SV: [sdiy] FORMANT VCO restoration and improvement

[HA.Synth at t-online.de]

Hallo Ingo,

wer "misst misst Mist" Sorry don't know it in english. Anyone?

Am Mittwoch, 18. Oktober 2006 20:05 schrieb Ingo Debus:
> Am 18.10.2006 um 10:52 schrieb HA.Synth at t-online.de:
> > 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.
>
> I'm curious, how did you measure this pulse, what kind of scope, and
> which probe? I can't remember having ever measured the output pulse
> of the 7413 in the Formant VCO (although I owned one many years ago),
> but I find it hard to believe that the pulse has such a low
> amplitude. Maybe the bandwidth of your probe or scope is too low? Did
> you use a 10:1 probe?

You are right here.

I have had a second look today. I think i have reached the limit of my 
equipment. I use a dual channel 15MHz scope from Metrix, build 1984. Probes 
from Monacor and Velmanns.

Now i can see the pulse. Nearly 3,5 V at the falling edge (output VCO).

Method:
VCO pitch 24kHz. Synced Channel A with the regular output of the VCO. saw or 
square. Time base set to 2uS. Now I can see the pulse thin as an hair. No 
chance to measure the width (time). 

Thanks to make me think again!


>
> > Third Question: Any other iprovements?
>
> I'd replace the coarse/fine potentiometer combo with something more
> practical. Perhaps a rotary switch with octaves or octaves/fifths
> steps and a pot with range of one octave.

Already done, works great. 
>
> Err, you're talking about the "old" Formant VCO from 1977, yes? Be
> carefulnot to fry the 726.

Yes I try.
>
>
> Ingo


Harald