Etherwave: Bad hum problem

I have recently bought a pre-owned Etherwave (standard). Unfortunately I did not test it properly beforehand. It has a 50 hz hum that ruins the sound. The hum is present when only connecting headphones, so it is not about the amp.

The hum is somewhat diminished if I touch the audio connection, and the same when I ground the unit to a waterpipe in the house, but neither way of grounding kills the hum enough to make the nice clean sound it should make. (When I play a note around 100Hz, I will get throbbing beatings because of the hum.)

I suspected it was the PSU, but I just got a new one (original), and it changed nothing.

What I find interesting is this: When I turn the Etherwave off, there is about one second, where sound is still heard, and THAT sound is clean and free of the hum-induced distortion. 

(It also sounds like there is always a secondary "ghost-note" that is a major third (four semitones) under the main tone. This may or may not be part of the hum problem, but we will see, if I ever get rid of the hum.)

I have opened up the unit to search for things broken or loose, but everything looks perfect inside. 

I will be very grateful for any help.

-Lars

Hi Lars!

I'm thinking it could still be a grounding issue.  Maybe the AC wiring isn't all that well grounded?  

It could also be the EWS is sitting near a strong AC source, like a power cable in the wall?  This was the case for mine, if I plugged it in the living room upstairs there was an AC cable in the wall going from the outlet up to the ceiling light, then over to the light switch.  It "motor-boated" like crazy when playing a pitch near 60Hz or 120Hz.  Have you tried moving it to a different outlet / location?

It might be a bad rectifier, or (most likely in my mind) a bad electrolytic filtering capacitor before the regulators.  Electrolytics don't last forever, and are one of the first things to go in older electronics (they sometimes swell their aluminum can when going bad).  Their ESR (equivalent series resistance) goes up as they go bad, so a quick check might be to wire a known good one in parallel (but be careful doing this, they are polar devices and can explode if wired up backwards).  Do you have any test equipment?  Ideally a scope?

Your amp is grounded.

Your headphones are not.

Plug into the amp and connect your headphones to the amp headphone output.

See if that solves the problem.

Grounding issues can be solved by using a long guitar cable with a substantial amount of it laying on the ground or floor. you may also want to run that cable so that it can touch your body - I stick a loop of it in my sock if I am having a grounding problem. Or you can just put your shoeless foot on the cable.

The theremin also should be grounded to the mic stand - you can run a wire from it to a water pipe. Or you can just see if putting your foot on the leg of the mic stand changes things.

It helps if you are also grounded - I take off my shoes and /or sit on the floor or ground.

Also, play on the ground floor of your building - surprisingly, your get better ground when you are on the ground..

Theremins are more sensitive to these issues than other electronic instruments.

50 HZ? If you are in Europe consult with Thierry Frenkel if these suggestions does not solve the problem.

Also, it may be the plug of the headphones.

Good luck.

One other troublesome thing, certain electronic switching power supplies and certain LED light transformers are noisy as hell - the antennas may pick them up.

I had to use batteries with my zoom r16 for years due to interference from their power supply. Finally they came out with a power supply that didn't interfere with my theremin. So now I don't have to spend so much for batteries.

You can see if you have any noisey wall warts or LED

lights.

"Hi Lars! I'm thinking it could still be a grounding issue.  Maybe the AC wiring isn't all that well grounded?   It could also be the EWS is sitting near a strong AC source, like a power cable in the wall?  This was the case for mine, if I plugged it in the living room upstairs there was an AC cable in the wall going from the outlet up to the ceiling light, then over to the light switch.  It "motor-boated" like crazy when playing a pitch near 60Hz or 120Hz.  Have you tried moving it to a different outlet / location? It might be a bad rectifier, or (most likely in my mind) a bad electrolytic filtering capacitor before the regulators.  Electrolytics don't last forever, and are one of the first things to go in older electronics (they sometimes swell their aluminum can when going bad).  Their ESR (equivalent series resistance) goes up as they go bad, so a quick check might be to wire a known good one in parallel (but be careful doing this, they are polar devices and can explode if wired up backwards).  Do you have any test equipment?  Ideally a scope?"

The AC wiring? This is Denmark, Europe, and we normally don´t have grounding in a wall outlet. The Moog PSU (for Europe has two prongs) but that is not normally a problem. I have played several Moog Etherwaves here that sounded fine. 

I have two rooms, and the problem is the same in both of them.

The electronic problem you speak of, sounds very interesting. The only equipment I have is a digital multimeter. I would need some help though, because I don't even know what a rectifier or a capacitor looks like. (But I can solder.)

"Your amp is grounded.Your headphones are not.Plug into the amp and connect your headphones to the amp headphone output.See if that solves the problem.Grounding issues can be solved by using a long guitar cable with a substantial amount of it laying on the ground or floor. you may also want to run that cable so that it can touch your body - I stick a loop of it in my sock if I am having a grounding problem. Or you can just put your shoeless foot on the cable.The theremin also should be grounded to the mic stand - you can run a wire from it to a water pipe. Or you can just see if putting your foot on the leg of the mic stand changes things.It helps if you are also grounded - I take off my shoes and /or sit on the floor or ground.Also, play on the ground floor of your building - surprisingly, your get better ground when you are on the ground..Theremins are more sensitive to these issues than other electronic instruments.50 HZ? If you are in Europe consult with Thierry Frenkel if these suggestions does not solve the problem.Also, it may be the plug of the headphones.Good luck."

 I think I have tried most things now, regarding grounding. As I wrote: Grounding diminishes the hum somewhat, but it is still bad.

Yes, Thierry sounds like the guy who would know. :-) (Yes, I am in Europe.) 

And no, it is not the headphones. 

(When I think about the fact, that we send 50Hz AC into the Etherwave, it does not seem very surprising to me, if a slight malfunction of the electronics can let a 50Hz hum through to the audio out.)

"The AC wiring? This is Denmark, Europe, and we normally don´t have grounding in a wall outlet. The Moog PSU (for Europe has two prongs) but that is not normally a problem. I have played several Moog Etherwaves here that sounded fine."  - Lars

I'm kind of shocked the Moog PSU doesn't have a ground, as a good ground is fairly necessary for the Theremin to reliably function.  

"The electronic problem you speak of, sounds very interesting. The only equipment I have is a digital multimeter. I would need some help though, because I don't even know what a rectifier or a capacitor looks like. (But I can solder.)"

The supply capacitors are the large blue cylinders in the image below:

You might want to inspect them closely for bulging, particularly the end with the rubber plug, though they can be bad even without visual indications of badness.  Note the outer marking on them with a minus or negative sign and an arrow: it is pointing to the lead that should remain at a lower voltage than the other end.  Also note the working voltage printed on the outside, for replacement you generally want the same or higher working voltage, though higher voltage parts can have higher ESR, and they are usually physically larger as well, which can present problems.  For a quick check get a known good electrolytic with the same or higher voltage, it doesn't matter about the capacitance as long as it is at least maybe 1/4 the value.  The EWS schematic tells me these should be 2200uF, so 470uF or larger.  Use alligator test clip leads, or solder the good cap across the suspected bad cap and see if it makes a difference.  If one cap is bad I would replace both with the full 2200uF units, as they likely come from the same batch.  Watch polarity!  Electrolytics can bulge, catch fire, or explode if wired up backwards and significant power is routed through them.  Reverse polarity will definitely damage them.

The caps shown above are axial, with one lead on each end.  I've seen some pics on the web of EW circuit boards with radial leads, where both leads come out of one end.  For testing it doesn't matter which type you use as they function the same.