I had a similar problem on 40m that only occurred when transmitting RTTY.
The K3 would indicate an over current condition and automatically reduce power 
as a result.  The PA also ran much hotter than on the other bands.

So hot in fact that switching diode (D10) burned out on the original PA during 
the welcome preamble of a RTTY net that I was about to run,  Gary was most 
helpful in getting me a replacement PA but this did not cure the problem.

Having had problems with the USB adapters kicking out on 40m RTTY due to RFI in 
the past and curing this problem by placing chokes at both ends of the USB 
cables, I decided to give the same treatment to the K3.

The cure was placing two toroids on the DC feed to the K3, one at the point 
where the DC cable leaves the power supply and the other just before the 
connection to the K3.   

Milton
AD5XD  

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