I just found out (and tested) that the original power supply from Z21 Control Station works in both 230 volts or 120 volts, perfectly and without problem, altought it is printed on the label only 230V input.....
the output DC voltage (Blackbox = 20V) remains the same no matter the AC input voltage.
Very good, because my unit is from Europe and my country is 120V.
This eliminates any improvisation!
In other early topic, I questioned myself 'why Roco didn't made a bivolt power supply, which is very popular nowadays'.... well, today I know: they did it! (but don't write)