Doepfer

How do I calibrate one of my 3 VCO's in my system,
the other two are tuned to middle C, so they are tune
with the rest of my gear.

But my 3rd VCO refuses to cooperate, turned to zero it
will only tune to G#2, and at ten on the pot it will only
tune to B2 - this is what I get pressing C on my keyboard.

Will my VCO need calibrating on the inside, even though I
let it warm up for 10-15 minutes, still no luck.  What's the
procedure for this.  I only have an electronic tuner.

Any info or help much appreciated.

Thanks.