Troubles with the pitch control on my DIY theremin

Hi everyone,

I hope there isn’t a similar topic, I searched but found nothing.
So the issue I have is that I tried to get the theremin my dad and I built together when I was 12 up and running again. It’s a Moog Etherwave DIY kit, I attached a picture of the circuit below.
It sort of works, but there is an issue with the pitch control. Like there is some sort of “gap” where there suddently is no sound, and I cant’ get to the lower notes, so I lack a full range. I tried everything but there was no way to hit the lower register. I don’t really know how to describe the issue with words, so I made I video showing said “gap” (I didn’t find how to attach it here so I uploaded it to youtube).
I guess there is an issue either with the way we welded the potentiometer back then, or maybe the oscillator has an issue I don’t know.
Also the potentiometer controlling the waveform doesn’t do much, so I don’t know about that.

If anyone can help me out it would be superb, I am a beginner at all that electronics stuff and I don’t really know what to do.

Thank you all in advance !

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If it has sat for a while I would suggest cleaning the pots with contact cleaner. I can’t see any damage in the picture but give the board and caps a close once over.

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I suggest two things: (1) don’t use cleaner ever unless you absoluteky must–gently work the pots, blow in air, work some more; (2) did you tune the oscillator(s)? if not, try lowering the pitch and see if the “gap” disappears. If all else fails, try pot cleaner–but be VERY penurious with it, mask up while spraying. Once done, repeat steps 1 and 2. Guess: it is posdible that there is an issue with the circuit–and there you might start by testing resistors that feed into the oscillator(s). I don’t have one of these toys, but oscillators are just that and should behave the way they do in any other Moog or other device.

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Check out these items from the Moog site:

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Welcome!
You’re right, overdoing the contact cleaner can make matters worse but using a cloth or cotton bud to ‘bleed’ off the excess can mitigate the volume. I’ve found both contact cleaner and a soaking in brake cleaner useful on crusted up vintage pots.
I myself subject my own Etherwave Plus to a regular cleaning but for this i use a vacuum cleaner and a special attachment.

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I thought cleaners were basically just clean canned compressed air, but I saw that there are a bunch of different kind. What do you suggest I look for that would’nt damage the components ?

I tried to tune the oscillators, it doesn’t do much. What happens is that the gap just shifts in space. It can be seen in the second half of the video I posted, I turn the knob and the gap just gets closer to the antenna.

Checked the items out, I don’t think they apply, as I do not have any CV output on this model. Those became a thing on later Etherwave iterations.

Problem solved !
I contacted Moogs tech support, and it turns out I just had to adjust the three variable inductors directly in order to tune the theremin.
Here is the guide if anyone has the same issue in the future:

(1) Make sure the instrument is on a mic stand or on a wooden stool (the instrument is heavily infulenced by its surrounding)

(2) Center the Pitch and Volume controls to 12 o’clock

(3) Adjust L11 with the trimmer tool until you have your prefered volume range
The instrument should be silent with your hand close to the antenna and at full volume with your hand away from the antenna

(4) Adjust L5 to set the high end of the instrument
Keep your hand about an inch away from the pitch antenna and adjust for the high freq

(5) Adjust L6 to set the range
The pitch should be silent when you are arms length away from the instrument

(6) Go back and forth between L5 and L6 unitl you have your desired range.

(7) Screw the lid back on and Enjoy! (the calibration will be slightly off, but adjust the front panel Volume and Pitch controls to desired position)

Keep in mind personal preference! Its ok to vary slightly from this calibration.

Side note: the tool I had was made of metal, so whenever I touched the inductors the instrument made very unpleasant ultrasounds. And as you can imagine, it was very impractical for tuning (but you have your hands on the circuit anyway so you have no real way of hearing the sound as you tune)
If you can use a plastic tool, your ears will be thankful.

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Good to hear you got your instrument up and running!

There’s some theremin miscellany here:

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This is now a mystery beyond me. As for what won’t damage components, think first about your lungs and eyes whatever you may choose that is not compressed air (or equivalent). Mask, ventillation and eye protection are a good rule to follow. Why not ask/research one of the online electronics sites. Key, here, is that unless you unsolder the part and clean it away from the PCB, you are spraying stuff onto the board and other parts, which could harm their function. Folks who work with similar electronics (current gen computer switching, etc.), may have suggestions. I’ve been cleaning up equipment that is old (70s to 90s and early 2000 simple keyboard amps), so they are not as delicate as what you are messing with. I use a WD-40 contact cleaner–but don’t recommend it for non-vintage equipment.

Hope this helps.

Jon

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