Campbell Scientific VSP3 Vosponder Voice Radio Interface - Manual (Page 48)

Appendix C. Audio Level Controls
C-2
at pin 6 of J6, increase the output level until peak clipping occurs then
back off on R24 approximately 20% (2-3 turns). This will provide for
optimum voice clarity.
R25 Receive Level – has a very broad acceptance range and is set for the
optimum level at the factory. Should adjustment of this control be
desired in the field a PC running a terminal program such as
HyperTerminal® is required.