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Step 1: Preparation
If you are preparing the cable for a connector, I strongly suggest you put any connector parts on now (the screw on part of an XLR, or casing of a 1/4" jack for example). Get into the habit of sliding these on before you start on the cable, or else you can bet it won't be long before you finish soldering your connector only to discover you forgot to put the connector casing on, and have to start all over again.
Once you have all the connector parts on that you need, you will need to strip your cable. This means removing the insulation from the end of the wire and exposing the copper core. You can either use a wire stripper, side cutters, or a knife to do this.
The obvious tool to choose to strip a wire would be......a wire stripper. There are many types of wire stripper, and most of them work the same. You simply put the wire in, and squeeze it and pull the end bit off. It will cut to a preset depth, and if you have chosen the right depth it will cut the insulation off perfectly. It is possible to choose the wrong depth and cut too deeply, or too shallow, but they are very easy to use.
On the other hand, some people (myself included) prefer to use a knife or side cutters. I use side cutters for small cable and a Stanley knife for bigger cables...and although I have a couple of wire strippers, I haven't used them for years. This may seem odd, but I've got my side cutters and knife with me anyway, and they do the job fine.
If you are using side cutters (as shown here), position them about 10mm (1/2 inch) from the end, and gently squeeze the cutters into the insulation to pierce it, but not far enough to cut the copper strands of the core. Open the cutters slightly so you can turn the wire and pierce the rest of the insulation. You may have to do this a few times to cut through all of the insulation, but it is better to cut too shallow and have to turn and cut again rather than cut the core and have to start again. Now you should be able to slide the insulation off with your cutters, or pull it off with your fingers. This may sound a tedious method, but in no time at all you will be able to do it in two cuts and a flick of the cutters.
I won't explain how I use a knife to do larger cable, as I'd hate someone to slice a finger or thumb open following my instructions. Using a sharp blade like that certainly does have it's risks, so stick with wire cutters or side cutters if you are at all unsure.
If your connector has been used before, make sure you remove any remnants of wire and solder from the contacts. Do this by putting the tip of your soldering iron into the hole and flicking the solder out when it has melted. Common Sense Alert! Please be careful when you flick melted solder...flick it away from you.
Step 2: Tinning
Whatever it is you are soldering, you should 'tin' both contacts before you attempt to solder them. This coats or fills the wires or connector contacts with solder so you can easily melt them together.
To tin a wire, apply the tip of your iron to the wire for a second or two, then apply the solder to the wire. The solder should flow freely onto the wire and coat it (if it's stranded wire the solder should flow into it, and fill the wire). You may need to snip the end off afterwards, particularly if you have put a little too much solder on and it has formed a little ball at the end of the wire.
Be careful not to overheat the wire, as the insulation will start to melt. On cheaper cable the insulation can 'shrink back' if heated too much, and expose more copper core that you intended. You can cut the wire back after you have tinned it, but it's best simply not to over heat it.
The larger the copper core, the longer it will take to heat up enough to draw the solder in, so use a higher temperature soldering iron for larger cables if you can.
To tin a contact on an audio XLR connector, hold the iron on the outside of the the contact for a second or two, then apply the solder into the cavity of the contact. Once again, the solder should flow freely and fill the contact. Connectors such as jacks have contacts that are just holes in a flat part of the connector. To tin these you put your iron on it, and apply the solder to where the iron is touching. The solder should flow and cover the hole.
Once you have tinned both parts, you are ready to solder them together.