Etherwave Pro problem

Now, Mr. Frenkel has a free minute and he'll tell you exactly what happens:

When you connect the pitch arm comprising 4 inductors and a small variable capacitor to your Epro, you add a series resonant circuit to the oscillator's parallel resonant circuit which helps for linearization. The resulting resonant frequency is either above or below the individual resonant frequencies since the equation is biquadratic and has theoretically 4, but in practice 2 real solutions.

By having the antenna circuit's resonant frequency slightly below the oscillator's free run frequency, the combined system should automatically jump onto the upper solution (since it is closer) and everything will be fine. 

If both resonant frequencies are too close, either because the oscillator's free run frequency is too low or the antenna circuit's resonant frequency is too high, the whole system will tend to jump on the lower solution, which is much more far away from the fixed oscillator's frequency, giving a very high pitch. By touching the antenna, you pull the antenna circuit's resonant frequency temporarily strongly downwards, which helps the system to jump up onto the "correct" resonance. It will then often remain there, as long as the oscillators run. That is neither a damage, nor a real problem. This effect can appear thanks to changes in the environment (temperature, humidity, etc.) or due to component aging which makes the resonant frequencies vary slightly.

If this phenomenon starts being annoying, it can be cured by either raising the variable pitch oscillator's free run frequency which is very difficult in the Epro, you'd have to use a frequency counter and fiddle with some of the jumpers and one variable capacitor on the pitch board. But that's not the usual way.

Or, and that's what we will do, you can easily lower slightly the resonant frequency of the pitch arm. It has for that purpose a small hole on its bottom side through which you can tune a very small variable capacitor. You will need a torch to find the capacitor's slot, it is much smaller than the hole. And you'll need a very small screw driver or preferably a plastic RF tuning tool with a ceramic point made for these small slots. Now you should try to adjust the pitch arm's resonant frequency with that by turning the variable capacitor in very, very small steps until the startup frequency of the whole system will be stable and not longer jump. But you shouldn't go too far because if you lower the pitch arm's resonant frequency too much, the linearity will not longer be optimal (the tone spacing in the highest octave will shrink) and you risk to come out of the tuning range of the pitch knob.

Don't hesitate to post your experience back here and/or to ask further questions.

Thanks for the help! I did play around with the pot. I fear I may have gone too far and I'm not sure where I was originally. 

Anyway, I did post a video. As you can see, a cable brings things to relative normality:

https://www.youtube.com/watch?v=mDQ04nWJe9I&feature=youtu.be

Glad you are back, Thierry ;-) Hope you had a nice vacation ;-)

By the above, is it correct to assume that the EW-Pro oscillator <-> antenna resonator is "tighter" (set to the state where the pitch oscillator frequency is extermely near to the antenna resonant frequency at the null position) than on most theremins? Is it this, in combination with an above average stable oscillator, which helps the instrument to have better linearity?

Fred.

Fred, the answer to all your questions above is "yes", but it's not only the proximity of the resonant frequencies. It's also the choice of the different L/C ratios. Do you remember my "thesis" about how this kind of linearization basically works? I started gaining insight by formulating mathematically this principle of linearization and verifying everything with the help of the EWS circuit. Only later I measured almost every aspect of the EPro's pitch circuit, too, and found everything confirmed.

Thank you Thierry,

IMO it would be great if someone was able to publish the schematic of this oscillator and the values for its resonators (tank and antenna)

The plots I have seen indicate great linearity when correctly tuned .. Having this data might enable others to either use the circuit as-is, or to used derived understanding to make better theremins.

Also, there is still this on-going "debate" about whether "real" linearity improvement is obtained usinf the dual resonator topology.. If anything is going to give a definitive "answer" then the E-Pro circuit probably can.

Oh well, one can but hope and wish ;-)

Fred.

So I couldn't really fix it on the arm pot. Given that the cable brings the pitch back down, is there some kind of coil or something I can add (hopefully inconspicuously) to the arm?

Hey -

Why not try some adhesive aluminium tape 'round the base of the antenna and perhaps extenfing a bit down the arm? - This should increase the capacitance a bit!

Cant see any harm could be done.. In fact parhaps just some aluminium tape stuck on the arm... But I suspect it wont be a good idea to make a loop round the arm as this could become an inductive load (shorting turn) seen by the coils, which would reduce Q.

All I think you need to do is to increase tha antenna capacitance some how without affecting its sensitivity - any small increase in the effective length (or circumference) of the antenna might do this - - A piece of coat-hanger wire cable-tied to the antenna so you can move it up/down increasing the length to taste.. Uou do not need whatever you use to electrically connect to the antenna - any conductive material close to the antenna which effectively increases its capacitance should do the job.

Fred.

Adhesive aluminum tape may help, but that remains a patch or a workaround.

I've (up to now) never seen a pitch arm with a broken coil (there are 4/5 of them inside), so I suspect that there has rather been a slight change in the VPO's free run frequency, which happens often and is normal over a long time. And that's exactly for that that Moog has put this small variable capacitor into the pitch arm which allows to adjust the arm's resonant frequency by about +/- 2kHz.

Retuning the pitch oscillators should normally not bee needed. Out of that, it's a rather complicated thing, fiddling around with jumpers on the pitch board to set the free run frequencies of the FPO and the VPO (without the arm connected) to 284kHz and 278kHz (pitch board rev B) or 282kHz and 276kHz (pitch board rev C), each within +/- 400Hz. The test points are on IC U7 - pin 1 for the VPO, pin 13 for the FPO.

Selecting these frequencies makes sure that there will be optimal linearity with the pitch knob centered, if and only if the pitch arm itself is correctly tuned with the small variable capacitor. Take care: there are also two variants of pitch arms, one suited for the rev B pitch boards and one for the rev C.

Merula, you could send me your pitch board and the arm in for diagnose and eventual adjusting/repair.