Thank you for your efforts.
I have now looked at the signal without the preamplifier and the circle via OsciStudio is still egg-like on may scope. Apart from that, I need my preamplifier because, as I said, the input signal is far too weak on this old thing and I don’t want to have to buy an expensive interface. Especially since, as we can see, it still doesn’t work.
I don’t know if it makes sense, but the problem must be the kind of that sound and/or static imaging, which my scope probably can’t handle.