Repair broken slider tabs and loosened knobs by reattaching them with electronics glue. Turn the mixer off (for powered mixers) and disconnect all wires/cables. Apply two to three drops of electronics glue onto the tab/knob and place it back onto the corresponding metal piece.
Remove the outer plastic casing of the mixer to access the reason for non-responsive buttons, sliders or knobs. Look at the wires running from the main control board to each control surface and input/output. Use the soldering iron to reattach frayed or detached wires with the electronic solder.
Look at the bulb (or transducer bulbs in some models) in each side of the mixer. Remove darkened bulbs and replace them with new bulbs. Replace the outer casing of the mixer and re-tighten it with the screwdriver.