Vintage Synth Repair

is this noise in tune with the other sounds the keyboard makes? you
say it is affected by the master volume, I think, but does it alter in
pitch with the tuning control? it could be leakage from the master
oscillator through a VCA that's gone bad, or it could be chorus-clock.
does the chorus effect still work properly? (one of my favourite
things on the rs09 is to put external audio through this)
does yours have the huge rocker switches or the LED buttons? could be
the electronics that scan these switches if it's the later model. any
case, check the power supply for failed components that could be
allowing this kind of crosstalk (parasitic oscillation) onto the audio
path. it's a linear power supply in these keyboards (thank heavens) so
it's not likely to be the power supply itself...
good luck, & don't forget to let us know what turns up.

duncan.