[Superman]

Even if the crackling does not occur when the knobs/switches are being moved, we cannot rule out dirty pots and switches. DeOxit has solve many mysterious problems with my electronics. Some sheet metal cover will likely need to be removed to access the contact portions of the pots and switches. Only a TINY amount of DeOxit is needed on each component. Such a small amount that it is very difficult to get only that small amount to come out of the can. Then work the pot/switch for a few seconds.