I could not get my Kenwood TKR-750 to pull down the RTCM pin 4 enough to trigger the COR. The pin4 voltage got down to 1.1v which is not enough. I fixed this by soldering a 540ohm resister from RTCM pin 4 to ground. It was just enough to bring it down to a switching point when the Kenwood sensed my 88.5Hz tone. Shaun zr5s