Etherwave Pro problem

So, after a fair while of not playing my theremin I switch it on and find that there is something wrong. Basically the point at which it is silent when nobody is near it is just a high pitched tone. In other words, you get to the lowest point in pitch and then it jumps to a very high pitch. Also, it seems hard to get the pitch to a satisfactory interval distance, intervals seem very close and pitch is only happening fairly close to the antenna. Anyone have any ideas what is wrong?

Merula said: "In other words, you get to the lowest point in pitch and then it jumps to a very high pitch."

From my experience most of the weakest links in theremin design are mechanical. If the above is describing what happens when adjusting the Pitch pot my first guess is age has begun to gum up the internal contacts. This is very common but I will not tell you how I would remedy this as the Epro is enveloped in mystical power as some would believe by the price they try to get when selling one. I have enough ghosts bothering me. (-'

Christopher

"Knowing absolutely nothing about theremins -- I'm buying my first one tomorrow -- I'm going to draw upon my decades of computer experience. 

Look for some kind of debris " - holopratstick

The incredible irony here is that, even though you havent the faintest idea what you are talknig about, and your decades of computer expierience is irrelevant, there is perhaps an extremely  tiny possibility that you are right!

IF an antenna is loaded by some conductive matter, then this could raise the pitch and reduce sensitivity - but any debris capable of doing this would need to be somewhat conductive.

       ABOUT CAPACITANCE :                                                                                                

To understand theremin operation you need to understand capacitance.. (My advice is to google capacitance and read everything you can about it)

The antenna "sees" a background capacitance determined by the proximity of conductive objects, and to a very (extremely) much lesser degree dielectric (non conducting) objects between the conductive objects and the antenna. Conductive objects which are connected to ground have by far the most influence, other conductive objects will be "noticed" because of their capacitive coupling to ground.

Two primary variables determine the amount of coupling to the antenna: the first is distance from the antenna, and the 2nd is the size of the object (or actually "overlapping area" of the antenna as a "plate" and the object as a "plate" of a capacitor).

All objects in the vacinity of the theremin contribute to the background capacitance, so a (say 1")  object close to the antenna will contribute a comparitively large capacitance compared to the same sized object further from the antenna -

but ALL conductive objects (which are not shielded by a larger grounded object between it and the antenna) at ALL distances from the antenna (unless shielded, and for practical purposes the curvature of the earth will do that if nothing else does ;) will contribute SOME capacitance - even if this is unmeasurable and doesnt alter the pitch..

Rule of thumb is that any conductive object, or any part of a conductive object, which has a line-of-sight to the antenna and which is not blocked by a grounded conductive object, WILL affect the capacitance seen by the antenna - and there are conditions where even line-of-sight is not required. (These conditions relate to the geometric relationships between all conductive objects in the theremins vicinity - for example, a grounded object 'blocked' from the antenna by another grounded, will still have an effect if it has a line-of-sight to some other conducting onject which is not grounded and has an unblocked line-of-sight to the antenna)

Probably the most common moving "floating objects" are people, anf their influence on the field can be 'modified' by their proximity to grounded objects they pass, even if these objects themselves are occluded.

Also, any non-grounded conductive object touching the antenna will increase the antennas surface area, and therebye increase its capacitive coupling to the 'background'

The Etherwave Pro is somewhat different to most theremins (I am no expert, let me say - Thierry is the expert, and he is the one who will probably know instantly both the cause of, and fix for the problem) in that the equalizing coils are in the antenna support arm, and the antenna is further from the body than most designs.. The sensitivity of this arm is, I think, likely to be greatly reduced at the theremin end, making it unlikely that any debris in the theremin will cause the problem.. Also, as the antenna is out on an insulated "limb" I doubt that there is much, if any likelyhood of conductive debris loading the antenna.

My guess ? (and its purely a guess)  - Some problem with the coils in the arm, or its been zapped by an electrostatic discharge. I would first try dissasembling and reassembling (remove, clean the contacts, reconnect) the antennas.

Oh, I must just add that Christophers guess is probably better than mine! ;-)

Fred.

" In other words, you get to the lowest point in pitch and then it jumps to a very high pitch. "

This actually sounds like a problem related to the tuning of the VFO against the eq coils, as in, either the variable oscillator has detuned or become faulty, or there is a problem related to the antenna resonator - not knowing the Pro (particularly not knowing which oscillator the user tuning control acts on) makes it difficult to guess further - But I would be cautious about running the instrument for too long in this state until Thierry has given an opinion.. I vaguely remember some comments about excessive current being drawn when the VFO was driving a wrongly tuned antenna resonator - dont remember if this was the E-Pro, but play safe! ;-)

Well, thanks for help so far. I guess I'll call Moog eventually if I can't find anything to fix myself. I don't really think it's a debris issue or things being too close. In all my years of theremin playing this is something obviously abnormal. Anyone with experience of the Etherwave Pro inside?

