[Elecraft] RX In/Out for External Filter

[Ken Widelitz]

The manual indicates you can "patch in" a narrowband filter between the RX
In/Out. 

When I did this with other radios, I had to remove the jumper between the RX
In/Out and insert the narrowband filter. There is no jumper on the K3, so
how does it know there is a filter there? 

The application is for a filter for the bottom of the 10 meter CW band so I
can use the other radio high in the CW band or for SSB.

73, Ken, K6LA / VY2TT