[Elecraft] K3 Rear panel PTT-in RCA jack doesn't work

[Jerry Flanders]

I am beginning to wonder if my K3 Rear panel PTT in RCA jack has a 
problem. It appears totally dead - I can't get it to switch the radio 
to tx. Also, it has no voltage across it that I can pull down by 
shorting the pins

I must have some configuration option screwed up, but have tried 
several changes to no avail.

My front mic socket PTT works, as does both the radio PTT command on 
the CAT and the RTS line when I set that option.

What am I doing wrong? How can I prove to myself that this jack is OK?

Jerry W4UK