Open theremin v3 pitch antenna problem

Posted: 6/3/2018 7:31:03 PM
tinkeringtanuki

From: Oakland

Joined: 6/3/2018

I am super excited I found this forum. I just recently have gotten into learning about theremins and the open theremin seemed like a good project. 

Instead of buying the kit I went ahead and had the PCBs made by a board house so I could get them in white and then I ordered all the components per the V3 BOM posted on the git for the project.

I have assembled 3 of them now as the first I overheated during reflow and thought that might be the cause of my issues. Unfortunatley, all three I have built seems to have the same behavior/issue:
I allowed them to warm up per the instructions and properly grounded it before running the calibration cycle.

The pitch calibration takes a long time and even after it completed it does not respond at all unless I actually touch the antenna. The theremin plays a seemingly random tone that will change if I tap the antenna.

The volume is able to calibrate perfectly without issue and functions normally. 

I am at a bit of a loss as to where to start troubleshooting. 
I have tried 3 boards and two arduinos without a change in behavior. 

Posted: 6/4/2018 10:31:13 AM
gerd

From: Germany (Black Forest)

Joined: 11/25/2017

Hello tinkeringtanuki,

do have ave an oscilloscope?
Can you measure the frequency of the pitch and volume oscillator?
It should be about 500kHz. Measure it at pin2 and pin12 of U5. The frequency should be more ore less the same as at the output of the reference oscillator U2 and U3 pin7.

gerd

Posted: 6/6/2018 4:07:44 AM
tinkeringtanuki

From: Oakland

Joined: 6/3/2018

Hello gerd,
Thank you for the help probing the board!
I was able to hook all four test points up to my scope and they were connected as follows:

U5 pin 2 -> channel 1 (yellow)      -  Only drops below 500khz if i actually touch the antenna
U2 pin 7 -> Channel 2 (light blue)  - Steady 500khz
U5 pin 12 -> Channel 3 (Purple)   -   
U3 pin 7 -> Channel 4 (dark blue)   - steady ~460Khz


It would appear that the issue is the 560Khz signal on U5 pin2 is my issue. How do I go about getting that down to where it should be?


I also snapped a pic of the scope while I was at it so I wouldn't have to write everything down:

Thanks again for the help getting me this far.

Posted: 6/6/2018 8:24:45 AM
gerd

From: Germany (Black Forest)

Joined: 11/25/2017

>U5 pin 2 -> channel 1 (yellow)      -  Only drops below 500khz if i actually touch the antenna
Pin 12 is the PITCH signal!

You have an inductor of 1mH. To achieve 500kHz you need a capacitance of 101pF (http://www.daycounter.com/Calculators/LC-Resonance-Calculator.phtml)
If you measure a frequency of 560kHz, yours is only 80pF.

The total value of C1,C2,C3 (all are in series) is 56pF.
But there is also the capacitance of the varcap D1 (BB914) which increases the total value to 80pF .. 115pF depending on the voltage over C6.

Test, if the varcap D1 works:
Connect the net C6/R3/R4 once to GND and then to VCC. The frequency at pin12 of U5 must jump between 560kHz and 470kHz.

gerd

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