Vintage Synth Repair

Hi,

I own a Korg VC-10 but when i turn the Output Balance to SIMULATOR i  
hear the Keyboard synth sound or Noise when i depress a key or keys, it 
should no make any sound .. only if i speak is when it should sound. If 
i speak there's no vocoder fx only internal synth/noise source.


What could be the problem ?


regards