Yeah Bernard, I suppose that over a period of years as the unstable PVC in that particular cord continued to decompose and release chlorine that the effect on the copper would spread farther inward from the surface. The wire would increasingly become cupric chloride compounds instead of pure copper. Fortunately the usual PVC jackets don't cause a problem and even decades-old cord still has bright copper when cut open.

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Enjoy the music, not the equipment.