29 October, 2013

Elecraft K3 modifications

There aren't that many modifications that you can do to the Elecraft K3. This is very different from the K2 as in my list I now have 138 different modifications for it. But Elecraft does have a few K3 enhancements and mods on their home page and here are two additional modifications that I have done to my K3.

Plug-in roofing filters on
main RX board
The first one is to add a wideband LC-filter (roofing filter). The filter was inspired by ideas from W5DHM with three tuned sections at the IF frequency of 8.215 kHz. It is to the right in the image. It is not the best of filters, and probably compromises performance somewhat, most likely because of its low image rejection 30 kHz away. It has however served me well as a receiver filter for the latest version of K1JT's software WSJT-X. That software processes a 4 kHz band for both the JT65 and JT9 digital modes, and the LC-filter has demonstrated to me the utility of having a wide roofing filter for reception of those modes. The filter also works well for listening to broadcast AM which was what W5DHM designed it for in the first place.

Back panel of the K3
The second modification allows for a connection to the P3 Panadapter without using the recommended KXV3A Interface. All that is needed for the P3 is simply a minicoax with proper terminations. On the underside of the main PCB it should have a plug that connects to the J66 P86 connector (see also page 10 in the KXV3 manual). In the other end it is soldered to a BNC connector on the back of the K3. I put mine in the hole reserved for the REF input, as I don't have the K3EXREF External Reference Input option. The BNC connector was marked with a label that says "IF out" as seen in the image.

Both of these modifications are temporary, and as a matter of fact the last one was just replaced by a KXV3A after several years of service. I needed the KXV3A for the PR6-10 preamplifier. I also plan some day to replace the wideband LC-filter with either the KFL3A-6K 6 kHz AM/ESSB filter or the KFL3B-FM 13 kHz FM filter.


  1. Hi Sverre, do you have a schematic of your LC roofing filter with component values please ? I am keen to make a wideband filter for my K3 to use on data modes such as FT8 and FT4.


    Bob Hammond G4DBW

    1. It's a long time since I bought the Elecraft 4 kHz filter, but I'll take a look at my notes to see if I can find filter details.

    2. I built the "K3 SWL Filter by Dave W5DHM" and I have the documentation in a Word-file that I must have received from him. I see that I replaced this LC filter already in 2014 with the Elecraft FM filter. Email me and I'll pass the document on.