hissing sound from my synth

[dougal]

hi guys, hoping someone can help me out here, long story short, i have a hissing sound from my synth, ( specs below in a sec ), ive connected it via headfones output to my desk. and as soon as i turn the channels on ( on the desk ), a very loud hissing is there, i did also try connecting via usb, installed the correct drivers, but the only thing that solved was it changed from a hiss, to a very low humming sound, ( i think i prefered the hiss to be fair, ) is there anything any1 can suggest to help stop this??, have i done something wrong somewhere??

what i have googled and tried
1: have a good ground on the desk,
2: swapped input channels to see if i had a faulty channel on the desk but all was ok,
3: bought different leads, longer, shorter, double screeded, etc etc, still no change,
4: updated all drivers on pc and for synth, still no change
5: all channels on the desk work perfectly with other inputs, eg mics, guitars, other keyboards etc etc

at a guess im thinking its the keyboard itself at fault, as nothing else causes me a prob,, my only issue is, i love the piano sound from the keyboard itself and would love this to be working as i play and record, would make life a lot easier, any suggestions would be deeply appreciated,,

specs

yamaha dgx-305 portable grand piano,,
desk is a tascam portastudio 488 ( old but reliable and crystal clear on everything else, )