Elecraft K2 KAF2 filter alignment

2 min | Bryan Nehl | 2020-10-18T05:02

I recently had my Elecraft K2 open doing some work on it. I thought I would take the opportunity to align the KAF2. I don’t have a spectrum analyzer. However, I was able to use a browser based audio spectrum analyzer. I connected the output of the K2 to the microphone in line of the station computer.

I used this browser based Sound Spectrum Analyzer to do the alignment. You will need to allow it to use your microphone.

I later found this more fancy Signal Generator and Spectrum Analyzer

Zero beat

Start with the KAF2 set to OFF. Preamp OFF and Attenuator OFF. Antenna, disconnected. RF Gain fully clockwise and AF Gain adjusted to register on spectrum display. Select widest filter option. S1 should be set to IN position.

In order to do the alignment properly, you need to be zero beat on a signal. I tuned the K2 to the birdy at 7Mhz. Then I activated the spot feature. I could clearly see 2 peaks on the spectrum analyzer. I carefully adjusted the VFO so that there was only 1 peak.

spectrum KAF2 turned off spectrum KAF2 turned off

Spectrum Analysis with KAF2 turned off
Notice all of the noise to the left of the peak.


Consult the Elecraft KAF2 manual. Make sure you have your filter configured for the two stage option. Check jumper W1. It should be open. Also, note that there are two pots on the filter to adjust. Turn the first stage of the KAF2 filter on (AF1). Adjust the R1 pot to peak the output.

KAF2 filter first stage tuned KAF2 filter first stage tuned

KAF2 filter first stage tuned
Note the dramatic drop in noise above the peak. Enable the second stage of the filter (AF2).

KAF2 filter second stage tuned KAF2 filter second stage tuned

KAF2 filter second stage tuned
After adjusting the second stage (with R2) the peak is even more pronounced than before.

There is some interaction between the stages. Alternate between adjusting the first and second stage until you are satisfied that it is calibrated for peak performance. The Elecraft manual suggests doing the BFO and filter cap setup CAL FIL at this time.