The single most common fault with a DX9
The single most common fault you will find with a DX9 is a dead internal battery. The DX9 internal battery is a CR2032 permanently soldered on to the mainboard which has an approximate 10 year lifespan. Once it fails, it can be the root cause of several issues and it is generally not worth trying to diagnose any DX9 problems prior to replacing the battery and then loading in a set of factory patches.
To check the internal battery on a DX9, press the Function button and then hit button 19 until the battery voltage is displayed. If the displayed value is not between 2.2 and 3.3 volts, there is a problem with the battery. Readings of 4 volts or more have been the subject of emails to me and also debate on forums where individuals have suggested failed diodes and overcharged batteries which are liable to explode. In my experience, neither is more likely than the battery simply being dead.