HP8559A Vertical Offset Problem

[Gregory]

Hello guys! I scored a HP8559A spectrum analyzer, but I'm unable to make the calibration procedure.
One of the steps says to use VERT POSN adjust to place the trace at the bottom of the display.
But it seems my trace is offset upwards, I can't get it to the bottom.

Now I'm trying to understand if it is a problem on the vertical amplifier or before it, like in the log/lin amplifiers

I recorded a video explaining the problem