IC melted down by connecting oscilloscope ground, but why?

Question

I am currently digging into electronics and today I experienced a strange problem.

I rebuild an oscillator circuit with the 30106 IC and I wanted to measure with my new oscilloscope (it is a china thing called DSO FNIRSI-150), whether there is something oscillating. As I connected only the oscilloscope's ground crocodile clip to the ground on the breadboard, the IC started to produce steam like an old locomotive and the 3 upper left pins of the IC produced so much heat that the breadboard was melting too in this area.

Why was this happening? - I could not find any explanation online. There are some threads discussing grounding problems with AC ground, but since the power supply of my very cheap oscilloscope does not utilize the ground connection of the power outlet, they might not be applicable here or am I wrong?

Also why does the IC melted down? - If I had (I don't know how) produced a short circuit with the ground of the switching power supply that powers the circuit and the ground of the oscilloscope, then why wasn't the oscilloscope damaged but the circuit IC?

I would highly appreciate any answer that gives me some clarity so that I won't burn any chips in the future :)

Here is an annotated image of the circuit:

Answer 1

The likely cause is that your oscilloscope ground and your circuit ground are not at the same potential.

Usually, a bench oscilloscope is grounded to the ground wire of your building's main power. There are a variety of reasons why it might not be. One common reason is the use of "cheat" adapters that allow a three pronged plug to connect to a two pronged socket (without grounding). I recommend that you not use cheat plugs, as there are serious dangers associated with their use, such as electrocution and fire.

A USB oscilloscope will probably be grounded to the ground of the computer to which it is connected.

The ground of your circuit will (almost certainly) be the ground of your power supply. If your circuit is stand-alone, using an isolated supply will likely solve the problem. An isolation transformer can effectively turn a non-isolated supply into an isolated supply.