Interfering sound on a TX816

Hello
My Yamaha TX816 interfering sound over all 8 modules, it sounds like a sinus tone of about 200-300hz (it is not the internal 440hz test tone generator)
Since it is on all modules, I suspect the fault with the, apparently not too good power supply, does anyone have an idea or maybe even the same problem already solved?

Thanks for your help and best regards
Miko