I just got a Korg C-15. The speakers are crackling the same way you might hear feedback in a guitar amp due to a guitar pedal with unclean power.
Has anyone run into this issue? Before I take it apart, I want to be sure I can fix it. Everything else works fine on it.
It could be a power issue or a problem with the speakers themselves. Before taking it apart, you might want to try plugging it into a different outlet or using a power conditioner to see if that helps. If the issue persists, checking the internal connections might be the next step. Good luck, and I hope you get it sorted out!
Most common factor is “dirty” or oxidized pots or sliders on volume or tone controls. However the noise is heard mostly when the parts are moved. Possibly some cackling when not being turned. Antioxidant or pot “cleaner” spray is used to clean those out, IF they are the open (unsealed) type. Also just compressed air can be used to blow out dust and crud from component openings. Air can’t treat any metal oxidation problem of course. The fancy expensive sealed type potentiometers are designed to keep out all humidity, dirt, liquids, etc. so typically can not be sprayed out. In either case you need to repeatedly slide or twist the noisy control back and forth over full range of motion while listening to the audio output to hear if it’s getting “cleaner” in response to the manipulation.
Another possible factor in aged gear can be with the output stage amp chips having dried out compound on the heat sinks (possible overheating). Typically easy and obvious how to remove and reattach the heat sinks. Usually one amp per speaker channel. You can use a cotton or sponge swab with alcohol solvent to wipe off the old compound. Let dry a moment and apply new proper heat sink compound fresh from the tube. Best not to use too much. It can ooze out sides of the sandwich when you reattach a heat sink, so be prepared to wipe off any excess.
Otherwise, inspect internal wires or audio cables which may be shielded type. Assure the shielding is not broken off, but properly grounded to the chasis or ground trace on the PCB as originally designed.
Also might want to look for signs of possible previous repairs or modification. One tell-tale sign is excessive solder flux vissible around solder joints that wasn’t sufficiently cleaned up. It can indicate a problem point, but also represents a possible problem of creating an unintended high impedance circuit short between nearby conductors. That can potentially cause a range of unwanted audio problems.
I’ll start by checking for dirty or oxidized pots and sliders, and try using antioxidant spray or compressed air. I’ll also inspect the heat sink compound on the output stage amp chips and ensure internal wires and audio cables are properly shielded and grounded.