I have a Simpson 260 series 3 that works on all functions except the RX 1 ohms position. I have check and replaced all batteries. Clean all contacts in and out. the other two ohms connections work. also check all resistors.
Look at the Pic below:
The unique paths for the Rx1 are the 3 switch contact positions number 10 (marked red) and a single wire (marked green).
Check them for discontinuity.