I'm using a HL2 with a 144/28 transverter to work weaks signals on 2 meters.
Therefore I have a VAC from RS to MSHV

Works fine but I also want to listen to the signal and 'assumed' the RX button in the monitor would let me listen to it but apparently is doesn't

What is the purpose of the RX button ?
Listening while TXing works fine!
73's TooN