A diode detector is three components, a low value capacitor, 1 NF, that connects the signal to the diode. The diode is also connected to your meter earth. The junction of the diode/cap then goes via a 10 K resistor to the meter live. Depending on which way the diode is connected the meter "live" could be + or -. If you want it +, then connect the diodes cathode (banded end) to live. The best sort of diode is a germanium point contact (lowest voltage drop), then a hot carrier diode then a silicon switching diode (highest voltage drop). Power rectifier diode will not work.
Frank