sorry I wasn't specific, the problem seems to be in the volume adjusters, both for the main volume and the sub woofer, they are both on the same chip.
It is hard to describe it, but it is like that: when you set the volume it can be very low or suddenly (with just touching the adjuster) too loud and then you need to "play" a bit with the adjuster to find the sweet spot.
I tried to clean the adjusters with some electronics specific oil spray, it made them less sensitive but didn't solved the problem.
I asked in a local electronics repair shop and told me it doesn't worth the cost to fix that.
Thanks for the response
