http://elecraft.85.s1.nabble.com/KPA500-suddenly-low-output-only-30-watts-tp7600292p7600305.html
When you key the K3, does an asterisk show up on the left side of the KPA’s LCD? If not, check the AUX or PTT connections to the KPA500.
Otherwise, let me know so I can follow up with you to get you going.