Vintage Synth Repair

Hi All,

My prophet 600 stopped making sound a few months back.  It was 
working fine before that.  I found if I cranked up the mixer trim & 
amp I could make out a sound & I was also able to distinguish any 
changes I made with each of the front pannel switches/POTs so they 
all work. I cannot make out any of the presets.

Any idea's I could try would be appreciated.

Thanks,

Steve