[Elecraft] K2 FM-ing and AM-ing on 80 Meters Problem Solved

[Don Ehrlich]

... in case anyone else is having a similar problem ....

My new K2 #4438 had both FM-ing and AM-ing on the low end of 80 meters 
but only above about 50 watts output..  Above 3700 and on all other 
bands it was perfect.  I had mentioned it on the reflector last week 
and received several comments and also a note from Ron ZL1TW who had 
an almost identical problem.

I found with Gary's help (Elecraft) that the problem originated in the 
T-R switch on the PA board.  I won't go into detail here but after a 
lot of testing I discovered that a small capacitor (I used 12 pF) 
connected from the cathodes of D12 and D14 to ground cured my problem. 
Ron ZL1TW tried it and also cured the FM-ing in his K2.

The cause of the problem was probably rf coupling through D14 due to 
higher than normal junction capacitance. This resulted in unusually 
high dc voltage on the cathodes and Q6, upsetting the action of the 
T-R switch during transmit.  This is only a guess ... and I have no 
idea why it occurred only on the low end of 80 meters.  All I know is 
that my K2, and Ron's K2 as well, are now playing the way they should.

Don  K7FJ