Open Theremin V3, volume oscillator not working

Posted: 8/7/2019 10:55:11 PM
Bernd

Joined: 8/7/2019

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

Posted: 8/8/2019 3:02:56 AM
gerd

From: Germany (Black Forest)

Joined: 11/25/2017

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

Posted: 8/8/2019 8:14:50 AM
Bernd

Joined: 8/7/2019

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

Posted: 8/8/2019 3:25:13 PM
gerd

From: Germany (Black Forest)

Joined: 11/25/2017

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

Posted: 8/21/2019 8:40:07 PM
Bernd

Joined: 8/7/2019

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:

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

The "set Frequency" has to be within the tuning range.

Posted: 8/27/2019 11:57:17 AM
june

Joined: 8/27/2019


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


Posted: 8/27/2019 12:29:23 PM
Bernd

Joined: 8/7/2019

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

Posted: 8/27/2019 2:46:07 PM
june

Joined: 8/27/2019

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

Posted: 8/28/2019 4:41:41 PM
Bernd

Joined: 8/7/2019

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

Posted: 8/29/2019 3:29:51 AM
june

Joined: 8/27/2019

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...

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