[Elecraft] K2 - Unstable power output on 40m (digital modes)

[Don Wilhelm]

Brian,

Do things settle down if you drive a dummy load instead of the antenna?  
If so, that would nail it as an RF Feedback.  If that is the case, use a 
good quality common mode choke at the junction of the coax with the 
ladderline.  See K9YC's RFI information at 
http://audiosystemsgroup.com/RFI-Ham.pdf - chapters 6 and 7 cover making 
effective feedline chokes.

Also investigate your KPA100 to see if it has the latest upgrade. The 
T/R switch was changed to eliminate instability on 40 meters.
Remove the right side panel of the K2 and peer between the KPA100 shield 
and board.  If you see blue toroid cores at RFC1 and L16, you have the 
updated version.  If you see red cores, update with KPA100UPKT.
You may also have to update the shield.  If you do not have shield clips 
to connect the shield to the sides of the base K2 and a shield over the 
speaker magnet, add the KPA100SHLDKT.

73,
Don W3FPR


On 10/25/2015 1:45 AM, Brian - N5BCN via Elecraft wrote:
> I'm having an issue with unstable power output using digital modes on 40m
> above about 20 watts.  All other bands seem to be ok.
>
> My antenna setup is as follows: a loop consisting of about 245 feet of wire,
> feed line consists of about 21 feet of 450 ohm ladderline to an Elecraft BL2
> balun, then about 3 feet of coax to KAT100/KPA100/K2.  I've tried the balun
> in 1:1 and 4:1 configs with no apparent change.  The feed point of the loop
> is in the attic above the shack, but the majority of the wire is outside
> running along a wood fence.
>
> No problems tuning any band including 40m.  When using digital modes, power
> output and ALC action (no bars) appears normal on 10 through 30m, and even
> works ok on 80m.  Haven't tried 160m.
>
>