Doepfer

With no input applied my VCFS is outputing a signal from the mix and up/down 
outputs, but not the audio out.   The frequency of the signal is controllable from the 
shift pot and of course can be modulated by the CV input.

I'm assuming this isn't normal but I may be wrong.  Does anyone have any ideas on 
what may be causing this or how I may by able to troubleshoot and fix it?

FWIW, when mixed with an input signal the results are not sonically unpleasant but I 
would prefer to have my VCFS in proper working order!