I’m not clear on you mean when you say the volume control does not seem responsive. But because the C-56p is a self-contained system there is not really anything that can go wrong that would not indicate something being broken. If you are unable to adjust the volume up or down it may simply be a worn volume fader. However,if the overall volume is low and distorted it is a sign that there is a more serious circuit failure.

A common problem I have seen with the C-50 series is for a handful of components on the main board to fail and start to leak. These components are known as electrolytic capacitors and tend to fail with age. The problem is two-fold because not only do these components stop working correctly but the substance that leaks out is corrosive to the surrounding parts and circuit traces. A technician will need to remove these capacitors and thoroughly clean the circuit board of any of the electrolytic fluid that has leaked out. After which any damaged circuit traces will need to repaired. Because of the corrosive nature of the fluid,this should be done as soon as possible to minimize the damage it can cause.

This type of problem will also show up in the headphones. If the problem does not show up in the headphones then the power supply / amplifier section needs to be checked. This section tends to heat up when in use, resulting in cycles of expansion and then contraction when the section cools down after you are done playing. Years of these expansion and contraction cycles eventually cause solder connections on the board to weaken. A technician would need to re-solder the connections to the amplifier and voltage regulator chips to correct the problem.

You can locate a Korg authorized service technician through the Service Locator at http://korg.com/Support

Please let me know if there anything else I can do to help you with this issue.