Old cable and voltage drop

[atferrari]

Two days ago, when applying +5V to a couple of LEDs, I measured an unexpected drop in the applied voltage of more than 0.3V.

After opening the PSU, checked the fixed regulator in charge, a 340 - 7805, and found it still happily giving the same 4,99V as when assembled 10 or 15 years ago.

I found that the reason of the voltage drop was the (multi-filament) cable which, over most of its length, after peeling it progressively, was seen covered with a sticky green / dark green substance.

Even the crocodile, after retiring the rubber-like protection was full of that gummy thing.

I recall having seen something like that before, when salvaging components from old (tubes era) equipment.

What is it? When, where and why to expect that happening again?

 

[BioniC187]

oxidation of the surface of the metal causes the green on the surface of the metal - surface reacts with the carbon dioxide in the air. its copper carbonate i think... don't remember clearly. as to why it becomes gummy like you said, possibly it absorbed some moisture from the air or something else. So expect it on exposed copper, and possibly more so in humid areas

 

[dr pepper]

It happens in valve radios on the IF and tuning coils, they call it green spot, and it can on fine wires open circuit the winding.
Some of the strands in your cable may be open circuited, however they will still conduct if they are in contact with those around them, unless the whole cable is blackened and oxidised which may be the case, if so the resistance of the cable will have increased and therefore its current capacity decreased, if its carrying any kind of current replace it.

 

[atferrari]

It happens in valve radios on the IF and tuning coils, they call it green spot, and it can on fine wires open circuit the winding.
Some of the strands in your cable may be open circuited, however they will still conduct if they are in contact with those around them, unless the whole cable is blackened and oxidised which may be the case, if so the resistance of the cable will have increased and therefore its current capacity decreased, if its carrying any kind of current replace it.

Had no doubt. Binned it on the spot

 

[4pyros]

It came from the oxygen made when the insulation on your wire starts to age. The oxygen corrodes the copper and mixes with the oils in the insulation and you have green slim. It happens to cheep speaker wire all the time.

 

[rumpfy]

For internal cable wiring material, the insulation material was either PVC (polyvinyl chloride) or rubber.
PVC contains heaps of chlorine attached chemically to the base plastic.
With rubber, this material was traditionally 'cross-linked' with sulphur.
Over time, the insulation material reacts with the metal (copper), and produces a copper salt. In the case of PVC the green salt is copper chloride. With rubber, there is a more insidious reaction. The sulphur used in the cross linking, reacts against copper to produce copper sulphide in the heat reaction of the cross linking. To protect the copper against reacting with the sulphur, it was traditionally coated with tin. There was no reaction between tin and sulphur, so everything was OK. BUT, over time, there is an inter-metallic reaction between the tin and the copper. Compounds of tin/copper form, and these are further oxidised to another tin/copper compound, neither of which are very good conductors of electricity.
So over time, the copper conductor develops a higher and higher resistance. The problem is worse with fine multi-strand conductors, as you note.
The modern PVC insulations are generally better than older formulations but still, the use of PVC is not liked by cable people. PVC is a complex formulation and includes a plasticiser and this is generally fairly volatile, so after time, the insulation goes hard and cracks. A low temperature PVC material is quite expensive, and other materials are often used nowadays. But cheap commercial insulations are just that;cheap. They will harden, corrode, and embrittle, in short time. High temperatures accelerate the process.
PVC is popular because it is cheap. But increasingly, commercial use of insulated conductor cable is calling for halogen free materials.