Etherwave Plus Sound Issues

Hi there, this is my first time posting, I'm looking for some repair info for the Moog Etherwave Plus.

I am the music curator at a museum which has a theremin interactive. The theremin itself is stock and has not been modified, other than some blocking to keep visitors from tearing the theremin itself out of the casework. I keep running into issues where over time the volume just steadily decreases over weeks/months. We turn the amp up to compensate and then one day it'll just stop working all together.

Because of this I've got a stock of about 5-7 theremins that I switch out. My local repair shop hasn't been able to get to these in a timely fashion so I thought I'd ask here if anyone knew what was happening. They have switched out 'bad coils' in the past, or so I've been told.

One of the last ones I bought was bad right out of the box and the most recent one I've had out of the floor for under a month and it just ceased working out of the blue. I opened it up and couldn't find any telltale signs of failure.

Anytime one fails I always check to make sure it's getting DC power, which they are, and that the amp and cabling is good. It's definitely an internal theremin issue.

Any insight would be appreciated!

Nick

Ah, actually, I was mistaken. I'm using the Etherwave Standard, not the Plus.

Nick

Nick,

Several theremins doing this same issue is interesting. It is a clue, but to what. Can you "post a picture" of the theremin in its normal location in the museum?

I would not trust your repair person unless they have prior good experience with theremins.

What country are you in, if the bottom of the world this could make sense, the same as people driving on the wrong side of the highway usually live in Europe.

The smarter people here need as many clues as you can give. I am sure there are tuning instructions here at TW that Thierry posted, you need to learn a good method of your own internal tuning, once understood it is not difficult. I think Thierry also drives on the wrong side of the road. Russia gets it right on the right side.

First thing a good engineer will say is you need an oscilloscope, I assure you a multimeter with a frequency counter works just fine. I need to post my method of simple tuning. Also I have a method of remote tuning as the theremin pitch often drifts over the course of the day, perfect for a museum. Most of my stuff has been put away so let us hope for input from others.

T

Ha, I'm in Colorado, USA.

I should note, the exhibit has been working for over 5 years now. The first year or two I didn't run into this at all. Nothing about the casework or design has changed. I'd say it's the last few years that I've been switching them out due to this particular issue. There is no consistency to how long they last, which is strange. I've had at least one go for a solid year before exhibiting this problem, and then this recent one was less than a month. I, of course, can't rule out bad power but we do run everything through standard surge protection for what it's worth.

I took my spare out of the packaging and fired it up and everything works wonderfully, nice and loud. I took the 'bad' theremin to my workspace, placed it on a mic stand in the middle of the room far away from everything and ran the usual volume calibration as described in the manual. I can still barely get any sound out of it. It's almost inaudible.

Very strange...

Nick

Hello Nick,

You gave another clue, Colorado so my thinking at first leans towards dry air and static discharge as the volume control loop is often grabbed.

Let us see where others take this conversation. I do have a method to protect against Electro Static Discharge that you can implement on your own.

T

Edit: One thing to mention is the printed circuit board copper traces are like tissue paper and will easily be damaged and lifting off the board from soldering iron heat, I prefer to clip a component lose from the top of the board and connect the new part to the remaining pins, maybe heat sink them at their base. The D1 diode is suspect along with the volume transistors for static discharge failure. Your guy doing the repairs probably learned this at your expense. Back in the day when I did TV repair 40 years ago I would just tell people it was beyond repair when I caused more damage, easy way out.

Try insulating the antennas, including the metal "plumbing" fittings.  It's probably ESD.  For the life of me I don't know why almost all Theremin antennas are bare metal, it's just asking for it IMO.

Hey Nick,

Another idea is to bypass the volume section completely. Just have volume on full when you attach the pin 12 of the LM13700 IC to the +12 side. In other words place a jumper wire (26 gauge) across the 8 pin header strip, one end in the VCA Out pin and the other end in the +12v pin on the same strip, this could bypass the problem on some of the unfixable theremins. In a museum volume control with kids I would think gets in the way. This strip is at the back of the board near the pitch antenna side. You would not need to remove the pc board for this fix. Even better remove the volume loop so kids are not grabbing it.

Maybe dewster will verify this approach, he is good at this theremin stuff.

T

Hello Nick,
because so many Etherwaves show here the same long-term effect (volume adjustment gets wrong) I think there maybe a temperature effect. For example, by intensive light spots. 

If we talk about the volume generator, it may be the reason for the lack of sound. At normal setting the frequency of the generator is 500-510 kHz. If the frequency decreases, the sound disappears first, and then begins to appear. Moreover, the response of the antenna volume will change to the opposite. This means that the theremin Will be silent when the arm is far away and sound when the arm is close to the antenna volume. This will be when the generator is set to about 470 kHz.

Hey Nick,Another idea is to bypass the volume section completely. Just have volume on full when you attach the pin 12 of the LM13700 IC to the +12 side. In other words place a jumper wire (26 gauge) across the 8 pin header strip, one end in the VCA Out pin and the other end in the +12v pin on the same strip,

I really like this idea! I might need some further help identifying which pin is pin 12, etc. Are they labeled anywhere? The VCA out and the +12 pin?

Since we run into an amplifier anyway it would be easy for me to adjust the master volume at that point.

Thanks,
Nick