On the radio, the HF+50 MHz antenna TX/RX relay is located on the HF-PA unit board. From there, a small coaxial cable carry the RX signal to the Main Unit board.
The modification consists in cutting this small coaxial and inserting in this line a SPDT mini relay which allows to select the received signal from
the main antenna or the signal from the newly added BNC connector. The relay is activated by a mini switch fitted at the back of the radio. See schematic below.
On the back side of the radio a BNC connector is added to connect the RX antenna and a switch is fitted to select between RX antenna or main antenna.
See the implementation below:
Inside the radio the small relay is glued onto the back side the radio. See the cabling sketch below:
You can also solder two 1N4007 diodes back to back to the RX antenna connector to protect your receiver from strong signals.