Left channel weak and scratchy?

I have a Pioneer Sx-1080 that I recently purchased on ebay. I bought a pair of JBL E30 to hook up to them. The problem is that most of the time when I turn it on, the left channel is very faint compared to the right and very scratchy and staticy. The right channel sometimes gets scratchy too, but not nearly as often. If I wiggle the volume or balance it will sometimes come on. Even when both seem to be working, the right seems to have more output than the left. I know nothing about electronics so I have no clue what this could be, although I have heard that sometimes contacts need to be cleaned. Could anyone shed some light on this situation for me? And does it sound like an expensive fix? Thanks.