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Subject: RE: MOTM-300 questions/problems...

From: "Dave Bradley" <daveb@...
Date: 1999-09-17

> 1. Tuning - has anybody been able to adjust the 1V/oct dead on? I know
> this is basically impossible but I'd think that over 5 octaves (which is
> the range I tune over), I should easily be able to tune so I get a slow
> phasing type beating between my reference oscillator (a digitial synth
> generating a sine) and the MOTM-300. The pot seems so touchy
> when you zero
> in on that "perfect tune" point that actually nailing it with a
> slight turn
> of the pot happens more by luck than by anything else - in other
> words you
> can't turn it small enough which is what you need to do. Has
> anybody else
> had this problem?

I built 4, and was able to get 3 of the 4 to track well. The 4th seems to
have a slight anamoly in the hi freq tracking which makes it less accurate
above 1-2 kHz. Maybe a slightly out of spec hi freq tracking resistor. (Most
older VCO designs had a separate trimmer for high frequency tracking - talk
about hard to adjust!)

The pots ARE a little touchy, even though they are multiturn. It's hard to
get it perfect. You learn to kind of 'lean' on the screwdriver instead of
trying to turn it when you get very close to where you want.

You also might consider trying to find 20 turn pots to replace the 10 turn
pots for more control.

> 2. Dirty coarse freq pot? - I noticed that when I turn the coarse
> freq pot
> I hear a little dirt in the sweep of the frequency of the VCO. I was
> hoping for a good clean sweep just as if a freq CV input sweep
> was driving
> it but it sounds to me like the coarse freq pot is dirty a bit.

I haven't noticed this, I'll recheck and report back. It may be a natural
consequence of having the pot sweep the entire frequency range - it's more
sensitive than a design where you have a range switch and a more limited
coarse sweep range.

> 3. Hard sync - it says in my manual that any waveform will work
> as a source
> for hard sync but after carefully looking over the schematic, it looks to
> me like only waveforms having a fast fall time will actually work (the
> pulse signal or the syncIO jack when set to soft). My experience
> confirms
> this. Have I completely missed the idea on this or is this truly the way
> it should work? Is the manual wrong?

You are correct, it will work best with a fast falling edge waveform. Soft
sync absolutely requires a fast falling edge. Paul recommends connecting the
sync jacks together, and setting 1 sw. to hard, the other to soft.

Dave Bradley
Principal Software Engineer
Engineering Animation, Inc.