Old RCX Wires Have Become Grey and Brittle I have a LEGO MINDSTORMS Robotics Invention System 2.0 (3804) that I acquired back in 2003 (17 years ago), and the wires have become grey and brittle.

The wires used to be black and smooth, and stayed that way for a solid 10 years until I stopped playing with this set in the fall of 2013. Last summer (July 2019), I got it out again for the first time in 6 years, only to find that the wires had become grey and brittle. I tested them, and they seem to be working, but I don't know how much longer that will be the case. At one point, I yanked on one of the wires a bit when detaching it from a motor, and it broke very easily. Clearly, even though the wires still work, they're not as versatile as they used to be.
Any ideas what caused this? Was it the long-term storage in the box? Was it that there hasn't been any current running through these wires in years? Or is it just inevitable that this will happen as the set ages? They've been in the box, sitting in a room at room temperature for several years.
Has anyone else with an old RCX-era MINDSTORMS set experienced this problem? Any suggestions on what can be done to stop or slow the deterioration of these wires, or perhaps even reverse the effects? It would make me sad if I couldn't use this set anymore. I had a satisfying fill of fun with this set from age 6 to 16, but I'd also love for my future posterity to get to enjoy it.
This is a common issue with the older Lego 9V cables. Lots of sources, including myself, have experienced the same thing - this particular chemistry of insulation ages poorly, no matter how well it's stored. Cables made later, circa 2004-2006 don't seem to have this issue - the insulation is less rubbery and more plasticy, and is still shiny like new to this day.
As far as repairing them goes, it's fairly simple and cheap, albeit a bit tedious. See this reddit thread and this YouTube video. The procedure which worked for me is as follows:

Use a thin flathead screwdriver or knife blade to pry off the back of the connector. There's a little tab that holds it in place.

The inside of the backplate has two ridges for guiding the wire out the back of the connector. Sand/cut these off - you'll need as much space to work with inside the connector as possible. I used a dremel.

The old wires are not soldered to the connectors so they are easy to remove. They are held in place by two tiny forks which are pushed through the insulation to connect the metal inside.

There is a little post in the center of the connector which the wire folded around. Twist it off with some needle-nose pliers.

Cut some 24-gauge speaker cable to size. Does not need to be speaker cable, but most speaker cable I've seen has two leads side by side, like the original RCX cables. You can even use two separate insulated wires if you can't find double wire.

Attach the new wires to the metal contact pads inside the connector. This is the hardest part, because the amount of space inside the connector is very small; remember you have to put the backplate back on when you're done. I could not achieve the sharp fold of the original wires, so don't bother trying. I also could not achieve the solderless connection with the little metal forks - so I soldered the new wires to the pads.
TIPS FOR SOLDERING:

Use high heat and as little time as possible to melt the solder to the wire and contact pad. Applying heat for more than 1-2 seconds will deform the plastic on the top of the connector. No pressure; if you need to add more heat/solder, just make sure the contact pad is cool to the touch before you try again.
I like to ball up the bare wire at the end of the speaker cable and stuff it into the back of one of the metal studs, then melt solder over top.
Don't bridge the gap between the two contact pads!


Once both wires are attached, add a small bead of plastic glue or epoxy on top of the two contact pads and the cable. The glue transfers force from the cable to the connector body instead of to your crappy solder joints.

Reattach the backplate. It should snap into place with a little force, use the screwdriver again if you need to. Mop up any of the glue that squishes out around the backplate - especially make sure the metal contacts on the underside of the connector are clean! Test to make sure it works.


As I said, it's tedious, but this procedure is effective and creates a strong connection between the cable and the connector body. I'm currently repairing about 40-50 wires using this method to get a little more life out of a Mindstorms classroom set.
The RCX light sensors from this era suffer a similar problem because their cables are attached to the sensor body. They can be successfully repaired in a similar way (refer to that YouTube channel I linked).