This schematic was for something else so ignore the red X'ed out stuff. Taking your keyboard apart with likely a philips screwdriver will void your warranty. Using a multimeter on the Volts DC setting as shown by the red arrow, see if you read 5 volts DC.
At this point, unplug the keyboard from your computer and
switch your multimeter to the "ohm/continuity/speaker symbol," the test leads should be attached in a way that does not require you to hold them with both hands so you have a free hand to perform a "wiggle test" such as using a female usb cable cut in half plugged in the end of the keyboard usb cable. An alligator clip can be used for this purpose on a test lead. Now you will be testing the 5V wire in series and then test the GND wire the same way from the keyboard controller to the keyboard's usb connector. Wiggle the keyboards usb cable, especially where it terminates to the USB plug and into the keyboard. To make things easier, use the "speaker symbol, beeper" setting which is ohms but with the buzzer to audibly alert you without looking over at the MM.
