this is a small modification i did to my masterkeyboard, in order to get rid of the midibox power supply.
after checking that the power supply of the keyboard ouputs 12V, i soldered to wire at its output, and let the wire exit from the master
the wire exits at the back and connects to the midibox
now my setup look like this:
and when i turn on the masterkeyboard also the midibox switch on.
very simple to do, but i assume i know how to use a tester, how to identify a power supply and how to not get an electroshock! 
