My headphones cable broke recently so I quickly whipped up a 3D printed repair cover (designed printed and assembled within an hour) to protect the soldered joint and provide strain relief. Here is a quick guide on how to re-solder and attach the protective cover (feat. helping Hans :) ).

Supplies

A broken cable

Wire cutters

Wire strippers (or anything which can strip wire such as flush cutters etc.)

Soldering iron + solder

Electrical tape

3D printer and material (ideally FDM/FFF)

A sentient 'helping hands' to hold the wires for you (optional)

Step 1: Find the Broken Thing

As nicely shown by helping Hans, here I am repairing my old headphones (again) where the old cable has fatigued. Its also helpful to be able to change the connector if it wears out, without having to replace the whole headset.

This can also be used for other types cables with more or less wires.

Step 2: Cut Out the Bad Bit

Use the wire cutters of your choice to cut out/off the broken bit (or use the superhuman strength of helping Hans). Here you can see the cable has failed in two places, so make sure all the bad bits are removed and what you are joining together is good working cable.

Step 3: Strip the Wires

Use the wire strippers (or other equivalent tool) to remove the outer sheathing layers. Strip 1cm or more of sheathing for each individual wire.

Step 4: Assemble and Solder the Wires

This is where a helping Hans, pun intended, comes in very useful (typically called helping hands if you were not aware). These allow you to hold the cables firmly together during soldering (this one also has a magnifying glass for smaller delicate soldering or working).

There are many approaches to soldering wires together, and I will admit I am a bit of a novice (as shown by some of the shabby soldering job). Rest assured though that the solder joint does not need to be exceptional, since the casing will protect and provide strain relief for the joint.

My technique is to twist the (matching) wires together, to apply a little solder to the tip of the iron, which has been cleaned and tinned, before touching the joint and applying more solder. Try to avoid heating it for too long as this can melt the nearby insulation.

Trim away the excess wire.

Step 5: Insulate the Solder Joint (and Repeat)

Here I have used electrical tape to insulate each strand of the wire (also trimming away the excess). Heat shrink could also be used if you have it.

Then repeat the process, soldering and insulating the remaining wires.

Step 6: Measure the Cable Width

The 3D printed cover is designed around a 3.5mm cable (as shown above). Measure the width of your cable and calculate the scaling factor as follows: measured width/3.5mm

The default cover also has a cavity length of 24mm (approximately 1 inch), so it will be able to cover a gap in the outer sheathing of that length. If you have a different sized joint which doesn't fit (or a lot more soldered wires which fill more volume) I have added a STEP and the Fusion360 design file so that you can alter the dimensions, or I may do it for you if you ask nicely :).

Step 7: Size and Print the Covers

The STL file is oriented in the correct position for printing and requires no support material. Simply scale the model using the scale factor calculated (measured width/3.5mm). You could also stretch the cover by scaling it in the long dimension.

Duplicate the object in the slicer to print both halves of the cover..

The cover should print fine with any material, although I used PLA for the ease of printing.

Step 8: Apply the Strain Relief Cover

The cover uses plastic 'teeth' on both sides of the joint to grip onto the outer sheathing firmly. One end of the cover has two teeth, while the other has one tooth.

To apply the cover correctly, the double teeth should mate with a single tooth on both ends.

Make sure the soldered wires fit inside the cavity (it probably won't fully close at first) and tighten two zip ties into the grooves on the ends and cut away the excess.

I also like to wrap a little electrical tape around the body to hold it together and make it a bit more waterproof.