Just built Etherwave doesn't work

Ubermann,
there is ample evidence that the scheme is operable (excluding too strong drift).
The next stage of adjustments should be performed not on the table but in completely assembled theremin, which must be placed away from objects.

I suppose the pink wires are "MGTF" and PCB was made by "laser iron technique"?  ;)

Thierry,
let me a bit correct you.

"magnetic coupling between the coils will further lower the resonant frequency of the antenna circuit"

Depends on mutual orientation of magnetic fields in adjacent coils. In PCB above the fields will be partially cancel each other so the total inductance of chain will be lower than the sum of individual inductances.

Only on "69" Kama Sutra' position the total inductance will be greater than the sum (in parallel placing).

Thierry,

I couldn't reproduce the sound attenuation. Every time I power on the circuit there is no sound appears excluding the hoarse sawing "ghost tone".

I measured the signal from volume oscillator and it's ok. The smooth steady sinusoid with frequency 545 KHz. Changing frequency of VO (from about 470 KHz till 570 KHz) doesn't help at all. I can see on screen of oscilloscope that the signal is changed but still no sound appears.

I checked the signal on pins of C18 and C17. Looks like there is just DC. My oscilloscope shows as below:

Just some noise and it cannot be measured as an AC signal.

Also I measured signal on cathode of D1:

The signal is noised and has a very small amplitude (about 10-12 mV).

Voltage on pin 13 of U3 is very close to 0. Also I checked the connection of volume antenna - it's ok.

Thus it's some strange there. It's just "three pines" (VO+detector+VAC) and some unknown problem covers there.

I don't understand what is the problem here because we have a correct signal with a correct frequency, and calculated SRF of the series L7-L9 corresponds to frequency of VO.

It's absolutely the same behavior as at last time when I built a circuit with "wrong" components. I could hear working etherwave not for long till it's muted completely and every new powering on or adjustments couldn't help.

I didn't try to replace C 17 and C18 by bigger capacities. Do you think it's still needed? The signal of VO looks stable...

Have you any ideas about the problem?

 P.S. I'll "wash" the circuit board's bottom later. My chemicals are temporarily not available for me.

ILYA,

Of course I understand the scheme is operable. It's not a secret for me. Secret is the root cause of my problem.

I'll make a shelf for the device and adjust it completely just when the circuit board became workable. Now it doesn't have a sense.

Yes, the wires are МГТФ and the PCB was made by "laser iron technique". But is it important here?

Forgot to say...

I used a capacitor 1000pF as C12 in detector. Thierry, I remember you told me the capacitance is not too critical here. Can it affect workability of the detector at all?

I have a filling that you use a "SRF" as the "self resonant frequency of the antenna circuit".

Let's first define the terms.
SRF=self resonance freq of the coil or other electronic component. That is an individual property of the 
real component, regardless of what is connected to.

The SRF affects to antenna circuit as an unwanted additional capacitance connected in parallel to coil(s).
Why is unwanted? Because it causes reduction of sensitivity. Because it shifts the resonant frequency away 
of calculated.

Back to your problems.

Firstly, if you use a standard 1:1 probe, your measurements are incorrect because of the capacitance of 
the cable and the input capacitance of oscilloscope (together can be around 100 pF) which have a great influence on scheme.

If possible, use the 1:10 probe or simplest DIY RC-RC devider. In the simplest case you can use a small (around 1 pF) capacitor on the end of the 1:1 probe (this "attenuator" should be calibrated before measurement).

With this "additional tool" check the AC voltages on both terminals of L7 (may be under temporaly 
disconnecting of С12, D1 and L8). Even without resonance of antenna circuit this voltages must be volts, not
millivolts.

(I hope you measure all the voltages with respect to theremin' ground). 

Yes, I incorrectly used term SRF as resonant frequency of antenna circuit. Sorry for that.

I switched to use 1:10 probe for measurements. Calibrated attenuator on test meander of oscilloscope, and measured voltages on terminals of L7 with respect to theremin's ground (GND pins of U1 and U2; as usual before).

Looks like I confused voltages I measured on VAC. Sorry. Actual are below:

Base Q7 - terminal L7:

1) (1:1) probe: 5.6 volts AC.

2) (1:10) probe: 560 millivolts AC.

Terminal L8 - terminal L7:

1) (1:1) probe: about 1.6 volts AC.

2) (1:10) probe: about 520 millivolts AC.

Probably the difference between values for (1:1) probe and (1:10) probe reflects the influence of oscilloscope's capacity to measurements you told above. I also couldn't see that it affects frequency of VO signal. The frequency was 545 KHz regardless the probe.

The only idea I have about the problem is resonant frequency of antenna circuit is shifted due to unknown reason. Because the signal of VO seems stable and correct (excluding it has another, not designed frequency).

I tried to replace L8 with 5mH J.W.Miller to decrease the RF of VAC. But it didn't help. Still no sound appears regardless adjusting of L11 and P1. Still no voltage changes between R14 pin and pin 13 of U3. So I replaced L8 back with 2.5mH J.W.Miller.

I wondered that etherwave having the problem works just first/first+second power on after it's assembled. And not for a long time - few second. For the period it sounds silenter and silenter for every next second and then completely mutes. But the frequency of VO wasn't changed manually during that time. After the device muted, I checked the frequency of VO and it was about 545 KHz value. Unfortunately I cannot check that is was the same from the beginning of power on. What can cause such behavior?

What do you think about the problem?

Mmm... So, have you any ideas or any suggestions about?

I still cannot find the root cause. I tried to replace C18 and C17 with 22uF to avoid possible self oscillations within the area. But this didn't help, of course. The volume oscillator looks stable and ok. The problem is not there...

I will write a reply during the weekend. My f...ing day job is actually eating me up 20/24 on 5 days a week. Please be patient