New Theremin Problem

Posted: 1/3/2020 4:39:45 AM

Joined: 1/3/2020

Hi all, 

I acquired a Moog Etherwave Plus theremin and when I plug it into my amp it emits an extremely unpleasant, constant high-pitched noise. Other attributes of the noise:

1. Its pitch does not change, regardless of my hand's position in relation to pitch antenna.
2. My amp works (I checked it with a bass guitar). I have it plugged into the "audio" jack on the theremin and the "input" jack on a Hartke HD25 amp. 
3. My left hand's distance from the volume antenna does affect volume to a degree, but it seems binary, either volume on or off. Though this quality might be a product of the extreme nature of the high pitched noise. 

I did not assemble this theremin, but I'm willing to open it up if it would help. 

I would appreciate any troubleshooting suggestions. Thanks in advance!

Posted: 1/3/2020 9:34:34 AM

From: Colmar, France

Joined: 12/31/2007

If you just acquired it, this would mean that it falls still under the manufacturer's or seller's warranty. So, it's most probably better not to open it and to send it back.

If you bought it used, you might lift the cover (2 screws on the front side and 2 on the back side) and check the electrical connection between the PCB and the pitch antenna, and then the three huge inductors on the right side of the pcb. With an ohm meter (and the theremin powered off), these should read ~30 ohm each, and a continuity check from the collector of Q1 to the pitch antenna should read 90 to 100 ohm (the total of these 3 inductors). 

Check and report back, so that we can decide about the next steps. 

Posted: 1/6/2020 12:43:57 AM

Joined: 1/3/2020

Thank you for the suggestion! The theremin is past its warranty period. After opening my theremin, I discovered the state shown in the photo visible at It seems the L3 inductor was broken at one end and then twisted away. I'm not experienced doing circuitry apart from a college course I took 10+ years ago, but as far I can tell, the other two inductors gave the expected ohm reading, and the continuity check failed. Is this something I can fix or is the point moot?


Posted: 1/6/2020 7:25:26 AM

From: Colmar, France

Joined: 12/31/2007

L3 is definitively dead and has to be replaced by a new Hammond 1535G inductor. Afterwards (due to the 20% tolerance of these), both pitch oscillators need full recalibration. If you can find and replace the inductor, I can help you with the adjustment stuff via Skype or FaceTime.
If you don’t feel comfortable with this kind of surgery, you might take the PCB off, pack it well and send it to France where it can be fixed and will be sent back ready for use.

Posted: 1/6/2020 10:21:49 AM

From: Minnesota USA

Joined: 11/27/2015


I have one of those inductors if you are located in the US.  Check your personal messages.

Posted: 1/8/2020 3:23:33 AM

Joined: 1/3/2020

Thanks, Thierry and piits8rh! I'm going to try to repair the inductor, and if that doesn't work, replace it. I'll report back in a couple of days how that goes and then we can work on recalibrating the pitch oscillators.  

Posted: 1/17/2020 2:05:18 PM

From: Halifax, Canada (east coast)

Joined: 7/28/2019

It looks to me like you should be able to straighten and solder a new lead onto that L3 without too much work. Maybe post on Facebook for an electronics geek friend to look over your shoulder - or better yet, just do it.  And as you say, if that fails you could just replace it.

After fixing it you may have to figure out a way to attach L3 mechanically to the board so it doesn't flop around - maybe one of these epoxy'd down with a padded zip tie around L3.

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