If it really is 75mV (which is unlikely) then a simple voltage divider can reduce the Arduino's 5V down to that level.
Double-check that your probe isn't overloading the circuit. Does it still function while you're probing it?
Have you heard of LANC controllers? That's an industry standard for controlling video recorders. The Tascam may be using that. Your description of the plug matches LANC.