We unscrewed the AC receptacle from the rear panel of the Juno-106 and pulled its wires through the cutout.

Now the terminals are easy to access.

As you can see, the white wire is already broken off its terminal.

After snipping off the black wire, and stripping and tinning both wires, we were ready to solder a 3-prong AC jack into this Roland Juno-106.

Here's the finished soldering work.  The jack's earth terminal is unused since we're replacing a two-prong jack.

And since the original jack was non-polarized, it doesn't matter which wire color goes to which terminal ‒ just as it doesn't matter which way you plug in the original, 2-wire cord.

The replacement jack snaps into the rear panel cutout without relying on screws.

Now any standard computer / peripheral power cord will fit this Roland Juno-106.

Before closing up the unit, we installed a blue cable tie to replace one we had previously cut.