Swapping power supply - need clarification on grounding (AG & DG)
The +12V and -12V are used to power the audio sections of the keyboard.

If you measure the voltage between +12V and -12V, you will find 24V total.

AG is the audio ground.  

If you measure from +12V (red lead to +) to AG, then you will find 12V.

If you measure from -12V (red lead to -) to AG, then you will find -12V.

This known as a "dual rail power supply."

I wouldn't use the Pico power supply.  The ground for the +-12V and the ground for the 5V is combined.  They are separate in your keyboard.  Besides which, I expect the computer power supply will have some "whine" kind of noises on the voltage output.  That wouldn't sound good.

The easiest (though ugliest) solution is to get two 12V DC wallwarts and one 5V DC wall wart.

Connect the 5V DC wallwart to the 5V and DC ground connections.

Connect the two 12V DC wall warts in series.  That is, the + wire of one to the - wire of the other.  That junction is AG - connect it to the AG connection of your keyboard.  You then have a +12V and a -12V - connect those to your keyboard and it should work.

You'll probably want to get old fashioned regulated wall warts.  The kind that provide a regulated voltage, but use a heavy transformer rather than the modern switching power supplies.  The modern ones are lighter, but they produce more electrical noise (whine and whistle.)

Your keyboard expects a clean DC supply, and modern power supplies may be noisier than the rest of the analog circuitry can deal with.

It might also handle it just fine - but the only way to know for sure is to try it out.  I tend to do what I know will work rather than try one questionable thing then try the sure thing.  It's cheaper.