Hello Peter,

I've checked the audio waveform out of the Intel NUC PC and the FST4W-120 start is indeed much slower to rise (~140mS) in comparison with WSPR. In the shots below I've altered the timebase to get the full rise of the FST4W onto the screen. It's the only difference that I can see between the two modes at start-up. My amplifier is based on the K3MF design here https://njdtechnologies.net/k3mf-2200-meter-amplifier/ with the FT-817 running at 10.136MHz and followed by a mixer with a GPS locked 10MHz LO all of which works fine on WSPR but trips almost every time with FST4W. I guess VOX might be a solution but at the moment I'm using the FT-817 in data mode with the audio connected to the rear which is my preferred set-up.   

  

I've also successfully used the FST4 modes but am distinctly uneasy about the effect the trips might have on my FETS and/or power supply! It's odd that nobody else from the LF/MF fraternity has mentioned it though. 

73 Chris G4SDG