I have a known good MagicForce 68-key keyboard. It comes with a cable and a USB Mini-B connector (NOT MicroUSB!).
If I use it with its original cable and connector, it works. I tried it once with another USB Mini-B cable and it did not work, but I believe that was a bad cable. I have since removed the connector and direct-soldered another USB cable to it and it doesn't want to work now -- but the cable I'm using it with is tested and known good.
What am I missing...?