Topband: Control cable black conductors

[Tom W8JI]

> All the other 4 conductors are bright clean copper when stripped.
> Could anyone tell me why some insulated copper conductors turn black in 
> this way and whether there's a better way of cleaning it off.


Water inside the insulation plus sulfur and/or irons that formed copper 
sulfide or covellite.

I've been successful using phosphoric acid. It is sold as a clear liquid 
wire or mag wheel cleaner around here. You'll know it by how red and painful 
it turns your hands.