Checking EM Thermin Circuit and Tuning/ An Induction Question

Please excuse my bad English. I learned it more than 25 years ago and this only during 3 years. I should have written "There is something wrong with your volume oscillator or with the vca processing circuit."

Finally, all would be simplier, if you had a 10 MHz hardware oscilloscope, a frequency counter and a webcam which would allow us to work on your problem in an interactive manner. But we do not have this, so we have to go on step by step.

My "philosophy" with problematic oscillating circuits is to get at first correct static DC parameters for the semiconductors without oscillating. If this is done, one may switch to the first AC step: Get it oscillating correctly under static conditions. And finally there will be the dynamic view: Modify the oscillation frequency with the volume antenna and pitch control in order to get out the correct control voltage for the vca.

With the help of an electrical engineer from my area, I actually discovered that the problem with the volume oscillator/ VCA was that I didn't have a bypass cap from pin 12 of U3 to ground. The older print of the schematic in the Do It Yourself article doesn't have it in there, but the hotrodding article does...perhaps it's a misprint in the original schematic?!? Now, I get wonderful volume response and no more ping like noises if I touch the volume antenna.

Thank you all very much for the time and effort you put in to helping me figure this out. I think that now I've ended up with a very effective and playable Theremin, now I've just got to learn how to play it :)

Thanks again,
Dan


Thierry- P.S.- Your English is not bad at all and is in fact extremely good. The misunderstanding earlier was because of my lack of knowledge, not your English skills! :)