Just built Etherwave doesn't work

Hi all.

I've built my Etherwave theremin using my own layout and unfortunately it doesn't work.

I suppose the theme was discussed few times but I hope you would help me with my case.

 

All components are placed on one one-side circuit board with jumpers. Placements of components are approximately the same as in original factory-built circuits from Etherwave kits.

I found three schemes in Internet:

1) From "Build your Etherwave theremin" document 1996.

2) From official "Hot-Rodding your Etherwave" document.

3) From unknown "Hot-Rodding" document.

All three schemes have some differents. For example C12 on schemes 1 and 2 have value 1000pF but on scheme 3 it has 100pF.

Some time ago I built a theremin using scheme 1 and another layout. The circuit was workable.

And new theremin I built using scheme 2 or 3 is unworkable.

 

Of course I had problems with component searching. I couldn't retrieve any J.W.Miller inductors and the variable inductors instead in the original scheme. So I replaced it by another components. Changes are below:

2.5mH J.W.Miller inductor -> 2.2mH axial inductor + 330uH axial inductor (in real it's about 310uH).

5mH L9 and 5mH L10 -> 10mH axial inductor.

10mH J.W.Miller -> 10mH axial inductor.

100uH variable Toko RWRS-T1015Z -> Toko A7BRS-T1040Z with Q=80.

47uH variable L5/L6 + 47uH const L12/L13 -> Toko A7BRS-T1040Z with Q=80.

68uH variable Toko 154ANS T1019Z -> Toko A119ANS-T1043Z with Q=80.

47uH variable L11 + 22uH/33uH const L14 -> Toko A119ANS-T1043Z with Q=80.

LM13600 -> LM13700 (but this doesn't matter as I know).

 

To debug my theremin I have just multimeter and no oscilloscope at all. I tried to compare voltages between my new theremin which is unworkable and my old theremin which is built by scheme 1 some time ago. Looks like my new Volume Oscillator works incorrectly because voltages from bases of Q6 and Q7 are about 0 and voltage of their emitters is about +0.02VDC.

I checked the layout many times for few days and couldn't find any mistake. I suppose the problem is somewhere else.

I tried to change Q6,Q7,Q8 but it doesn't help. I tried to use C13,C14,C15 from scheme 1 because the Volume Oscillator should on any of three schemes should generate 470kHz frequency signal. And it doesn't help too.

As I can see by my multimeter the voltage on pin 13 of U3 is too small (about 0.03V) and almost doesn't change when I bring my hand over the volume antenna.

So I can't understand why the schemes 2 and 3 doesn't work.

 

QUESTIONS:

1) Could you please help be to find out the problem. Why my new theremin doesn't work? What's wrong with my Volume Oscillator?

2) What the scheme is correct, 2 or 3?

3) What the capacitance should be for C12? 1000pF or 100pF?

 

Thanks in advance.

 

Ubermann.

 

I forgot to say an important thing.

When I just built the theremin and powered it on, it was continuously playing with a maximum loud. I could change frequency by bringing my hand near pitch antenna (so I think his part or circuit works correctly) but no reaction was from volume antenna when I brought my hand near it. Just continuing max lout playing.

On the next day I powered on the theremin again and it started continuously playing but with some silenter. And silenter on the next day and so on. Now it doesn't work at all (no sound). That's why I think the problem should be in volume control circuit part. What do you think about it?

 

+Some additional question:

Is it normal that U1 and U2 are hot after the theremin was working for 5-10 min? In my old theremin which was built by scheme 1 (from my last post) these components heats even some more progressive. And in the old theremin U1 was even overloaded (it had 12.6VAC input and about 6VDC output instead of 12VDC because of load).

 

Thanks.

 

Ubermann.

1. Building /debugging a theremin circuit without frequency counter and oscilloscope will most times not work. Checking DC levels in a RF circuit is only a rough approach but does not allow any conclusions.

2. Your circuit must have an important problem, the voltage regulators are normally warm but never so hot that you can't touch them. The current drawn by the Etherwave's circuit is less than 30mA for each. So, even if you have a high input voltage of 19VDC at the buffer capacitors, the power dissipation of each regulator is only about 200mW. If much more current is drawn, it's (if you made sure that you didn't accidentally switch the in and out pins of U1and U2) most times due to a faulty U3. 

3. Your wild and unreflected replacing of the original coils by puzzling others together will give unpredictable results. The correct theremin operation does not only depend on the inductance value, but also on parasitic winding capacitance and the DC resistance (if the latter is for example too high or too low for the tunable inductors of all 3 oscillators, they risk to modulate the RF with a low frequency between 300 and 600Hz which gives ghost tones, bad or no volume response at all and can cause saturation/overheating of the transistors). And the "big" coils L1 to l4 and L7 to L10 must forcibly be 3-Pi wound. A little search in the forums here would have helped you to find distributors of the original coils and/or several equivalent types which are safe for replacement. Only the variable TOKO 10mm inductors are really hard to find. They may be replaced by the Coilcraft Slot TEN series, but this requires some modified resistor values to shift the oscillators' transistor boasting. The values have also been posted several times in these forums.

