Another VCO that works (fwd)

Joachim Verghese jocke at
Thu Sep 21 15:28:48 CEST 1995

On Wed, 20 Sep 1995 gstopp at wrote:

>      I was able to get the Chroma VCO to work properly. Yes, it was cockpit 
>      error. Let me just say that this VCO is a lot more sensitive to solder 
>      flux effects than the others for some reason (charge pump maybe?).

Even Fender had great problems with flux effects. I seem to recall that
they used water soluble flux (despite disapproval from ARP engineers)
which after washing made the VCOs permanently unstable. They had to throw 
away hundreds of Chroma voice boards (all those CEM3350s?!?!).

Anyway, I made some tests with my charge-pump VCO, and found that it's
the exp converter which limits the low frequency performance. I injected
control currents directly into the inverting input of the integrator
op-amp, and the VCO produced a good sawtooth down to 50 mHz. With the
1000pF integrating cap, this corresponds to a control current of only
a few hundred pA, which approaches the input bias current of the op-amp. 
Maybe a MAT-01/CA3140 would perform better than the 3086/TL082 for
the exp conv.


PS Thanks Gene, for the EN stuff you faxed me.

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