EM theremin volume circuit problems

Hello again everyone,

during the last summer/autumn, I managed (with the help of TW forums! Thanks for helping me) to build an EM theremin. Waveform aside (adjustable from "harsh" to "so harsh" with the waveform control), it came out quite OK.

It's been a while since I last touched it, and now I have found the volume control not working (the .

What I checked:

- the EM theremin article says one should adjust the volume coil so that the voltage at pin 12 of the LM13600N goes to 0 with the one's hand away from the antenna; however, no matter that I did to the coil, the voltage remains something about -9 to -10 volts. I could not identify a pattern - the fluctuations seem to be rather random.

- then I took the 'scope. The volume oscillator runs nicely at some 500kHz; the adjustment range one could get with the volume tuning pot is rather narrow - maybe +-1kHz or so; this bugged me a little, but I don't know if this really shouldn't be so.

- unlike the pitch oscillator, which obviously squeezes and stretches its sinusoid when I move my hand from/to the antenna, the volume oscillator frequency does not react at all to my hand. Rather, the only thing that changes is that about 1V of DC offset adds to the sinusoid when I touch the antenna.

Any suggestions?

It is normal that the volume oscillator's frequency remains constant. Approaching the hand to the volume loop will modify the resonance current through the big coils, which is detected by the diode D1 and then amplified by the 1st half of the LM13600/13700.

First thing is to do a DC resistance check from the volume loop over the coils until the tank circuit of the volume oscillator. Each 2.5mH coil should have around 8ohms, each 5mH coil around 15ohms. If that is ok and the problem persists, connect your oscilloscope to the anode of D1. You should see only DC around 0V which can go down to -4V in resonance (try tuning the volume oscillator until you find this point). Then, approaching your hand to the loop will make this voltage go back to 0V. If you see not only DC but spikes or a sine wave on the oscilloscope, D1 is defective (sine wave = open or short, spikes = reverse breakdown) and must be replaced.

If you get this 0 to -4V response, follow the signal and look around the LM13600/13700 where it gets lost.

Keep us updated!

Hi Guys

We are experiencing the exact same issue at the moment. 

I have enclosed pictures, can you give any idea about where to run these tests at? it is a generally generic theremin. I checked for continuity at a couple places, we have the o-scope and everything we need, just trying to figure out where to run the tests at.

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Hi Guys

We are experiencing the exact same issue at the moment. 

I have enclosed pictures, can you give any idea about where to run these tests at? it is a generally generic theremin. I checked for continuity at a couple places, we have the o-scope and everything we need, just trying to figure out where to run the tests at.

[

trying to post pictures the best i can, it's a little jacked at the moment