the voltage could not have changed but, if your keyboard drew more current then what the adapter was designed to output, you may have blown the adapter and hence, no power on the Portastudio.
Sometimes there may be nothing visible to the human eye when opening up something to look for a problem. If a transistor or diode failed, looking at it wouldn't tell you a thing. It has to be checked by a technician or a knowledgeable friend with a multi-meter who can do some basic electronic trouble shooting.
The best way to check it is to plug it into the wall, (the adapter) and take a DC voltmeter and see if there is an output of the proper voltage. If it reads 0 volts, you've fried it.
Good luck.