[Elecraft] K3-KPA500: simplex comms

[Franki ON5ZO]

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