[Elecraft] K2 RF Feedback

[Stewart Baker]

Hi John,

The capacitor goes between pins 1 & 4 of U3 (SSM2165). It should be mounted on 
the bottom side of the KSB2 board. The RF choke value is not critical and can be 
anywhere between 1mH and 3mH. If you use one of a reasonable size it can be 
easily fitted by unsoldering and lifting out one end of C32 . I then put a small 
length of component wire into the empty hole and soldered the RF choke between 
the floating end of C32 and the component wire.
The reference to PI filter is because the new circuit when drawn resembles the 
Greek letter PI (can't find it on my keyboard !)
The circuit consists of C33 as the left hand bar of the character, the RF choke 
as the crossbar, and the new (0.001u) capacitor as the right hand bar.
Most Amateur radio textbooks will show examples of PI circuits, the most famous 
being the PI network useful for impedance matching.

I have found that RF feedback can be a difficult beast to track down. Sometimes 
it can occur when the antenna/feeder arrangement changes only very slightly.
A setup that works well one day may play up the next if for example if you tidy 
up the the shack wiring (an activity never performed here ). 
The other area that the K2 is very sensitive to is magnetic coupling from Power 
supply transformers. This can badly affect both transmitted and received 
signals. It is worth looking at earlier posts on this subject.

73
Stewart G3RXQ


On Wed, 14 Jul 2004 10:41:40 -0500, John Rader wrote:
> Hello Stewart,
> Thanks for the suggestion.
> I occasionally get RF into my audio as well. I am going to put a choke in
> series with C32 as you suggest.  How critical is the chokes value?
> Most audio reports are good, but occasionally I recieve a bad one.
>> I have also added a 0.001uF cap directly across the compressor input pins.
>> This PI filtering should stop any RF "dead in it's tracks".
> Do You remember which pin numbers on the compressor chip you placed the cap
> across? Can you explain the PI filter as well.
> Thanks,
> John K5XTX
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