A grld-dip oscillator (g.d.o.) is basically a variable high frequency oscillator with a d-c micro ammeter in the grid return circuit' to Indicate relative power. The selected plug-in tank coil « mounted externally to serve as a "probe" that can be coupled appropriately to the circuit or source in question; a complete set of plug-in coils is provided to cover a wide range of frequencies from 400 kc to 250 mc. The tank capacitor is variable and calibrated for eight frequency ranges, one frequency range for every coil provided, As a g.d.o. the 710 can be used to determine the resonant frequency of de-energized resonant circuits or self-resonant components. Indirectly, Therefore, it can also be used to determine value of capacitance, inductance, or Q by procedures that will be described. Since If Is basically a v. f.o., the 710 may also be used as a signal or marker generator. By switching off the oscillator plate supply, the 710 becomes a tuned r-f diode detector with a meter in the diode load circuit. As such, it can be used to determine the frequency of rf energy sources, with the plate supply switched on again, but a headphone plugged into the phone jack, the 710 becomes an oscillating detector. This provides a very sensitive method for determining the frequency of unknown r-f energy sources, namely that of “beating" the unknown r-f energy picked up by the "probe" call against the frequency generated by the Internal variable oscillator.