Checking EM Thermin Circuit and Tuning/ An Induction Question

Posted: 2/14/2008 4:16:33 AM

From: Colmar, France

Joined: 12/31/2007

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.
Posted: 2/14/2008 9:01:55 PM

Joined: 9/25/2007

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,

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! :)
Posted: 7/2/2013 7:08:29 AM

From: Krasnoyarsk, Krasnoyarskiy

Joined: 7/2/2013

When building a theremin (based on hotrodding article) i faced with the same problem.
My volume circle does not work. On Q6 and Q7 emitter voltage in the range of -0.06 to -0.1 V (changing when reconnecting the power).
Pitch works fine (when grounding 12 pin of U3).

If unsolder Q7 and Q8, the voltage on the emitter of Q6 is -0.65V. If the solder Q7 and unsolder Q6, then it will be the same voltage. (On the collector voltage of about 12V. On the base of 0V).
In what could be the problem?

P.S. I don't have oscillator:(
P.P.S Sorry for my bad english

Posted: 7/2/2013 8:47:13 AM

From: Colmar, France

Joined: 12/31/2007

vasily, just measuring DC voltages around a ~500kHz oscillator circuit is not a good idea for "debugging" such a circuit since most multimeters will not show correct values if the DC is superposed by RF oscillations. Using an oscilloscope, together with a 1:10 probe (which reduces the capacitive load of the probe from ~40pF to only ~5pF) will be more helpful.

If you have no oscilloscope, you can always put an AM radio, tuned to the lowest medium wave frequency (530 to 550kHz), besides your circuit and play around with L11 to increase the volume oscillator's frequency temporarily until you get a signal in the radio. This would prove that the oscillator works, while measuring the voltage at the Emitters of Q6/Q7 is of no use. It may vary from -0.2V to +0.2V when the oscillator works (while -0.6x V will clearly indicate that the oscillator does not work), due to component tolerances.

Then you might connect your multimeter to the anode of D1 (thats the side without the ring). There you will most probably find around 0V. Tune L11 until this voltage goes down to around -4V (with all things as far away from the volume loop as possible). Then you should be able to vary this voltage by approaching your hand between 0 and ca. -4V. If you can't get this result, try shorting L10. This signal is then amplified by one half of the LM 13600 or 13700 and you will find -12V at pin 12 with 0V at D1 and +12V with -4V at D1.

At this point, everything should work fine.


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