Step 1: The parts
A diode. Germanium point contact, OA79 or 1N34 is the preferred and traditional choice. But a modern substitute will be a schottky (gold bonded) diode. These have a low forward voltage, below 250 millivolts or so, versus the 600 or so of the silicon diode.
A few capacitors, chip type, value not critical, around 1 nf to 100 nf. (or 1000 pf to 0.1 microfarad)
One resistor, 1 Megohm.
One stereo earphone jack.
One pin from a turned pin IC socket. This forms the 'hot' end of the probe, so that various pins can be inserted for probing. A short piece from a needle for exploratory work. A piece of wire soldered to the circuit board when using it for a tuning up session, requiring it to be in place for a long time.
The circuit is traditional, as below:
C1 R<--||-------/\/\/\/\-------------o + | | --- | ^ | /-\ diode === C2 | | | |<--------------------------------o -
Step 2: Connect it up
The socket, capacitor and supporting board will go inside the barrel of the jack.
Step 3: Take the jack apart
We need just the outer shell and the cover - It is best that the cover of the jack is metal, otherwise the shielding will not be perfect.
Step 4: Form the body
A molded mono earphone lead was used to get the signal out from the probe.
Step 5: Put it all together
A HAM friend of mine measured about 5pf of capacitance for this probe, which is pretty good.
In use, a sharp probe is inserted into the turned pin socket. The ground lead is wire wrapped around the barrel of the jack. This is necessary if accurate measurements are to be made in a noisy environment.
I hope this will be of use to the HAMs among us.