I'm using an HP54501A 100 MHz scope that I got off of ebay. I'm relatively new to these things. It's worked fine for several months. I've always been surprised that it could work w/o a shared ground with the circuit... that doesn't seem possible. But it did.
One day, I started it up, accidentally auto-scaled (something I hadn't done before), readjusted to my desired resolution, and then took a measurement. Now everything reads as a very slight sine wave around 0 volts. Even DC sources.
The internet says my oscilloscope's ground (which is connected to my wall socket) needs to be connected to my circuit's ground (which is connected to a battery). That seems reasonable to me, but I have no idea how to accomplish that. Could that be the problem? If so, how do I connect those two grounds. If not, does it just sound like a broken oscilloscope?