The data sheet for my oscilloscope says that the vertical input sensitivity is 1 mV/div to 10 V/div in 1 MΩ mode. But I turn the Vertical Scale control fully clockwise and I can only get down to 10 mV/div. What am I doing wrong?

You probably aren't doing anything wrong. But you do have a 10X probe attached to that channel, right?

The 1 mV/div specs relate to the oscilloscope inputs. If the signal’s source impedance is relatively low (<< 1 MOhm), the signal’s frequency is relatively low (less than a few MHz), and the voltage is within the input range of the oscilloscope, you can connect the signal directly to the oscilloscope with a cable and the vertical scale does go down to 1 mV/div in 1 MOhm mode.

However, when you attach an attenuating probe, the mV/div scale factor always increases by that same factor. This is a standard feature of oscilloscopes that automatically sense the probe attenuation.

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