[Elecraft] KX3 - Hi Sig warning

[Graham g3tct]

Bit of a weird effect found here. I am using an external 2m transverter 
to 28MHz. I noticed that when switching between VFOs using the A/B 
switch from (eg) 145.7875FM to 144.430 usb, the KX3 says “Hi Sig” and 
disconnects the preamp. This still happens with nothing connected to the 
KX3's antenna socket. It also happens on 50MHz and 21MHz – I haven't 
checked all the bands.

I found the VFO with FM mode has to be 120kHz or more above or below the 
freq of the VFO in USB mode. It only happens on the transition from FM 
to USB, it doesn't happen for FM to other modes. And it doesn't happen 
on the transition from USB to FM. As I say, weird.

Graham