Home Forums General DIY Pedal Discussion How to build an Audio Probe

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    Barry
    Keymaster

    In this post, I will show step by step how to build an Audio Probe. Using an Audio Probe is described in another thread, located on the Old Site HERE

    To build an audio probe, you will need a few basic supplies. These are:

    • 100nF (.1uF) Capacitor, preferably ceramic disc type, as it is likely to be small. 16-100V rating is fine.
    • A few feet of stranded wire, preferably 2 colors such as red and black or red and green. Gauge is not critical, but we’ll say in the 18-24 gauge range is fine.
    • A female 1/4 inch diameter Audio jack that is designed to be soldered onto the end of a wire; not the type that mounts into an enclosure
    • An alligator clip or similar type device to hold the ground wire onto the circuit’s ground
    • A paper clip, piece of solid core wire; something to use as the probe tip which won’t bend very easily. If you have an old multimeter test lead, that would be perfect, and it already has the wire attached. In this post, I use a straightened large paper clip.
    • Some heat shrink tubing (not necessarily required, but will help prevent a short and will provide wire stress relief.
    These parts are seen below:

     

    The female audio jack unscrews, to reveal the terminals. Mine is a stereo jack, so it has 3 lugs. We will only use 2 of those. Solder the ground wire to the lug for the sleeve, as seen below. You will notice in this picture that I put some green shrink tubing on the 2 wires, which I actually ended up removing a few steps later, so for now, ignore the heat shrink tubing, and just note that the ground wire is now soldered directly to the sleeve lug of the jack.

    Next, solder one lead of the capacitor to the tip lug of the female audio jack as seen below:

    Then solder the probe wire to the other lead of the capacitor:

    As I mentioned previously, I removed the green heat shrink. This was so I could take a larger diameter of heat shrink tubing, and use it to cover the solder joints of the capacitor. Here, I am about to slip the white heat shrink over solder joints and capacitor:

    And after heating the tubing a bit, it shrunk down over the cap and is insulating the solder joint from touching the adjacent ground wire.

    For some stress relief on the wires, I put a couple pieces of heat shrink tubing over both wires before screwing the female. Here’s a picture of everything about to be screwed together:

    And a picture of this end, assembled:

    Next, we’ll solder the alligator clip to the end of the ground wire. I recommend that you use the alligator clip’s clamping clips to grab the insulation of the wire as this will prevent the wire from being stressed right at the solder joint. And here’s a picture with that step completed:

    If you used a multi-meter test lead for the audio probe wire, then you are now done, and can ignore the rest of these instructions with the exception of the last paragraph which provides some guidance for safety. I had a multi-meter test lead that I intended to use for my audio probe, but since I could not find it tonight, I decided that I would make my own using a large paper clip and some heat shrink tubing, as described next.

    We are adding something rigid to the end of the wire that can be grabbed and aimed rather than trying to probe with something flimsy. I used a straightened paper clip, and bent the last quarter inch back onto itself, and did the same with the stripped wire. I locked them together and then soldered at a higher temperature than I typically use for circuit boards. This is because the large paper clip is fairly thick, and its length tends to pull heat away from the solder joint. We don’t want a cold solder joint on our test probe, that’s for sure. Don’t worry, the paper clip can withstand much more heat than many electronic components. Once you’re done, turn that soldering iron back down before you overheat your next circuit board.

    Here’s a picture with the paper clip and wire soldered together. I didn’t worry too much about cleaning up the residual flux, but I suppose you could if you wish.

    Next, I used 2 different diameters of heat shrink tubing to cover most of the length of the paper clip. This is to reduce the chances that it will contact other components by accident as I am probing around inside an enclosure, and to increase my ability to grip the probe since the paper clip by itself is pretty skinny and is fairly slippery. The heat shrink tubing may also provide some stress relief to the wire right at the solder joint by adding a little more stiffness.

    Here’s a picture of the competed Audio Probe. By the way, I tried it out, and it works!  :)

    Obviously, this audio probe is designed to accept a guitar cable. You could also take an old guitar cable, cut off one end, and solder on a ground lead, a capacitor and a probe lead to that. Then you would plug the 1/4 inch male plug directly into your amp while testing.

    Personally, I think the design in the photo essay above is a little nicer, as you don’t need to destroy one of your guitar cables, you can hook up any length guitar cable you wish, which is nice if your amp is 25 feet from your soldering iron, and this design takes up almost no space when not in use.

    Courtesy of Bruce R.

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