Problems in home-made Etherwave theremin

Posted: 5/9/2014 12:18:41 PM

Joined: 10/15/2013

Hi all.


I built an Etherwave theremin with my own layout. All basic functionality works fine. But unfortunately there are two problems which I need help to resolve.


The theremin is a bit modified (in defference of scheme from HotRodding document):

1) LM78M12 and LM79M12 instead of LM78L12 and LM79L12 correspondingly.

2) 100uF capacitances instead of C11 and C10. And pin + of C11 is grounded.

3) Instead of inductors L8-L10 I use just L9 to reach resonance.

4) 100uF capacitance instead of C13 and 47uF instead of C18 and C17.

5) 100uF capacitance instead of C4.


Problems I have:

1) L5,L6 and L11 are adjusted now. But regardless distance from volume antenna to my hand, I cannot mute the sound completely. Adjusting L11 or P1 (volume tuning control) doesn't help. When I touches the volume antenna there is very silent sound and it reacts on pitch changing. But if I touched the antenna then the sound can be noised (not always) and if I keep my hand very close but don't touch then the sound is always not noised.

I measured the voltage on pin13 and pin12 of U3 when loudness is changed:

pin13: is equal to -0.056V when loudest sound and equal to +0.019 when maximum silent (my hand is very close to antenna but don't touch).

pin12: is equal to +6.07V when loudest sound and equal to -11.51 when maximum silent (my hand is very close to antenna but don't touch).

Voltage of output of U2 is equal to -11.98V. If I connect pin12 of U3 to output of U2 then the sound mutes completely.

Could you please suggest me how to fix it.


2) When I power on the theremin I can hear some noise sound and the noise continuosly disappears. But if I turn volume tuning control to right half of circle then the noise appears again and become much stronger. If then I turn the control back then the noise disappears again. The noise appears/diappears continuosly (not suddenly) while colume tuning control is chaging position. Changing all another controls don't make any influence to the noise. But if I turn pitch control to zero-beats (no sound at all) then the noise diappears too.

Looks like there are some self-generations in the circuit of theremin. Changing C13, C17, C18 doesn't help. Using oscilloscope I can see that signals from all three oscillators are ok. The noise appears on pins 2 and 3 of U3 (see pictures below).

Noised pin3: