The EM Theremin - Pitch and Volume Antenna: not working.

Posted: 10/23/2013 12:32:13 AM

From: Germany

Joined: 9/16/2013

Hi everyone,

i just finished building the Em Theremin with the Article by Robert Moog. I used some golden Coils from "Kaschke" here. For the Pitch and Volume Antenna i use the Bourns 6300 Series.

When i turn it on i here nothing. As suggested in the article i used a multimeter with the following results:

Outputs of U1 and U2: +- 12 V

Collector of Q1 to Q8: 12V DC, Emitters of Q1 to Q4 and Q7: about 0,6 V DC, Emitters of Q5 and Q8: -2,5 VDC

AC Voltage across L5, L6 and L11: <1 VAC (should be 10 or so)

DC Voltage at R24: -0,2 V (should be around -0,5V)

I checked the VCA with Pin 12 of U3 to ground: I hear a very weak oscillating signal which can be tuned by the Pitch Tuner Poti and the slug of L6. And, (and seems strange to me) it can also be tuned by turning the slug of L11 but nothing happens when i turn L5 (which should be changing something, because its the VPO, right?) When i connect pin 12 to -12V the signal should dissapear. But its still there...

And, when Pin 1 is normally connected, the Pitch and Volume antenna do not work and i can here nothing but wind.

I double and triple checked the circuit but i cannot find anything. And i have no oscilloscope, but i hope i can fix it wothout one.

Any suggestions how i can find out in which part (VPO, FPO, VCA etc...) the problem is?




Posted: 10/23/2013 6:50:40 PM

From: Austria

Joined: 10/23/2013

Emitter Voltage of Q1/Q2 and Q2/Q4 should be -0,6 V (negativ!)

Q6/Q7 around +0,2V

Try this coils instead of yours:  Conrad - Item no. 516317(AM Filterspulenbausatz 1,8 Mhz) 

58 to 59 turnes are about 47 µH

I used this coils in 2 EW Theremins.

It is hard to find any errors without oscilloscope.





Posted: 10/23/2013 9:53:15 PM

From: Germany

Joined: 9/16/2013

I measured it again, the emitter voltage is -0,6 VDC. I just forgot the -.

Thanks for your advice with the Conrad link. It's quiet hard to find some equivalent coils here in Germany. If i don't find a solution i'll definetly try those coil-kits.


Finally, the pitch area works. A 3904 was obviously broken. The range of the Pitch is not good (a half octave or so) and the sensivity is about 10 cm... but i'll focus on the volume area first on which i'm still struggeling with...


Posted: 10/25/2013 10:50:27 AM

From: Austria

Joined: 10/23/2013

For tuning see

What is your voltage at emitters of Volume Oscillator ?

It should be around +0, 2V

Posted: 11/6/2013 2:55:01 PM

From: Germany

Joined: 9/16/2013

Thx betabox, but i got the volume circuit working. But it only works with my test antenna wire, which is a isolated wire with a crocodile clip connected directly to L10 hanging out of the enclosure (which is completly made of wood). I bended a copper tube (10 mm diameter, 20 cm long) but i did not manage to get a tone out of this one. My Coils (Kaschke, see above) worked fine for pitch. On the Volume side they are oscillating with about 500 kHz. My question: is it possible that they are still to weak to work with my bended copper tube? (weak Q Factor = low range? capacitance?) And betabox: i ordered those coil-kits i'll definetly try them out!

Second: When i turn the Theremin on (with added switch) the postive power regulator (7812) is delivering a wrong voltage to the circuit (+0,7 VDC) and the theremin is of course not working. Then i unplug my wall-wart (Hugh and Kettner PSA 812, Secondary: 13VAC, 600mA) wait a second and plug it in again: works fine. Any suggestions what can be the problem?


thx, Tino.

Posted: 11/6/2013 8:08:36 PM

From: Austria

Joined: 10/23/2013

Newer schematic of the Em Theremin

L10 and L4 are eliminated

My volume antenna is about 50 cm long (unbended)

Is your power supply grounded at output side? 

This is very important, because the Theremin cannot work if not grounded 

13 V Ac could be to less. The DC  input voltage of the regulators must be at least 2.5 V  higher than the output voltage.

I do not have any idea what causes the very high current drain when switching your device on, maybe one of the oscillators will not start oscillating at power on 


Posted: 11/6/2013 9:14:24 PM

From: Germany

Joined: 9/16/2013

Thx betabox, but i do not see that L10 and L4 are eliminated in the HotRod-Article. They are still there and have same values.

My Volume Antenna is less then 50 cm: around 30 cm. And bended in the classic way.

Power Supply is grounded: The Theremin worked already but sometimes i get those very low voltages. The current is ok.

When the theremin works i got + and - 11,77 VDC in the whole circuit. Could that be to less?

Posted: 11/7/2013 2:32:10 PM

From: Austria

Joined: 10/23/2013

" ... but i do not see that L10 and L4 are eliminated in the HotRod-Article"

Sorry it was my fault. They are not longer present on newer devices from Moog, which using a shorter pitch antenna (40 cm)

I built 2 Theremins based on this schematic without L10 and L4. My antenna coils are from Hammond (1535 series).

L5, L6 and L11 are the coils from Conrad (see above)

L12, L13, L14 are Neosid coils (also available from Conrad)

The first Theremin has been built on a prototype board about 6 month ago, for the socond one I used this PCB (but beware there is an error, read postings below, just finished, see below)

Both Theremins are modified with the Yet Another Etherwave Standard Bass Mod

Antenna lenghts are a little bit different, the first one needs a 48 cm pitch antenna, the second one a 40 cm to get linear for 5 to 6 octaves. My antennas are made of 10 mm chrome-plated copper pipe.

How did you arrange your antenna coils? They should in a 90 degree position.

11,8 V should be enough.

It could be possible that your problem are L5, L6, L11. I did not find any datasheets about your Kaschke coils.

Of course I do not know anything about my selfmade coils, with the exception of the inductance, but they are working :-)


Homemade Theremin 2013


Posted: 11/7/2013 8:53:29 PM

From: Northern NJ, USA

Joined: 2/17/2012

"Both Theremins are modified with the Yet Another Etherwave Standard Bass Mod"  - betabox

What do you think about the YAEWSBM mod?

Posted: 11/8/2013 8:38:39 PM

From: Austria

Joined: 10/23/2013

What do you think about the YAEWSBM mod?

Your Bass Mod is working perfect, thank you very much,

I got a bass response down to 15 Hz and a very good linearity for 6 1/2 playable octaves.




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