I hope something isn't broken (smoked)...

A few weeks ago I bought a second hand KPA500. I ordered the KPAK3AUX cable and 
hooked it up. Comms were working both ways: amp follows rig, rig follows amp.

The first real RF test was last week in the ARRL DX contest. I noticed that 
sometimes I heard a cracking noise in the headphones, which made the KPA500's 
display flicker. In many cases the display was set to "18.1 MHZ" and the amp 
lost control. From then on, the rig switched bands by 'RF sensing' i.s.o. 
following the K3. If I changed bands on the rig, the amp would follow but not 
the other way round. Resetting the amp solved the problem.

Today I had the same problem, but now it worse. Now there is only one way 
communication from rig to amp. And when I switch between operate/stdby on the 
amp, the K3's display doesn't reflect the status with a message. So the amp is 
unable to talk back to the rig. I found out that by switching off the K3, I 
regain access to the amp's band buttons. But then the PTT asterix shows (with 
the rig OFF!). For a while at least (few seconds), then the amp toggles the 
bands itself in random order, only to freeze on a given band. But when I turn 
the K3 on, the amp immediately switches to the band the rig is set to.

I assume the root cause is the crack/rattle I hear when sending, which started 
it all. The noise occurs only when I key the rig for longer periods. Short 
burst like "call 5NN kW" don't trigger it. Long transmissions do (long CQs, 
ragchew etc).

I'm afraid something is broken. I hope there's an easy fix and an explanation?

- Franki ON5ZO
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