Merula,

I know that Randy George does.... and of course, Thierry. :)

Hi Merula, 

There is a common problem with the Etherwave Pro depending on how the pitch oscillators are tuned.  If what you are describing is related to this issue then the high frequency tone that you are hearing when you reach the lowest point in your playing arc should be the same frequency that would sound if you would disconnect the pitch antenna arm from the instrument.  

In my instrument, this problem regularly occurred when first powering on my instrument after it had not been played for a while. The first thing to do when ever the instrument was powered on and before it warmed up would be for me to touch the antenna. Then afterwards, as long as I didn't power the instrument off, it would behave normally.

 For a long time after initially purchasing my E-Pro, this did not happen with my instrument. It only started happening after a few years, and afterwards I just got used to touching the antenna every time I first switched it on.  I'm not entirely sure how touching it actually resets the circuit, but I am sure it has something to do with a jump in the capacitance of the circuit when touched... 

Fred, BTW, that was an excellent description of capacitance you wrote. I think it would help if more people understood the principles and theory behind the invisible interaction with the theremin pitch circuit.

"Fred, BTW, that was an excellent description of capacitance you wrote. I think it would help if more people understood the principles and theory behind the invisible interaction with the theremin pitch circuit." - Randy

Thanks Randy,

I agree - although im not sure what my primary motivation is for repeatedly hammering this stuff.. More to do with scientific education than improving theremin playing, I think ;-) But I do strongly believe that de-mystifying stuff should eventually result in musicians being able to make better use of their kit, resulting in better performances.

I have tidied the description up a bit..

Fred.

"In my instrument, this problem regularly occurred when first powering on my instrument after it had not been played for a while. The first thing to do when ever the instrument was powered on and before it warmed up would be for me to touch the antenna. Then afterwards, as long as I didn't power the instrument off, it would behave normally.

 For a long time after initially purchasing my E-Pro, this did not happen with my instrument. It only started happening after a few years, and afterwards I just got used to touching the antenna every time I first switched it on.  I'm not entirely sure how touching it actually resets the circuit, but I am sure it has something to do with ESD and a jump in the capacitance of the circuit when touched... "  - randy george

Good lord!  I wonder what causes this?  It seems likely that there is a static charge is on the antenna that gets bled off when you touch it.  If so, this is possibly an argument against a series capacitance between the tank and antenna.

"Good lord!  I wonder what causes this?  It seems likely that there is a static charge is on the antenna that gets bled off when you touch it.  If so, this is possibly an argument against a series capacitance between the tank and antenna." - Dewster

I really have no idea about what this fault could relate to - The Epro is something of a mystery generally..

But I am inclined to think that charge accumulated on the antenna / antenna capacitor (if there is one ;-) isnt going to cause this, or at least on its own..

I think that an AC coupled antenna will still function if there is a DC charge on it with respect to ground - I dont really see how the oscillator coupling will be affected...

But, with regard to having a series antenna capacitor, this practice could certainly cause problems if there is no discharge path (ESD protection / discharge tube) to prevent excessive voltage from developing on the antenna side of the capacitor and exceeding the breakdown voltage of this capacitor. It would be quite easy to accumulate such a charge even if the antenna wasnt connected I think - Inside a padded case, for example, where friction of triboelectric materials could produce charge that transfers to the instrument (and particularly to exposed conductive parts like terminals abs connectors), and when the instrument is removed and connected to the supply, this charge is retained by components that dont have a leakage path.

One can blow up an instrument by connecting it to ground as easily as you can by zapping it with ESD - IF there are "charge islands" which dont have discharge routes or overvoltage protection.

I am inclined to thinking though that the E-Pro problem may be oscillator or logic related - a kind of bistable state reset by loading the antenna.. I have seen this with misbehaving SC oscillators (particularly in the EPE 2008) which had a series L added between the rank and antenna - The oscillator / antenna could flip into a sort of "positive feedback" mode of operation where the antenna resonator was operating in its capacitive region, and would flip back again when capacitance was reduced.. behave for a while.. be extremely linear.. but be dangerously unpredictable.

I never got to the bottom of what was going on, because I saw rhe flaw in the design (the mismatch between tank and antenna resonator components values) and fixed it. (well actually I soon abanoned this oscillator for use with series antenna LC)

I suspect that the E-Pro uses a feedback mechanism to improve linearity (this is pure guessing!) and that mechanism may be faulty.

Is Mr Frenkel on holiday ? It really may be worth PM'ing him on this - There is no documentation on the E-Pro innards, and Thierry probably had more knowlege (certainly with regard to faults) on this instrument than perhaps even anyone at Moog.

Fred.

Mr. Frenkel has been travelling trough southern France since last thursday, giving lectures and concerts in several conservatories. He's back now at his day job and he will give an exhaustive reply as soon as he'll have a free minute. :-)