[Elecraft] K3 RF Feedback Problem

[Dave G4AON]

You "can't" have RF feedback if you are using a dummy load, unless one 
of the connectors/leads inside the K3 isn't properly connected - 
allowing high levels or RF inside the cabinet.

As per Stewart's comment, have you tried a directly connected mic and 
also are you sure that power supply isn't causing a problem?

73 Dave, G4AON
K3/100 #80
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Have you tried just running the mic into your K3 without the
external station controller ?