> Last but not least, i have a 100Mhz analogue German made scope.
> Can i touch the probe directly to the mains line or should i use a cap
> for the protection of the scope's input?

You should check the voltage rating of the scope first. Measuring through
a cap won't protect the input. And you have the problem of where to
connect the scope's ground. Simply touching Active with the probe isn't
enough

THE SAFEST WAY IS WITH AN ISOLATING TRANSFORMER

to power either the circuit or the scope

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