[Elecraft] K3 HI CUR on 20m Only

[Chris Soames]

Hi, hopefully someone can point me in the right direction to cure the 
problem that has manifested itself on my K3.
When I TX using more than 11 Watts on 20metres shows HI CUR on the display.
When I TX using more than 20 Watts on 20 Metres the display shows HI CUR 
for a second or two the the rig switches itself off.
I have tried 2 different antennas and a DUMMY load and the results are 
always the same.
I have tried both antenna connectors, I have also checked the power 
supply and it is feeding 13V into the rig.

I do have the internal antenna tuner fitted, When I tune on 20 metres I 
do not  get the SWR reading at the top of the display. All I get is -- 
at the top and 5.0 W at the bottom with reference to tuning.

With the internal ATU bypassed I get exactly the same problem on 20 only 
HI CUR and switches off.

All other Bands at all power levels are fine, the problem is 20metres only.

Where to look?

Help!!!

Chris G0TZZ