I have a Sommerkamp FT-277B and a President Jackson. I tried one time to build a Frequency counter  what shows me the frequency in receiving mode. With the newest firmware you can switch the mode between: Normal, FT-277 Low, FT-277 High and Jackson. The ZF Frequency are in the Firmware, see the Source code.

I build it on a universal board. This picture shows the start up

After the first screen it shows the CPU Clock

The first Test on the Jackson

Here is the Manual: Anleitung

Here is the Partslist for Reichelt: Bauteileliste

Here you can download the Schematics: Universal frequency counter circurit

Here you can download the Firmware:  Firmware