I'm tempted to make and post a YouTube video showing my method in this but before I lose all my marbles can I make a quick check with reality?
So if you have a sound source (probably a synth but to narrow it down I ended up trying this with my mobile). If I plug in my Sony headphones, that have a 3.5mm TRS jack at either end of the cable, and play some music on the phone (it was actually Transvision Vamp but could have been anything) then, sure, I hear Wendy's dulcet tones in the headphones. So the signal is routing correctly and the cable is working.
If I now unplug at the headphone end, clip the ground clip of a scope onto the Shield of TRS and then touch/clip the probe of the scope to Tip or Ring and press "Auto" on the scope I should just see "music" which stops and starts as I start/stop the track on the phone displayed on the DSO shouldn't I? Only thing is, I don't. There's a noise flaw but I was expecting a wildly varying signal. But there's virtually nothing.
Can I assume there's nothing more to this - like the signal has to actually play into a load to be monitored or something like that is there? If the pins are just open to the world I should still see the signal (even an FFT if I enable that) on the scope?
Scope is Hantek DS02D15 by the way.
I just don't know what I'm missing or what I'm doing wrong or failing to do.