Problems in home-made Etherwave theremin

Hi all.

I built an Etherwave theremin with my own layout. All basic functionality works fine. But unfortunately there are two problems which I need help to resolve.

The theremin is a bit modified (in defference of scheme from HotRodding document):

1) LM78M12 and LM79M12 instead of LM78L12 and LM79L12 correspondingly.

2) 100uF capacitances instead of C11 and C10. And pin + of C11 is grounded.

3) Instead of inductors L8-L10 I use just L9 to reach resonance.

4) 100uF capacitance instead of C13 and 47uF instead of C18 and C17.

5) 100uF capacitance instead of C4.

Problems I have:

1) L5,L6 and L11 are adjusted now. But regardless distance from volume antenna to my hand, I cannot mute the sound completely. Adjusting L11 or P1 (volume tuning control) doesn't help. When I touches the volume antenna there is very silent sound and it reacts on pitch changing. But if I touched the antenna then the sound can be noised (not always) and if I keep my hand very close but don't touch then the sound is always not noised.

I measured the voltage on pin13 and pin12 of U3 when loudness is changed:

pin13: is equal to -0.056V when loudest sound and equal to +0.019 when maximum silent (my hand is very close to antenna but don't touch).

pin12: is equal to +6.07V when loudest sound and equal to -11.51 when maximum silent (my hand is very close to antenna but don't touch).

Voltage of output of U2 is equal to -11.98V. If I connect pin12 of U3 to output of U2 then the sound mutes completely.

Could you please suggest me how to fix it.

2) When I power on the theremin I can hear some noise sound and the noise continuosly disappears. But if I turn volume tuning control to right half of circle then the noise appears again and become much stronger. If then I turn the control back then the noise disappears again. The noise appears/diappears continuosly (not suddenly) while colume tuning control is chaging position. Changing all another controls don't make any influence to the noise. But if I turn pitch control to zero-beats (no sound at all) then the noise diappears too.

Looks like there are some self-generations in the circuit of theremin. Changing C13, C17, C18 doesn't help. Using oscilloscope I can see that signals from all three oscillators are ok. The noise appears on pins 2 and 3 of U3 (see pictures below).

Noised pin3:

pin3 without noise:

If I ground pin12 of U3 then the noise still present.

Where do the generations cover? What do you think? How to fix that?

Thanks is advance.

Hello 

Cut your link between R14 and Pin 13 of U3 on both ends on your board and connect R14  with a wire direct to U3.

On your photo I belive to see that you lead  this  signal near the two oscillators. This my cause a lot of interference between volume antenna detection circuit and oscillatores.

Hi Ubermann

C10 should be 470nF - 100uF is WAY too big! C17 should be 1uF not 100uF!

I am working my way through your mods, but these two struck me as important!..

The "noise" on the waveforms is AFAICS normal HF - its a real sloppy integrator on the mixer - you may be advised to increase C24 to perhaps 100nF (thats nF not uF !)


"2) 100uF capacitances instead of C11 and C10. And pin + of C11 is grounded."

Ok - now I am confused! - Could you place a link to where your schematic comes from? - because I am using the one linked from TWs technical pages (this one) and C11 isnt grounded!

C11 should be 470nF not 100uF - on the schematic I am viewing, one end is connected to +12V (this end could be connected to ground - its simply AC coupling to ground anyway).

- If we are using different schematics, then what I said in my last could be wrong!

Im not going to spend any more time on this until I know we are referring to the same schematic!

Update ->

Ok - Have just looked at the EM theremin schematics - I think you have used these! This schematic is extremely similar, but is the earliest version of the EW.

I suspect that the poorer quality of the copy may have caused you to misread some component values - my advice is for you to print up the "hotrodding manual" linked above*, and compare the values in that schematic against the ones in your circuit - I think that you have seen things like .47uF and not seen the decimal point, so fitted 47uF.

*I now think you would be better served by comparing the values to the parts list in the EM article below.

Once you have confirmed the values and changed those you got wrong, it may work! -

Fred.

I have just found this reprint of the EM article which is a lot cleaner than the scanned copy I had been looking at.

