In a typical situation you can not allocate a particular XLR cable to just one mike, and cables move tasks freely in a studio or live rig.
Please confirm that the XLR cable in question was connecting to a microphone, not between two other components.And, I repeat: an open shield on even one side can lead to hash/noise in the mic.
It was indeed a cable going to a microphone! (So: connected to nothing else.)At the moment pin 1 of the XLR on the microphone input was connected to the shell of the connector (=mains ground), a clear mains hum was audible. (I suppose this was due to a design fault in the mixer. This happened on every channel.)
... chassis ground connected to audio ground is the norm in most vintage supplies.
What's the best procedure for an XLR cable connecting a tube mic power supply to a mixer? The cable is not directly connecting to a microphone. Is it more akin to a cable between two powered units, even though it is not carrying line level?