A relatively accurate mili-amp meter.? Any meter of reasonable quality should to fine.? There are other more complicated ways to set it, but a mili-amp meter is easiest, and most good meters are capable of mili-amps readings.
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You can probably download a service manual from the web, they are available at no cost.? That will contain the directions of how to do it.
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If for any reason you need to access the hidden menu, be certain your radio is connected to a 13.8 VDC power supply.? If you do not, your volt meter will be out of calibration on the FT-817 display.