AFOL Master Builders Program

9 Volt Cables – Replacing Broken Wires.

The 9 volt train system was introduced by LEGO back in 1991, replacing the existing 12 volt system that had been around for a number of years. Along with a move from inner ‘power rails’ to running power through the main rail itself LEGO switched from using a conventional style ‘plug & socket’ arrangement to using a ‘brick’ style plug that attaches to special power studs.

While this was certainly an improvement in playability, in the longer term the lack of screws made rewiring these plugs a lot more difficult than their predecessors.

Moving forward to 2007, LEGO have again overhauled their cabling system, this time moving to a 4-core system, allowing for both power and data (Power Functions). As the old cables are no longer in production, they are becoming more and more a precious commodity. Unfortunately, as time does with many things, many of these cables are breaking down. In particular, the sheathing is notorious for splitting and breaking off. Fortunately though, while these brick plugs don’t have screws, they are not a sealed unit, so repairs are possible.

Opening the plug
At the back and front of the plug you will find small click-in lugs that hold the lower outer shell to the upper shell. Using a very small screwdriver, lever these free on one side only, the easiest is usually the cable side. Once both are free the under shell will come free.

How it works:
Once you remove the base you will see the two important parts of the plug.

1.The ‘Hump’ - This lump acts as a retention device to stop damage to the plug from people (i.e. children) who pull on the cord to detach it.
2.The ‘Teeth’ – These metal prongs bite into the cable, making the connection from the cable to the inner metal shell.

Replacing the cable
There are many options in terms of cable type you can use, the best we’ve found for the task is 14 strand light duty Speaker wire. Once you’ve measured and cut off the required length you need, shave down the last 10mm of the cable without actually exposing the wire.

The cable sheath needs to be thinner on the top, bottom and sides to enable the Teeth to penetrate the cable cleanly and for the cable to fit over the Hump with enough clearance to close the two halves together properly. This may take a number of attempts to get right, so we’d recommend always allowing an extra few centimetres of cable to work with.

When the cable is ready, use a screwdriver to push the cable gently into place. Once it is bent over the Hump, similarly to the old cable, replace the lower half of the shell and apply even pressure until it lock into place.

Polarity & Testing
Always make sure that you have inserted the cable the correct way up, we normally recommend making sure the cable strip is on the bottom at both ends. Once in, that’s the hard part done, now test it!

There are a couple of ways to test, the usual one people use is, once you have completed both ends of the cable you plug it into a battery box and a light or motor and see if it works. This can pose a problem in that if it doesn’t, you then have to work out which end is the faulty one. The other, smarter, way is to use a multimeter and test the circuit once you’ve put a plug on one end. You may need to expose a small section of the open cable end for testing purposes, but you can then cut a clean end once done.

Notes:
Length - While you could make a cable as long as you like, you are going to lose power the longer it is. LEGO only made these cables up to Three metres in length, so we don’t recommend going much beyond this.

Colour – LEGO 9v Plugs all came in Black… except for the ones that were yellow… but all used Black wiring. While typical speaker wire comes in Grey, The LEGO purists out there may want to try and source all black cable. Just be aware of the polarity of the cable when putting on the second plug.