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.