4. All 3 schemes are correct. They reflect different evolutions of the Etherwave circuit.

5. Both values of C12 (100pF or 1000pF) work fine. It's just a non-critical loading capacitor for D1 which detects the RF current through L7 to get a raw volume control voltage. 

Thank you very much, Thiery.

 

Actually I wasn't supposing that inductor's DC resistance of variable inductors are so critical for the oscillators.

Don't you know what the value should be for original TOKO 10mm inductors? I couldn't find the value in datasheet.

I agree that I undoubtedly need for oscilloscope but unfortunately I have no possibility to retrieve it now. So I just hoped that somebody have had such problem and would give me an advise about that.

As soon as I retrieve an oscilloscope I'll try to adjust my theremin circuit.

 

I live in Russia and here is difficult to get J.W.Miller inductors. It would be much easyer to buy original Etherwave theremin kit. But the hope is alive and may be I will get the inductors. Time will show ...

I'm not so familiar with 3-pi wound inductors. Don't you know what the fundamental mechanism works there? I mean why exactly 3 wounds (in J.W.Miller 6306,6302,6304)? I suppose, windings of one inductor should make some noise suppression effect from one to another?

 

About hot U1 and U2, the problem became non-actual for my new circuit because it heats but not so much to I can't touch it. That was relevant for my old version circuit - probably there was some another mistake. Anyway thank you for the answer.

 

Why 3-Pi wound inductors? It's all about reducing parasitic capacitance. The more windings you have and the more they are wound in flat layers one above another, the more capacitance will be effective between the windings. Inter-winding capacitance translates as a virtual capacitor in parallel to the coil, transforming it into a parallel resonant circuit. You cannot eliminate this effect, but you can reduce it considerably by making diagonally crossed winding layers and by splitting up the winding into several packets. 

A general rule says that the coil's SRF (self-resonating frequency of this virtual parallel resonant circuit) should be at least 2 to 3 times higher than the operating frequency. This is given for example for the 10mH inductors go the Miller 630x series and their equivalents of the Hammond 153x series: they have a SRF of about 790kHz which is high enough to ensure correct operation at about 285kHz.

Thank you again.

 

Now it's clear. But what about DC resistance of L5, L6, L11? If quality-factor of my variable inductors are less then the original (80 instead of 100) and it's the same inductance then my inductors have bigger DC resistance, I think. So to reduce it I'd add some additional parallel resistor to the coils.

What do you think about the way? Is there any better way to resolve the problem of variable inductors?

 

Putting resistors in parallel to the coils is a very bad idea since it will shift the transistors' working points and increase the losses of the tank circuit, causing a too small amplitude or no oscillation at all while the transistors will heat up. Out of that, you will know nothing about the resistance to use. The Q-Factor is determined at a specific frequency (i.e. 455kHz) which does not tell anything about the real world behavior when running at 285kHz, especially in the Moog circuit where a high oscillation amplitude occurs (about 24Vpp) while the "normal" amplitude in radios is a few hundred millivolts only.

The variable TOKO inductors are still available here: http://www.jabdog.com/toko-ez.htm (part n° EZ-1017 for the 47uH). They have the advantage that you do not need to change any resistor values which is required when using the (almost equivalent) Coilcraft slot TEN inductors.

Or you may send me a private message and then I could send you 3 of these TOKO variable 47uH inductors, together with 2 fitting 47uH fixed inductors and one fitting 33uH inductor so that you can follow the schematic n° 3. Shipping from France to Russia (at least to SPB or Moscow) does usually not take more than 10 days.

Thank you.

 

I.m temporarily on vacation and have some problems with internet access. I'll wright you a private message when I have more or less normal internet access.

 

I'm gonna to assemble few theremins according scheme 2 or 3 so I need more components that you suggested.

 

You are welcome.

In case you need larger quantities than my small spare part stock, I could order everything and send it to you. I know that many distributors will not ship to Russia for various reasons, thus I'm ready to help.

Hi all again.

Looks like the problem about unworkable volume oscillator is resolved by replacing:

1) A119ANS-T1043Z coil -> Toko 154ANS-T1017Z + 70F225AI.

2) 2.2mH axial inductor + 330uH axial inductor -> 2.5mH J.W.Miller inductor.

 

To get necessary details at retail in Russia is difficult but possible (for future builders, do not use Russian Post for deliveries - I had a fight for my delivery; transport companies are much better). Now going to build new circuit board for new components...

I just have few new questions about the circuit:

1) What is the role of C2 and C6 capacitors?

2) How does the volume osc detector works? (R14, D1, C12, L7)

3) How does the pitch detector works? (D4, R23, R24, C23)

4) I saw a photo of industrial-version of etherwave circuit board and there are another coils are used instead of L1-4 and L7-L10. What the coils are used there? (what values and models)

5) What roles of C29 and C30 in power supply? Is it not enough to use C19 and C20 for that?

6) We can see a difference between connecting C11 in schemes 2 and 3 above (to +12V or to GND). What is the better?

 

Thanks in advance.