Thanks guys. None of the gear I buy use a SMPS, they're old 70's amps with transformers. And, as I say, I do use an isolation transformer with the amps anyway. But looking at the diagrams that guy shows on the video, explaining how the short can be created when the device isn't isolated, it just seemed that a short would be possible again once I connect up a signal generator to the amp. Or am I still not getting something? I just want to know if the danger is still there.
The port you connect your signal source to has nothing to do with the Mains.
If the audio amplifiers you are working with have power transformers, the Mains connection is confined to the on/off switch & the power transformer primary.
The metal chassis, cabinet or "earthy" part of any PCBs are not connected to the Active or Neutral Mains lines, but, if they use a three conductor power cord, the chassis will probably be connected to the building's Protective Earth.
With modern devices, the special situation of trying to look at SMPS with a 'scope
can give rise to the problem referred to in the video, but the DC supplies at the output of the SMPS are transformer protected by the higher frequency transformer used in SMPS.
WARNING:- Some manufacturers of 1970s/80s TVs were somewhat less than scrupulous at
physically separating Mains wiring from other wiring, but electrically the isolation still exists.
"Transformerless" devices were formerly common amongst domestic radios & TV sets in some countries, including the UK, & with them, there is a good chance that the chassis may be inadvertently connected to the Active conductor, so if you connect a "ground" lead connected to the building PE as would be the case with a 'scope to that chassis, you will get a very large spark, & (if you have one), your RCD will instantly operate.
Even with such devices, however, if they had provision for an audio input from a record player (traditionally marked PU), both sides of the connection would be isolated from the chassis using large value, high voltage rated capacitors, as the wiring from the PU cartridge of record players is not in any way Mains rated, quite apart from the high likelihood of the cartridge disappearing in a puff of smoke!