I have a cheap/simple guitar amp here that has a quite noticeable 60Hz hum, and it's not dependent on the volume setting, or the house it's in.
So I took it apart. The mains transformer is bolted to a metal plate that's also the front panel (with the knobs/inputs/etc). Mains earth GND is connected to it, but I unhooked that for the sake of using my Tek2430 on it. And I can quite clearly see the hum, it's +/-25mA on the speaker, not a simple 60Hz sine wave.
There's 2 DC COM connections to this metal plate as-well. 1 is from the TDA2003 amp IC, it's metal package tab is bolted to the plate.
The other is just another a wire on the other end of the PCB ground trace, it's bolted to the plate.
So when I unhooked that wire, the hum goes away and it's no longer on the scope (fades into the microV noise anyways).
The metal of the transformer is ohmically OL from pri/sec. windings.
So why would that wire even be there ? Just as over-the-top safety in case the TDA2003 came loose ?
I'm not hooking it back.