&gt; I hooked the ground crocodile clamp of my scope to what I thought was the ground but which in fact was V- of the DC supply voltage of the neon light installation that was use to drive the display.<p>Yeao!  I own a couple of high-voltage differential probes for this very purpose.  Even though most single-ended scope probes are rated to 300V Cat II, I won&#x27;t go anywhere near main&#x27;s power with them.