> Two of the VCO's on the Mini Model D appear to have some kind of
off-beat waveform.
This is the second waveform from the left for VCO1 and VCO2. The third
VCO simply has an inverted sawtooth. The odd shape is created simply;
the triangle wave and sawtooth are added together with some resistors
prior to the volume pots.
> I thought the Mini used the modular 921 VCO boards..
There are three types mini VCO boards. The first is all discrete and
appears to be very rare [RA Moogs only??], the second has op-amp summing
stages but with a discrete core and waveshaping, the third is op-amp
based and uses the heated uA726 in the expo. One PCB contains all three
VCOs in each case. For each 'upgrade' the VCOs got more stable.
Considering the difference in circuitry they do sound remarkably similar
to each other.
The 921 does share some similarity to the second set of mini VCOs, in
particular the CV and expo stages. Although, it has a load more features
like variable sync and sinewave generation with a clever two stage
method.
Tony