get a momentary on off switch from radio shack, unsolder the hot going to the jack and solder it to one lug on the switch, now solder a wire from that same lug to the hot lug on the jcak, now take the other lug on the kill switch and solder a wire from that to the ground lug on the jack. that should be it, I put a kill swith in my RG 7420 not that long ago and I love it.