A while ago i build a the etherwave schematic. Everything is working except for the volume regulation. The volume oscillator build around L11 and L14 is oscillating at +/- 500 kHz. i can alter that frquency by turning the 5k volume pot or L11, but the volume dous not change. I can controle the volume by manipulating the tension directly on the vca out pin so i know the lm13700 is working properly.
What could be the couse of this problem?
Also can someone explain me what D1+C12 and D5 are axactly doing in this sircuit?
D1 and C12 form a simple rectifier to get a DC voltage which is proportional to the resonant current through L7. This will be then amplified by U3b. R37/D5 linearize the volume response for small input voltages, making the transition between no sound and a very few sound smoother.
Maximum volume is attempt when the resonant current through the series resonant circuit formed by L7 to L11 and the free-field capacitance of the volume loop is maximal. In that case you will measure a voltage of about -4V at the node D1/C12/R14 towards ground. Tuning L11 correctly consists in adjusting the volume oscillator so that its free-run frequency equals the self-resonant frequency of the above series tank circuit. If you can't tune them for maximum resonance, then ...
... either the series resonant circuit is interrupted (check DC resistance from the base of Q7 to the volume loop: should be between 30 and 40 Ohms)
... or there is too much parasitic capacitance around the volume loop, lowering too much the self resonant frequency (set the theremin up on a mic stand and make sure that there are no metallic parts up to 1m above and below and up to 50cm around the volume loop when trying to tune L11 for resonance). I've seen a few theremin-noobs who tried to tune their instrument while it was sitting on a metallic workbench with the volume loop just a few centimeters above...
... or D1 is defective and will never produce a DC voltage, even when there is resonant current through L7. A simple multimeter diode test will not forcibly reveal a problem. I had already some 1N4148 which looked well when checked with a multimeter but which didn't work in the circuit until I saw using an oscilloscope that they had a reverse breakdown at ~-25V although they should keep up to -75V
If everything is ok, you should be able to see a voltage of 0V at the node D1/C12/R14 which goes through a maximum of -4V and back to 0V when tuning L11 slowly from one end to another. At the moment when you have -4V, you should see +12V at PIN12 of U3b and hear sound.
Thank you for the fast reply. The volume control circuit is mutch clearer to me now.
After more measuring I found out there was a (very hard to see) bad contact in a solder joint (between R14 and U3-b). I also found out that U3 was not responding to voltage changes on pin13. In the beginning my PCB had a design flaw where the node R14, R35, R37 touched the -12v signal. I fixed it but I didn't think of the possibility that it could have damaged the U3 IC. I replaced U3 and the theremin responds to both the volume and the pitch antenna now.
The 'antennas' are only test wires for now. My next step is constructing the antennas and the cabinet.
Do you have any idea where I can find the exact dimensions for the antennas? In an older document I can find 18 inch for the pitch antenna and 19 inch in a u-bend for the volume antenna, but I want to make the more round volume antenna they use on the modern etherwaves.
p.s.: Sorry for the late reply. Seeing that the last post in this section dated from a while ago, I didn't expect a reply so soon.
As a fellow DIY'er, Ive learned the hard way that it really pays off to have the cabinet and other accoutrements ready before building the electronics. Thing like theremins are damn near impossible to tune and debug on the bench and without the real antenna. Get all this ready now, put the circuit in whatever cabinet you intend it to live permanently, then resume tuning/troubleshooting. I think you will have a much easier task.
Think I may have shorted something, because the volume antenna doesn't respond and the actual volume is really low.
My amp doesn't have an earth and the power adapter from M00g doesn't have the earth connected. When I was trying to tune L11, I think something must of happened as the amount of crackle you got from touching the volume antenna nearly popped my amp speaker out.
So, I've rigged up a temporary earth from the earth lug on the audio jack, into a plug to provide earthing. This has got rid of the crackling and popping, so I'm well grounded, but still no real response from the volume antenna.
So I replaced U3 and pin 12 gives -11v and when I short out C28 get 0v
And the volume IS REALLY LOUD!
I replaced D1 as I didn't get any -v reading on the negative side and that's where I am so far.
I tuned L11 into 450MHz and L5 into 285MHz and have adjusted L6 for zero beat and get a correct range over the pitch antenna but still no volume.
When tuning L11 you eventually get very distorted notes and turning the Volume CCW across to CW gives real distortion across the pitches – I don't think that's normal.
Latest volt readings are;
L9 4.8v 3.5v
L8 3.6v 3.3v
L7 3.1v - 011.7mv
D1 3.0v 043.6mv
C12 042.3mv -012.9mv
Top Middle Bottom
Q6 11.5v - 170.1mv 141.8mv
Q7 12.6v - 010.5mv 141.1mv
Please advise as what I could try next.
Well - at first I though the LM13700 was dead, so I spent some money and ordered a slew of them from Arrow (if you are in Philadelphia and need some we can meet - I don't read the email I registered with much, so keep trying).
Next step was trying to adjust L11 - of course I broke the slug...
Now I replaced L11 and it still did not work.
I tested diode D1 with a hand meter, and of course it showed it was 'good'. I reread this post and decided that it must be the issue.
I found an 1N4148 in some electronics junk my father had given me and...
Now to get it 'tuned' up so that I can return it to my son's school so that they can teach with it.