Using Osciloscope on Hot Chassis

[Tubejunke]

I need to know exactly what method must me followed to do this without blowing up your scope. I already knew that you couldn't just go in there and clip probe's ground lead to chassis and start poking around, but I still didn't know what I am to do.

In this particular case, I am working with a 50s Zenith Transoceanic radio. It has a floating ground, so I connected not to the chassis but to the grounding point. Hot end of the probe was not connected nor was connection to the scope at the input terminal.

As soon as I touched the probe connector to the input terminal of the scope a spark flew. Luckily for me the scope is OK. Possibly because I was working with a Variac on a reduced voltage as I suspected trouble.

What do I need to do here folks? Thanks for any input!

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