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Originally Posted by TinCanAlley
Some said it was ok not to since it had a transformer for all sources of power, while others said it was better safe than sorry.
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Yes, it's a better safe than sorry thing. The issue could arise if you accidentally touched something that is on the line side of the transformer. If that happens, it will short out your probe ground to line voltage.
However, if you were to accidentally touch the line side of the transformer with you hand, you could then be connected to the breaker yourself, and that could hurt or kill you. This is generally why some people recommend an isolation transformer no matter what. I do enough electronic repairs that I bought one and just use it.
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Maybe someone can tell me if I have this correct.... I want to scope the B+ DC coupled. If I then do AC, won't that show me ripple? If so, how much ripple is too much ripple?
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I feel like I would start getting suspicious if it was more than 100mV, as that might indicate weak filter caps. If it's much over 150-200mV that would make me concerned, and any much higher than that I would turn it off and start hunting for the bad component. Certainly if it was over 1V or more.
In terms of AC Vs DC probing: The DC probing doesn't show the ripple because you have to crank up the volts/div so high that you lose the visual resolution to see the ripple. Looking at AC only means the DC component is gone, and you are then just looking at the waveform centered in the middle of the scope (vertically). So that's why you see the ripple in AC coupled.
I don't know what that particular set looks like, but my 1975 Sony 9" is about the older thing I have and it's B+ is rock solid at 130V with less than 20mV of ripple, so I know it's possible.