The 9-band graphic EQ is an active filter bank. The FX processor is a digital module (often a single DSP chip). The service manual may not include the DSP’s internal code but will show the codec and analog I/O around it.
The X2222 is part of the XENYX series, and some service information is shared with models like the X2442USB or X2222USB (the USB version has additional digital sections). behringer xenyx x2222 service manual
As a mechanical interface, faders inevitably wear out or become scratchy. The manual lists the exact resistance values and tapers (logarithmic vs. linear) required for replacements. The 9-band graphic EQ is an active filter bank