[Elecraft] KPA500 - AC hum in the monitor

[Jack Berry]

Hoping you all will have some good ideas on a cure for this problem.
I am hearing AC hum in my monitor headphones - ONLY when transmitting. 

The hum comes and goes instantly when the KPA500 is switched on/off. Same with other headphones.  No question it's related to the amp but not necessarily the fault of the amp.
 
I have gone over every connection related to the amp with no change. 
Fortunately it hasn't made its way to my signal. 

Help wanted!

Jack - WE5ST