Soundlab MK II VCO range????

[elmegil]

I'm building a SLMKII, and everything has gone pretty well, except that at their lowest settings, with no other modulation, I can't get the oscillators to go below about 150Hz.

I was initially confused that when the coarse pot was at either end, rather than the +/- 7V I expected based on the voltage divider, they actually were more like +/- 5.5V. However, that should be more than enough range, I don't think I have any oscillator that tracks over 10 octaves anyway. Doing some math and including the 100K resistor to virtual ground into the op amp clarified for me why the voltage differed, at least I think....

The only time I've gotten any truly low notes out of either oscillator is when I've been modulating with the S&H and adding in significant negative voltage down from the already low -5.5V.

I have not yet calibrated anything, but I did ensure that my HF comp trimmer was at max resistance. Since I normally start at 80Hz or 100Hz, I'm a bit unsure about whether calibration would help any anyway though I suppose I will give it a try later.


Anyone else building this one see this behavior? I have not had this problem with any other MFOS oscillator I've built, not two of the original SL, nor the Ultimate. I've double checked most of what seem to be the relevant components, and haven't found an error yet, but I'll keep looking....