I have a question about proper oscilloscope connection for doing work on vintage stereo equipment (120v AC input to +-76v DC board voltage).
Our house wiring is a 1950’s 2 wire system with no ground wire to the outlets. The only earth ground in the house is a bare copper wire from the breaker box to water main. There is also an earth ground at the service transformer in the alley.
I need to know how to properly connect an oscilloscope and DUT in this environment to prevent ground looping. Obviously, neither the scope nor DUT is earth grounded when plugged in? What’s the potential difference between the two (are they both floating)? If I’m probing, am I the only earth ground in this situation? I don’t know how isolating the DUT would accomplish anything? Would a battery operated scope be an option (still needs an earth ground) and what about probe isolation? Is a differential probe the only real option? I want to be able to do this safely, but reasonably too.