Vintage Synth Repair

Hi group:
I dealing with a problem with my Roland MKS-20.
When I play hard on the master keyboard, the audio outputs (balanced, 
unbanlaced and phones) on the MKS-20 becomes distorted.
The distortion seems to be greater when Chorus LFO it's at its peak, 
but even with the Chorus section bypassed distortion stays.
At low MIDI velocities (playing softly on the keyboard) audio it's 
clean.
Any help will be appreciated.
Also a service manual for the MKS-20 in pdf format, will be welcomed.
Best regards.
José
Portugal