Etherewave Pitch Tuning Problem

Hi all,
   I just finished building an etherwave kit and I'm on the final step of tuning it. The volume side of the instrument seems to be behaving as expected - the volume goes silent when my hand gets close to the antenna, and adjusting the volume knob changes the cutoff distance as expected.
   On the pitch side, the theremin is producing a constant high pitch, and waving my hands around the pitch antenna produces no effect. Changing the pitch tuning knob changes the constant high pitch, but that's about it. I have tried it in different rooms, on different circuits, with and without the lid. My guess is that I have to go in and adjust the L5 and L6 oscillators (which I thought were supposed to come tuned from the factory) but I have a few questions to run by more experienced people online before I mess anything up. So, a few questions:

a) Does my problem suggest an easier solution, like an obvious/common construction mistake?
b) The tuning instructions for the pitch circuit say to short capacitor C28. This is a tiny capacitor with virtually nothing to clip leads to, and besides this practical obstacle, I have no idea why you would want to short it before tuning. What's going on here?
c) This is more of a theory question - there's a grounded piece of aluminum foil that sits right under the lead to the pitch antenna. It seems to me like this is equivalent to always holding your hand a couple cm away from the antenna, and maybe would produce a constant high pitch. Why is the foil there and what does it do, when the instrument is behaving normally? 

Thanks for any help, and I hope my questions make sense.
--Henry

"b) The tuning instructions for the pitch circuit say to short capacitor C28. This is a tiny capacitor with virtually nothing to clip leads to, and besides this practical obstacle, I have no idea why you would want to short it before tuning. What's going on here?"  - clueless

You can short it out at the header, pins 1 and 2.  This defeats the volume loop and makes the audio on all the time, so tuning is a bit easier, but you don't have to do it.

"c) This is more of a theory question - there's a grounded piece of aluminum foil that sits right under the lead to the pitch antenna. It seems to me like this is equivalent to always holding your hand a couple cm away from the antenna, and maybe would produce a constant high pitch. Why is the foil there and what does it do, when the instrument is behaving normally?"

The foil and wire form a small capacitor which adds to the antenna intrinsic capacitance.  I wouldn't worry about that until you're a tuning guru (which I am not).

I think you're going to have to play around with the pitch coils.  Mark their current positions with a magic marker and go to town.  Maybe try L6 first.  If you have a DMM you might want to check the continuity of the chokes leading to the antenna, they're quite fragile and can be easily damaged.

dewster as you mentioned before, check the continuity of the three pitch side coils in series before adjusting anything.

Amazing how they can be so fragile.

Christopher

Thanks guys, that's all very helpful! It turned out to be a bad inductor - in fact, I can even see the broken lead. I'm going to try to repair it by soldering in a jumper, otherwise I'll have to wrestle with moog's customer service (or an electronics shop) for spares.
--Henry

Glad you found the problem!  You definitely should get a replacement choke (try for two!) from Moog, as they've become rather rare on the market.

Is getting parts from Moog easy? I am curious.

If I was to substitute with only what is available today I would try a 10 mh choke that has the highest current rating I could find. This gives it properties I like. Below is 450 ma. Even more ma is better.

https://www.ebay.com/itm/CADDELL-BURNS-1662-2-Shielded-Choke-10mH-10-450mA-AXIAL-NEW/201443982599?hash=item2ee6ff3d07:g:S7kAAOxyf1dTKH~z

dewster when a choke says it is Shielded and it has no obvious metal can around it how are they doing it. Is this more an inductive closed loop or capacitive shield from a special outer coating.

Christopher,   has  always wanted to know.

"If I was to substitute with only what is available today I would try a 10 mh choke that has the highest current rating I could find. This gives it properties I like. Below is 450 ma. Even more ma is better."  - oldtemecula

Current rating has nothing to do with it.  I can't find the 1662-2 datasheet from a quick search, but it seems to have a minimum self-resonance frequency of 200kHz, which probably isn't high enough.  There aren't a lot of inductors out there that have good enough specs to be Theremin EQ inductors.

"when a choke says it is Shielded and it has no obvious metal can around it how are they doing it. Is this more an inductive closed loop or capacitive shield from a special outer coating."

For audio, shielding often means electrostatic, but for an RF choke I assume it's inductive (ferrite core concentrates the majority of the magnetic field).

dewster said: Current rating has nothing to do with it.  I can't find the 1662-2 datasheet from a quick search, but it seems to have a minimum self-resonance frequency of 200 kHz, which probably isn't high enough.  There aren't a lot of inductors out there that have good enough specs to be Theremin EQ inductors.

If this is all someone has it is worth trying, it would work. Otherwise gut the EWS box and put my Electrodeum in it. My guess the SRF is 100 kHz. I want this value to be as far from my own operating frequency either high or low. Higher current chokes have larger diameter wire which has less skin resistance. What I want to achieve is antenna circuit resonance with the largest p-p voltage on the Pitch Antenna.

Moog must place a custom order in quantities of this coil, again are getting parts from Moog easy?

We will never agree as you over complicate which is better so we present ideas from a different approach.

In my oscillators I always get shielded chokes hoping it reduces coil interaction.

Christopher

"If this is all someone has it is worth trying, it would work."  - oltemecula

You seem to be recommending he buy it, and it probably wouldn't work.

"My guess the SRF is 100 kHz. I want this value to be as far from my own operating frequency either high or low."

You need SRF to be significantly above the operating frequency, otherwise the coil behaves more like a capacitor (the phase flips 180 degrees).

"Higher current chokes have larger diameter wire which has less skin resistance."

Skin effect isn't the limiting factor to Q, I believe RF transmission is.

"We will never agree as you over complicate which is better so we present ideas from a different approach."

What color is the sky in your world?  We're talking coils here, not how many angels will fit on the head of a pin.

Linearization inductors need to have a specific inductance, SRF, and DCR, because these are the important factors here. Ideal inductors with infinite SRF (no parasitic capacitance) and zero DCR (highest Q factor, no energy loss) do not exist. That’s why a Theremin designer like Bob Moog has to go with a real world product and to adapt the oscillator circuit design to the weaknesses of the selected inductors.
So, you can not take whatever inductor to replace a broken one, it has ideally to be the same manufacturer (brand) and model, or at least one with identical inductance, SRF and DCR.
In the Etherwave Standard and Plus, Hammond 1535G inductors (10mH,710kHz,31R) are used in series with the pitch antenna, and Hammond 1535D (5mH,1MHz,14R) and 1535B (2.5mH,1.3MHz,7R) on the Volume side.
The Hammond 1535 series inductors have recently become obsolete, but most distributors have still sufficient stock. The only alternative, the Bourns/J.W.Miller 630x series, has already become obsolete almost 10 years ago, they are very hard to find.
Thus, be careful with and preserve what you have! I just hope that Moog music inc has enough stock, because it will be extremely difficult to design another robust analog “bread and butter” Theremin like the Etherwave which will remain on market during 25 years with today’s available parts.