SV: [sdiy] FORMANT VCO restoration and improvement

[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.

> 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.
 
> Third Question: Any other iprovements?

Well, changing the T1 switch NPN tranny
into a PNP instead (minimoog type solution).

KD