@hagosrush How old are the speakers? Normally a pot takes a while to get dirty unless it is in a really humid or dusty environment. I'm guessing not very old. Just like lovinthehd said Deoxit works well and is easy to use. Basically disconnect the speaker cables, spray it on, let it sit for a couple of seconds, turn the dial a few times, repeat this process a few times, wait for it to dry completely (hairdryer helps), reconnect and you should be good to go. If that doesn't help you will need to get the dial replaced.