One thing to consider is RF junk coming from nearby light fixtures (both LED and even fluorescent if using electronic ballast).
I've got some el-cheapo LED lights over my bench and they radiate a *lot* of junk. Easily picked up by a floating scope probe.
Let me get this straight, just so I'm not confused with what you're asking...
You're waving around an oscilloscope probe that isn't connected to anything at the probe end, just open, which runs to the (relatively) high impedance oscilloscope input and you're seeing 60 Hz hum/noise of a few hundred mV on your scope, correct?
Why would this be surprising?
It looks as if you have a good shot at identifying your noisier items.
It's not quite as bad as it looks though... Because you have a loose capacitive coupling between the scope probe and the outlets, higher frequencies will be coupled a lot more efficiently than the fundamental 60Hz mains frequency, so will look much bigger in comparison. (Note that this is not a suggestion to start probing the socket contacts directly!).
What you are doing right now is what you want - emphasizing the noise (sources) vs the mains fundamental.
Do you think it's more likely that there's a specific item causing this oscillation, or a problem with the wiring, or something else I need to keep hunting for?
The fact that it seems to be synchronized to the power line means it is probably coming from some kind of switching power supply
It looks as if you have a good shot at identifying your noisier items.
It's not quite as bad as it looks though... Because you have a loose capacitive coupling between the scope probe and the outlets, higher frequencies will be coupled a lot more efficiently than the fundamental 60Hz mains frequency, so will look much bigger in comparison. (Note that this is not a suggestion to start probing the socket contacts directly!).
What you are doing right now is what you want - emphasizing the noise (sources) vs the mains fundamental.
In most of those cases, there was nothing but the scope plugged into the outlets in question. The noisiest one from upstairs did have a wall-wart plugged into it, which you could probably guess by looking. I should probably test that one again with that removed. Maybe I should plug the scope into a different outlet than the one under test, but the oscillation was still present in other places without the scope present, so I dunno if it matters? ...and no, I have no interest in connecting directly.
Do you think it's more likely that there's a specific item causing this oscillation, or a problem with the wiring, or something else I need to keep hunting for?
Triac dimmers are nastier than switching supplies BTW
I did consider the scope as a possible source, but testing from the strip or outlet it was connected to, to another outlet across the room gave the same results.
Keep in mind that there is more than one Outlet on each circuit in your house. So with the scope plugged into one side of the room and testing on the other side of the room you're probably still on the same circuit.
so that may be your scope noise that you're seeing on the same circuit thinking that because it's a different Outlet it's not.
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I did consider the scope as a possible source, but testing from the strip or outlet it was connected to, to another outlet across the room gave the same results.
Unfortunately "gave same results" is not proper baseline measurement. If scope is possible source, you can exclude it *only* when you see clean sine in some measurement. Do you have hefty isolation transformer? Maybe you can borrow such? Or with your scope go to place where you know - power is cleeeean
you could also set your scope up in one spot, and then go and turn off all of your circuit breakers except for the one powering the scope and see if you still have the noise or not.
If it goes away then turn your circuits on one by one until it comes back and there's your culprit.
If it doesn't go away try changing which circuit your scope is on and turn everything off again.
If it still doesn't go away then it's something to do with your scope or there's some other source and it just happens to be picked up
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There will always be some high frequency noise imposed unless you're WAY out in the middle of nowhere, I'm not sure why you're surprised. We shouldn't expect anything approaching a clean sine wave.
If you're meaning those seemingly short bursts of high frequency riding on the side of single cycles, it could be anything. The fact that it seems to be synchronized to the power line means it is probably coming from some kind of switching power supply, seemingly basically in a sleep-burst mode, rather than interference from, say, a cell phone, or something that would be more randomly placed on the 60 Hz wave.
Welcome to the world of EMI !
The biggest culprits are SMPS and any mains switching like dimmers that use phase control for both RF and mains induced muck but especially those tiny warts without input common mode filtering.
Live with it, learn to recognize EMI types and develop measurement techniques to minimize its influence on the fundamental frequencies of interest.
Spring connector ground probe connections jump to mind.
Welcome to the world of EMI !
The biggest culprits are SMPS and any mains switching like dimmers that use phase control for both RF and mains induced muck but especially those tiny warts without input common mode filtering.
Live with it, learn to recognize EMI types and develop measurement techniques to minimize its influence on the fundamental frequencies of interest.
Spring connector ground probe connections jump to mind.
No, you're missing the point. The measuring technique isn't the issue. The inevitable issue is that the existence of the parasitic on my household electricity is coming through into tube guitar amplification. This is disturbing, annoying, and potentially dangerous. The scope seeing it is a good thing.
I can never know if it's perfect without a clean power source. This isn't a headache I can ignore.