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Evonet

Cambridge
CB24 8TX, UK.

Repairing a damaged flex

09/10/2017

Flexible cables get damage – by getting jammed in a door, chewed by a dog, accidentally cut, and sometimes, they’re not long enough – or you may need to pass a flex with plugs on both ends through a small hole.

You can of course go to a hardware store and buy a product specifically to join two cables – but they only work for two (or sometimes three) wires, and they are bulky.

Today, we’re going to learn a better way, which you can use if you have a soldering iron or some small-diameter copper tubing and some heat-shrink sleeving.  With care, you can use the method for almost any kind of cable, from mains cables, to USB cables to HDMI.

You will need the following tools:

  • Wire cutters

  • Wire stripper (or Stanley knife – be careful!)

  • Copper tube (various sizes), or soldering iron and solder

  • Cutting mat

  • A piece of heat-shrink sleeving large enough to go over the whole cable, about 4” (10cm) long.

  • Pieces of heat-shrink sleeving about ¾” (16mm) long the right size to go over each of the wires in the flex.

Even if the wire is only partly damaged, cut it through completely.  If there is a damaged section, cut it off.  (Of course, your flex is going to end up a little shorter.)

Thread the larger heat-shrink sleeving onto one of the pieces, and slide it well away from the end.  (You can put some sleeving on the other piece too, if you want to be doubly-sure you’ve got a safe result.)

Cut and strip the wires on the two cut ends like this (roughly life-size):

(Use a similar approach if you have more than two wires in the flex to join – just make sure the longest on one side is the shortest on the other, the next longest on one side the next shortest on the other, and so on.)

Twist each of the copper wires so that the strands stay together. 

Thread each of your smaller pieces of heat-shrink sleeving onto a longer tail on one of your pieces of wire. 

 

Crimping method

To crimp, select an appropriate size of copper tube, and cut about 13mm off, using a Stanley knife blade rolling the tube on a cutting mat.  Repeat for as many wires as there are in the cable.  Brass tube may also be used, but you will need to saw it.

Carefully push one wire into the tube, making sure that you include all the strands.  Crimp the tube about three times, by squeezing the tube with the wire cutters, sufficiently to grip the wire firmly, but not so firmly as to cut the tube!   (You can also use a professional crimp tool, but it may be difficult to find the right size jaws.)

Make sure that you have everything you’re going to need threaded onto the flex, because you’re now going to join the two halves.

Put the second piece of wire into the tube, and crimp it like the first.  Give a strong tug on the wire, to make sure that it is held tight.  If the wire comes out, start again with a new piece of tubing.  Repeat for all the other wires in the cable.

Note: For large high current wires, choose a piece of tubing of sufficient diameter to take both wires side by side, so that you do not rely on the current capacity of the tube itself.

Soldering  method

Twist each of the copper wires so that the strands stay together.  Working over your work surface (in case you drip some hot solder), tin them with the soldering iron and some solder, so that all the strands are covered with solder.  Melt a tiny bit of solder on the tip of the soldering iron, and touch the tip on the copper.  Touch the solder on the other side of the copper and let it melt in between the strands.

Blow on them to let them cool for a few seconds.

Thread one of your smaller pieces of heat-shrink sleeving onto a longer tail on one of your pieces of wire.  (If you didn’t let the wire cool after tinning it, the sleeving will shrink as you put it on!)

Make sure that you have everything you’re going to need threaded onto the flex, because you’re now going to join the two halves.

As you need three hands for this next step, get a friend to help.  Have him/her hold the two parts of the flex on the work surface, so that the first two wires touch, like this:

Taking care not to burn your fingers (or those of your helper!) touch the soldering iron on the two wires where they touch.  If necessary, add a little bit of solder – but try to avoid sharp spikes or edges that might cut the sleeving.  Take the soldering iron away without moving the wires, and wait a few seconds for the joint to cool.

Once you are happy with each joint, slide the small heat-shrink sleeving over the joint, and heat it with the heat gun from both sides until it shrinks evenly over the joint.  Repeat for each of the other wires.

If you have judged your lengths correctly, you should now be able to slide the large piece of sleeving over the whole joint, leaving at least half an inch extra at each end, and shrink it into place.  (If you have a second large piece on the other end, wait for the first to cool before you slide this on and shrink it into place.)

Your cable is now as good as new!

Chris Moller, MIET, 09Oct2017

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