) to my FT290R2, but for some reason never did get around to trying it.
One reason may have been that K40s sounded very harsh on FM which they were never designed for, so it would have made it necessary to carry the original microphone around as well, for FM operation.
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