I have a peavey cs-800x that i used to DJ

The CS800 designs have not changed mucch in the last 20+ years. There is a resistor inside that can fail that can cause this symptom. If you have not worked on these before, I'd suggest that you have this serviced. As it stands the parts cost should be in the $20 range. If the outputs have finally failed, the parts cost will go up significantly. WIthout taking measurements inside the unit, I can't say specifically what is wrong.

Hope this helps,

Dan

check first for loose connections or bad connectors. you might try to replace the cables just to check as isolating each channel. hope this helps