This is very simple yet very useful tool for debugging new pedals. You need one 1/4″ mono plug, one 100nF capacitor, alligator clip and some wire. Connect alligator clip to jack shield and 100nF cap to jack tip. When debugging some circuit connect clip to circuit ground and use other end of cap (or wire connected to other end of cap) to follow signal path through circuit. Plug the probe into amp and plug guitar cord to pedal input (pedal output stays unplugged). Make someone play guitar while you’re probing circuit and once you find a spot where nothing is heard through amp you found a possible problem.
If you have a cap small enough you can fit into plug casing. It will look nicer and cap will be protected inside. It can be seen on this photo how I did it. If you can’t find any slave around to play guitar while you’re probing the circuit it’s not a bad idea to connect guitar cable to sound card output and just play some music on computer.
The other way to make this useful tool is using 1/4″ jack socket and regular guitar cable. Advantage over previous design is that you need shorter probe leads and you can use as long guitar cable as you need.