Hi Antonio,
As a starting point (w/o the oscilloscope) compare the negative outputs of the two power supplies by measuring the voltage across them ... this may reveal the problem.
Should you want to see how the 24v works on the tool, you can always just put two 12v car batteries in series ... not good for regular use or for the batteries, but it may give you an idea of the improvement the 24v will give you and indicate whether its worth your effort.
