If it "keys up" without a mic plugged in, try plugging the mic into the radio and see if it stops. Some radios require the mic to be plugged into it to operate properly. If it is keyed up with the mic plugged into it, try unplugging the mic and see if it stops. If it does stop when you unplug the mic, either the switch in the mic is stuck/went bad or there is a wire shorted in the mic cord to radio plug. Make sure pins 3 and 4 on the mic plug are not shorted. A continuity meter will tell you. If the mic is keyed, pins 3 and 4 will show a short, if the mic is not keyed, pins 3 and 4 should not show a short. If the mic is functioning normally but the radio is still keyed up then there is something in the radio that is shorted.
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If it "keys up" without a mic plugged in, try plugging the mic into the radio and see if it stops. Some radios require the mic to be plugged into it to operate properly. If it is keyed up with the mic plugged into it, try unplugging the mic and see if it stops. If it does stop when you unplug the mic, either the switch in the mic is stuck/went bad or there is a wire shorted in the mic cord to radio plug. Make sure pins 3 and 4 on the mic plug are not shorted. A continuity meter will tell you. If the mic is keyed, pins 3 and 4 will show a short, if the mic is not keyed, pins 3 and 4 should not show a short. If the mic is functioning normally but the radio is still keyed up then there is something in the radio that is shorted.