[Elecraft] queer behaviour !

[f9oj.7]

Hello
A good friend of mine, F6CVM, has a K3 as well as an Icom transceiver. He 
uses an old Heath SB 230 as an amplifier.
When he connects Icom/SB 230, no problem.
But if he connects K3/SB 230, as soon as he plugs the cinch into the K3 key 
out output , (with no drive), the  amplifier sets on the transmit position !
Did anyone meet the same problem or has a clue about it ?
Thanks for any idea...
Best 73 to all
Jacques de F9OJ