You can get the complete schematic set from Peavey customer service.
What is wrong? Isolate the problem.
Have you tried a different speaker and cord?
Plug a signal into the POWER AMP IN jack on the front. Does that come out the speaker clear or distorted? If distorted, your 400BH has a problem. If clean the power amp is OK and the mixer portion has the problem.
Likewise, plug a cord into the MAIN OUT on the front and send it to the input of some other amp and speaker for a listen. CLear or distorted?
How about the monitor out, does it sound clear through another amp?
Do ALL the channel inputs ahve the same distortion, or only one?