VCS issues *Solved*

forshee · Joined: Dec 16, 2010 Posts: 46 Location: Hopkins, MN

Hi all! I'm having some odd issues with the VCS. The fall time works fine but the rise time has no effect. I thought it was the wiring so triple check that and it seems good. And I'd still think it was wiring but I noticed that turning the rise cv all the way positive makes the output snap to +5v with nothing at the cv in. Any ideas on what would make this happen? Would the 2N3904's having the wrong pin out do this?

Also could someone care to explain why that 330R is in the feedback loop in the processor?

Thanks!

andrewF · Posted: Wed Apr 13, 2011 4:55 pm Post subject:

Hi

to e-m

re: 330R
from Ken's DC Mixer page:

forshee · Joined: Dec 16, 2010 Posts: 46 Location: Hopkins, MN

Checked the 2N3904s I had and they all seem to have the same pin out as on the board. And the LED works as expected and that circuit uses a 2N3904 from the same batch. I'm not great with discrete transistors but the led shouldn't change smoothly with the pins reversed right?

At pin 1 of the TL074 I'm noticing it snap to 14.4 with the rise cv knob one way and -13.2 the other. It seems like its acting as an integrator. I'm assuming this is completely wrong since I don't see that in the rise section. I'll try re-flowing the joints around that section a cold joint on that 820k might do this.

And after a second look I am getting rise times they just seem to go from zero to a few seconds with no in between.

And thanks for explaining that 330r. Never thought about driving multiple jacks. Pretty neat trick. Every time I think something Ken dose is weird I end up learning something.

forshee · Joined: Dec 16, 2010 Posts: 46 Location: Hopkins, MN

Ok it was the 820k. I pulled a trace and thought I fixed it with a jumper. Well that trace was connected to two things. Occam's razor.

Now what would make the processor work as expected but with no where near unity gain?

Also is there any protocol on marking threads solved on EM?

andrewF · Posted: Thu Apr 14, 2011 4:59 pm Post subject:

Reading back thru, it did sound like you had a comparator in there somewhere, glad you found it.
No protocol, but its a good idea. I guess you could edit the thread title by adding "SOLVED"

With the processor check out its heritage:
First the CGS VCO;
http://www.cgs.synth.net/modules/pic/schem_cgs48_vco.gif
Then the Serge CV Processor;
http://www.cgs.synth.net/synth/serge/pic/dual_processor_schem.jpg

forshee · Joined: Dec 16, 2010 Posts: 46 Location: Hopkins, MN

Thanks. Yeah this is my first RoHS board and first time I've ever pulled a trace.

Shouldn't be too hard to fix the processor but my power supply was craping out on me. New bench PS should be here soon.

I'm excited to try this as an envelope extractor (and a filter, and a VC LFO and a. . . ).