All outs and ins work fine except from the main output. When connecting a cable-jack into the main output there's a instant grounding-noise, which is measured to be strongest around 50hz.
I can hear this grounding-noise in the headphone out as well, if a cable-jack is connected to the main output. For now I use the headphone out as the main out, (though this is not optimal and I'd like to get the main output fixed).
1. This happens regardless of whether the other end of the cable is connected to something, but the grounding noise gets much stronger when the other end of the cable is connected to e.g audio interface, monitor/speaker or mixer.
2. If connecting only a jack-adapter (no jack in the other end) to the main output, there's no grounding noise.
3. The grounding noise follows the volume adjustment. If the volume knob is turned all the way down or up, the grounding noise cannot be heard. Anywhere in between highest and lowest volume setting, the grounding noise follows the main volume.
4. The grounding noise follows the High, Low, and Off-switch as well. If this switch is set to Off, the grounding noise can no longer be heard in the headphone output. With the High and Low-settings the noise follows the general volume and can be heard in the headphone ouput as well.
I see that the main output on the Polysix got it's own unique grounding connection, going to a board located directly over it. This is the same for the headphone ouput. The rest of the ins and outs are all connected to the same ground which goes to the chassis/casing.
Here's a photo of the main output in my Polysix. It looks like there is some oxidation on the metal of this output, and the plastic seems to be unoriginal compared to the other ins and outs which got a pin sticking up in each corner.
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Some more info that might be relevant:
The seller lives in the same country as me. He denies ever having heard any grounding noises from the main output...
Since this is a Japanese import, it is rated for 100V. I currently run it on 110V through a step down converter, (but I'm planning on swapping the transformer to one rated for 240V).
Since this is a Japanese import it has no ground wiring in, as there's no (or very few) electrical outlets in Japan with grounding.
I have provided more text and photos in a thread on Gearslutz: https://www.gearslutz.com/board/electronic-music-instruments-electronic-music-production/975189-korg-polysix-ground-hum-hiss-output-help.html
Any suggestions to how I can get the main output working?