SV: [sdiy] FORMANT VCO restoration and improvement

[HA.Synth at t-online.de]

Hallo Karl,

Am Dienstag, 17. Oktober 2006 15:56 schrieb karl dalen:
> --- HA.Synth at t-online.de skrev:
> > I am doing some restoration work on my Elektor FORMANT and started with
> > the VCO. I have also done some improvement. So far i did:
> >
> > replaced OpAmps to better types where appropriate.
> > Increase signal output to +/- 5V
> > Added voltage reference +/- 10V to critical points
> > Added sync input
> > Added lin FM
> > and some other minor changes.
> >
> > Two Questions here
> >
> > Now i want to add sync out. Therefore i wanted to use pin 8 of the 7413
> > Schmitt Trigger. Just added a source follower here. But it didn't work. I
> > can't see the pulse at the scope. Not at pin 8 nore at the output of the
> > source follower. Any ideas or help here? Maybe the pulse is to short/weak
> > to trigger my scope? Derive sync out from another waveform? Which?
>
> I dont have the schematics at hand but shurely the 7413 will be able to
> deliver a sync pulse, if your scope dont trigger increase the freq of the
> VCO until you see some green lines on the scope then try to sync to that.
> The pulse is narrow.

I found the pulse with the method Paul suggested. It is about 40mV. I think it 
is easier to derive sync out from an other waveform then amplifying this 
pulse.

>
> > The load capacitor and the transistor switch are driven against the +5V
> > rail. Is it useful or recommended to put an +5V refernce here instaed?
>
> It is, it will improve amplitude of the saw and the freq of the osc.

I don't understand this. How can the amplitude or frequence be improved when 
the voltage stays the same? Maybe wrong english understanding on my side? I 
read improve == larger amplitude, higher freq?? Or is the meaning better 
shape and decoupling from, the the rali to avoid soft sync via rail?


>
> > Third Question: Any other iprovements?
>
> Well, changing the T1 switch NPN tranny
> into a PNP instead (minimoog type solution).

What is the improvement here? 
>
> KD