Added ->

Look at the parts list in the above - some components (most) are specifically selected for optimum operation.. For example, C17 and C10 are specified as Tantalum.. This is no accident - Tantalum has better high frequency behavior than other electrolytics (or certainly did have when the article was written).. Replacing these components with large electrolytics can be a really bad idea!

Tanatalum became extremely expensive, and small values were often replaced with cheap ceramics which have better HF characteristics, newer electrolytics  appeared with reasonable HF performance, and these were used - But in general, the bigger the capacitance the poorer its HF specification, particularly with electrolytics! (this is the reason why one sometimes sees a big electrolytic with a parallel small ceramic - the ceramic is there to shunt high frequency which the large electrolytic hardly touches due mainly to its high series inductance).

ok - I have just clicked on your username and seen your prior posting - I forget names, so thought you were a newbee..

I am really puzzled - you seem to have had access to both EM and EW articles, so I have no idea what hybrid you have concocted!

And I am more mystified now as to why you put 100uF capacitors where you did!

Perhaps you could paste the ACTUAL schematic of what you have done, complete with values and type of components used, so that those who feel inclined to helping you can actually do this without wasting their time guessing?

Also, it seems this is really a continuation of this thread you started months ago:

http://www.thereminworld.com/Forums/T/29123/just-built-etherwave-doesnt-work

It would have saved me a lot of time if you had just continued on this thread, rather than starting a new one - History is an important part of all debugging processes!

Fred.

FredM,

I didn't misread anything from circuit schema. I made the changes intentionally. These capacitances are not critical to increase some. It wouldn't affect theremin's workability. Moreover, as I understood at my last thread (you mentioned above), the changes can fix some problems with parasitic self-generations within circuit parts. Some of my changes I performed in trying to fix the noise I described in this thread.

Take a look at schema №3 from first post at my last thread: http://www.thereminworld.com/Forums/T/29123/just-built-etherwave-doesnt-work.

There schema has some changes against original "Hot-Rodding" schema. As I understood using 100uF as C10 and C11 as described in the schema №3 is more stable that .47uF.

And this is not a continuation of original thread. This is a new one Etherwave (for myself now). The old one was completely workable and adjusted. So it cannot be considered as continuation of one theremin building-adjusting.

By the way I fixed both the problems by replacing LM78M12 and LM79M12 with LM78L12 and LM79L12. It seems like more powerful voltage regulators generate noise within power supply. When I disconnected pin2 of U3 I could see that the noise persists on pin3 but signals from both pith oscillators are "clean". That's why I suspected power supply circuit.

So now the theremin works fine. Thanks for participation in the thread.

The conclusion of the thread: don't use LM78M12 and LM79M12 instead of LM78L12 and LM79L12.

Great!

Glad you got it working.. I still dont understand how 100uF is better than 470nF (and am quite surprised it works) and, to be honest, I have not found 78L/79L regulators less noisy than standard 78/79 regulators - but regulators can vary quite a lot from different manufacturers.. For the regulator noise to be so large into the decoupling / smoothing capacitances on the circuit, I suspect they were not neeting their specification.

Sorry I misunderstood - But hey - Telepathy isnt something I have fully mastered yet! ;-)

Fred.

Actually I don't understand why do the regulators generate noise. Probably it has some bigger internal capacitance causing auto generations within power supply and volume tuning circuit. I don't know... But the noise appearance depends only on P1 knob position.

And also I don't understand how do the regulators affect sound level. I tried to connect R14 directly to pin13 of U3 according to betabox's suggestion. And the effect still persists - sound is not muted by moving hand close to the volume antenna or even touching that. Although the output voltage of power supply with LM78M12 and LM79M12 is absolutely the same as with LM78L12 and LM79L12 (as expected).

I used the more powerful regulators to avoid possible overheating of U1 and U2 when any additional hot-rodding PCB is attached. Hope power of LM78L12 and LM79L12 will be enough...

I don't know how do exactly some parts of circuit work but I think the bigger capacitances damp possible auto generations better within its circuit part. Anyway the approach is approved practically.

Datasheet of 78/79(M)xx recommends a Tantalium C of 1uF or standard aluminium electrolytic of 25 uF at output to GND for filtering. 

For 78/79L series a 

 0.1uF (as used in the EW Theremin) is enough. 

Maybe this is the reason