Topband: Control cable black conductors

[Dave G4GED]

Hi and thanks for reading.

I'm trying to splice a damaged (rodents) RX ant control cable.
It has 7 insulated, stranded copper conductors all inside a PVC jacket.
Problem is, when stripped of their insulation, 3 (black, brown and 
green), of the copper conductors have become coated in a black film? So 
to effect jointing and soldering, require cleaning.
I've tried IPA 170 and several contact cleaners but none remove it.
So far, only Emery Cloth will do the job but it's very difficult to 
clean each strand without breaking some and therefore weakening the joint.
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.

Thanks in advance.
Dave

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