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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.

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    \$\begingroup\$ I'm not sure ground loops are what you should worry about here. \$\endgroup\$ – Hearth Feb 13 at 18:31
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Grounding loops are not your problem. The real problem is the unknown potential between the scope and the DUT. Without an Earth GND yes both are floating. (Does your scope even accept a 2 prong input without an Earth GND? Usually, most scopes have the chassis connected to Earth GND.)

If you just probe your DUT with the scope floating, you will attach the scope GND to the DUT GND, making that the common ground. The problem is that if the two are at a different potential then you are not protected from shock and may damage the equipment.

Read this document by Tektronix.

"The four safe solutions for these issues fall into four categories: battery-powered oscilloscopes, differential measurement systems, isolated-input oscilloscopes, and monolithic isolation amplifiers." source

Alternatively, if you can guarantee the Vrms of your DUT signal you could do a differential measurement using 2 single ended probes by connecting their GNDs together and putting one on DUT GND and 1 on the signal. Then doing a math function on the Oscope to subtract the two.

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  • \$\begingroup\$ This is why I'm considering a battery powered scope, to alleviate the grounding concerns. Would that solve the power problem? \$\endgroup\$ – DMoss Feb 13 at 22:05
  • \$\begingroup\$ Read the Tek document I link. With a battery powered Oscope you are not guaranteed safety. The chassis of the Oscope will be floating or connected to the DUT GND. Without the EARTH GND protection, you could get shocked. Your signal of +/-76V is still fairly high and even a few mAs at those voltages could be dangerous. So, in general, that approach is not safe. \$\endgroup\$ – EasyOhm Feb 13 at 22:59
  • \$\begingroup\$ So if I read that right, if I have an earth ground to connect the scope (battery powered) chassis to that should make it safe. ? \$\endgroup\$ – DMoss Feb 13 at 23:19
  • \$\begingroup\$ The document is for the TDS3000 Series Oscope, yours may be different. In general, however, having the Oscope connected to earth GND should make it safe as it would cause the DUT GND and Oscope GND to become earth referenced. You can double check w multimeter between Earth GND pin on the Oscope and the GND clip on the probe. You should see a short. \$\endgroup\$ – EasyOhm Feb 14 at 1:05
  • \$\begingroup\$ That makes sense to me now, both the DUT and scope would have the same GND then too. That's good, right?. I can clamp to the earth GND in the house, its accessible. Thanks for your help. \$\endgroup\$ – DMoss Feb 14 at 4:36

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