Humming JX-10 even without power

Hey everybody,
I have a Roland JX-10 (Super JX) that has kind of a loud 50Hz hum leaking into the signal. I already replaced the PSU caps to make sure they are not the reason.
The hum also appears when the unit is not powered OFF, but the power cable is connected. So I thought It might be some kind a grounding issue. So I only connect the hot signal of the TS cable to my sound card, but it’s still humming. So apparently not a ground loop.
Any ideas how the ground noise can get into the signal path?

Thanks a lot!