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Alinco DX77
Electret microphones
There is a jumper inside the Alinco DX-77T to provide +8v bias on the microphone audio + line. If you enable this Jumper you can use Electret microphones with out having to build a separate voltage insertion circuit.
JP1 is just to the rear of VR14 the Microphone Gain control on the PC board. You can find JP1 on the schematic (which came with the radio) Just above VR14, and just left of TP1. The bias circuit consists of R338, R339 and C281 all of which are already present on the board. Actual bias voltage will be less than 8v depending on the load presented by your microphone.
I used a magnifying glass, and a sharp grounded soldering iron to bridge the solder pads JP1. Be sure to open up the original hand MIC and install a capacitor in the audio line to avoid having DC running through the coil of the dynamic microphone.