Open Theremin V3, volume oscillator not working

Hello,

I built the V3  of the open Theremin but unfortunetely it does not work yet - after calibrating I hear absolutely nothing.
I did some tests with my oscilloscope and found out, that the volume oscillator does not work properly.
I can see the frequency coming from U5, pin 12 but there is nothing at pin2.
Also I see a sine wave on TP6 (at the antenna) but again nothing at TP7.
The reference oscillator outputs at WP3 and WP4 are fine. So I guess there must be something wrong with the volume oscillator just behind the antenna.

Has anyone an idea what to check next?

Bernd

Hej Bernd,

The DC voltage at U12 Pin 5 and 6, and both pins of R5, R6, L2 should be something about 2 ... 3V.
We had similar Problems when I built up this theremin with a student. One problem was that she assembled the wrong capacitor. And as I remember, there was a short circuit directly from the volume antenna to GND of the PCB, because we've used a washer.
Remove the volume antenna, screw and washer. Can you now measure a signal at U12 pin 6?
It's not easy to measure the capacitance in the circuit. So I would suggest toto remove C8..C12. Double check the capacitors and then assemble only C8,C9 and C10. (Later on, if it works, the others)

gerd

Hej Gerd,

thanks for your quick response!
A short circuit at the PCB was the decisive hint. In my case it was not the washer but the banana plug itself which was slightly to big and touched ground
.
Now I can see a sine wave on both antenna-testpoints and also propper outputs of the oscillators at U5, pin 12 and 2.
DC Voltage at U5 (4069) pin 5 and 6 is about 1.5V.
There still seems to be a problem: It does not stop calibrating and sound starts only after I shortly touch the volume antenna but I can't controle the volume, neither with my hands nor with the trimmer. Also the "register" potentiometer has no function.

Bernd

Hej Bernd,

the frequency of the antenna and the reference oscillator must be similar (maybe +-10%). So can you measure the frequency of pin 11 & 12 and 2 & 3 of U4.

During the calibration sequence the voltage at D1 and D2 must by a dynamic value.

gerd

Finaly I got it working now.
The two oscillators have to be calibrated with C1/C14 and C7/C8.
There is a serial monitoring implemented in the firware. So you can see what happens during the calibration process by using "serial monitor" within the Arduino application:

====================== 
PITCH CALIBRATION 
Arudino Freq: 15993471.00 
Pitch Set Frequency: 499504 
Frequency tuning range: 499063 to 538151

Hello

I'm having a similar problem.
I built the V3 but volume is not working. Pitch is working. I've checked my soldering , grounding wire, antenna is connected in the same way as the instruction and I think it's OK...
Is there anything else I can check without an oscilloscope?

The serial monitor is looking like this.

VOLUME CALIBRATION

Volume Set Frequency: 460482
Frequency tuning range: 463706 to 463722

DAC value L: 0 Freq L: 3238
DAC value H: 4095 Freq H: 3239

CALIBRATION COMPTLETED

Hi June,

try to use another capacitator instead of 330pF for C8. I think I changed to 270pF or even 220pF. See how this will change the values in the serial monitor.
I am wondering about this very small "Frequency tuning range" of 16Hz.
I also had problems with a short circuit from the antenna plugs to ground. The washers were to big and had contact to parts of the PCB. SO, if you have a multimeter just check between ground and the antenna plugs.
Bernd

Hi Bernd,

Thank you for your reply.

I will check the antenna and ground first tomorrow.
I wonder if that happens using a washer that comes with the kit.

Changing capacitor is a challenge for me. I have to get one first...

June

Hi June,

if you got the OpenTheremin kit from gaudi.ch you can contact Urs and ask for support. He is very helpful.
http://www.gaudi.ch/OpenTheremin/index.php/home/contact

Bernd

I checked between ground and the antenna plugs. They were not shortened.
I ordered some capacitors so I will try replacing it.
I wish this very small "Frequency tuning range" doesn't mean any hardware failure...