Without access to a Spectrum Analyser or an Oscilloscope it can often be quite difficult to get circuits to work. This circuit is for a basic 10K probe that can be used to detect the presence of RF and even give some indication of the level of RF. It will not tell you the frequency (you need a good RF frequency counter for that), but at least you will know if the circuit is oscillating or not.
This probe is useful for any low level RF work, and simply connects to your multimeter. The voltage shown will not be accurate, since this is a rectifier probe, but the measurements are good enough for you to be able to determine where the RF stops, or if a stage is not giving the gain you think it should.
C1 = 10pf. C2 = 1nf. Both cermic disc type.
D1 = 1N34 or similar Germanium point contact detector diode or a "Zero Bias" silicon diode.
R1 = 10K 5% (¼W should be plenty).
A length of coax (enough to give you a reasonable distance between meter and item being tested) The coax can be anything that you have to hand. In fact, high capacitance cable that is useless for anything else can be put to good use.
2 plugs to suit your meter, 1 red (+) the other black (-).
A short length of stranded hook up wire (30cm or so).
A crocodile clip.
An old, large bodied pen (a large marker pen is ideal, as long as C1 & C2 will fit in).
A copper tack or small piece of brass rod, sharpened.
Some heat shrink sleaving (large enough to fit the O/D of the coax or insulation tape.
Connect it up to your multimeter, clip the crocodile clip to the ground of the circuit you want to check and probe away. With DC-Volts selected you can get a reasonable indication of the RF voltage level.
To use the RF Probe for signal tracing in a malfunctioning RF circuit or a homebrew circuit, connect the aligator clip to a convenient "ground" or "common" point in your circuit. Often this is the chassis. Most of the time, you'll be probing at the base/gate, emitter/source, or collector/drain of a transistor, one either side of a coupling capacitor or transformer, or at the input or output of an IC. Because the circuits' RF must overcome the diode's barrier potential (of 0.25V, for a 1N34A), voltages much less than that won't read at all, and voltages less than about a volt won't read very accurately. Typically, RF and post-mixer-amps in receivers don't have enough RF voltage, unless you inject a very strong signal at the input.
If you want the probe only for HF circuits then you could increase C1 a little (to say 47pf) so the sensitivity becomes more uniform at lower frequencies.
For use with lower frequencies (a few MHz only), C1 can be increased further in value, but I would not go above 100pF.