DIY Etherwave volume never quiet

Hi,

I have built an etherwave theremin from the Hot rodding article. The PCB is a two sided one copied from pictures about an original EM theremin.

I read articles from Thierry about changing components. Using the Slot-Ten 5-10, only L7, L8 and L9 on the volume side and L2, L3, L4 on the pitch side, with some resistor changing R2, R6, R16 1,5kOhms, R15, R19 330Ohms.

The problem is the volume never reach zero. I hear a small sound when I touch the volume antenna. I tried to adjust L11 and the volume knob but it don't solve the problem.

Any ideas what to try?

firstly check the values of R35 and R36.

Try shorting the volume loop with a 1nF capacitor to GND. This should definitively mute the instrument. If not, the detected resonance current is too high. Take care, in opposite to the pitch oscillator, the volume loop is connected through the coils to the base and not to the collector of the oscillator transistor. There should not be more than 7Vpp @ around 500kHz (check with an oscilloscope).

Try replacing D1. Often, these will still show ok when checked with a multimeter but have a too high reverse current, though.

Another problem could be a grounding or stray capacitance issue. But I can’t guess that without seeing your PCB layout.

Thanks for the help, but these options don't solved the problem. Do you think a bad line on a pcb cause problems like this?

I like to show you the layout but I can't upload that to this website. If you give an e-mail contact, I send it to you.

"... the volume never reach zero" -- Dr Spider

check the values of R35 and R36.

Checked the resistors, but there's no problem: R35: 4,7M; R36: 1M

I'd try to reduce R36 (or R35) a bit.
Possible your LM13700 sample has out of range parameters.  I faced  with it, when ordered them on AliExpress.

I use 13600 instead of 13700, but this matters or not I don't know.

Reduced the resistors values by connecting a bigger in parallel, but nothing changes.