static electricity spark now etherwave is silent

"Fred, where would one fit such a component (excuse the ignorance) and would it change the playing characteristics of the instrument?" - Roy

One lead of the ESD tube would connect directly to the antenna with as short a wire as possible, the other to the common ground point where the supply comes into the theremin, again with as short a wire as possible - if short isnt possible, it should be heavy wire with low inductance - good quality copper. Each antenna wired in this way with its own ESD tube.

I have generally used simple 90V neon tubes which are far inferior to those I have shown above - but fine if one caters for their charactaristics in the design - they have a capacitance of 3 to 5 pF..

The parts shown above have a capacitance of less than 2pF, so are better suited to retro fitting, but re-tuning will be required and I cannot say what the effect would be of the extra 2pF on say the linearity of say an EW - It will have an effect.

Fred.

ps.. DO NOT run the ESD tubes ground connection to any other grounding point on the theremin!!! - It MUST connect to the central earth return.. running it any other path (or particularly to the theremins PCB) could actually create more devastation than not fitting it!

The ground terminal on the power supply connector is the only absolutely safe connection point.

One also needs to match the tube to the antenna voltage - if the antenna voltage goes to 100v peak (200V P-P) then the tubes discharge voltage must be a little higher (say 120V)

The above is what I deem MINIMUM protection - There should really be additional low voltage protection close to the sensitive electronics down-stream of the front end.. I usually implement this with fast diodes clamping excessive voltages to the supply rails.. the discharge tubes are there to ensure that these protection devices are not overloaded.. But on their own, the ESD tubes are probably enough - I am just a bit pedantic! (I try to blow my circuits up with a HV zapper before ive even got thenm fully working, LOL - zapping the antennas and the knobs and the leads... A bit silly really - I love HV sparks! - and love beating them at their game even more ;-)

"(I try to blow my circuits up with a HV zapper before ive even got thenm fully working, LOL - zapping the antennas and the knobs and the leads... A bit silly really - I love HV sparks! - and love beating them at their game even more ;-)'  -- FredM

We did this at the Telecom I worked for (calibrated zap gun) - LEDs hanging out of the metal faceplate were often the worst, sending the processor into la la land (call in the lightpipes).

Those are some interesting Bourns parts Fred!  First time I've seen them. 

Aside: hard clamping when there is LC resonance involved can sometimes cause trouble of it's own.  We had diode clamps on the logic side of DSL transformers that would go low impedance with overload and cause the transformers to ring at an even higher voltage due to the impulse and lowering increasing of net Q, which caused the windings to blow out.  Protection is sometimes counter-intuitive, and when it is it's often counter-productive (not that it shouldn't be pursued).

Theremin repair progress report, of sorts, from Bob in Ohio...

I thought I had it licked! I purchased a DMM from local electronics store ("Radio Shack") and used it to test the the diode D1 and it indeed tested bad, with a high transmittance reading where it should have been "0F" but after replacing the diode the Theremin is still silent. Sigh. While I was at the store I picked up the transistor which was suggested as a suspect also. It looks a little tight in there to replace. It seems odd that more than one part would fail. Maybe I can test it with the DMM as suggested above. Thanks!

Bob 

...Temporarily shorting C28 does result in sound...

Theremin repair progress report, of sorts, from Bob in Ohio...

I thought I had it licked! I purchased a DMM from local electronics store ("Radio Shack") and used it to test the the diode D1 and it indeed tested bad, with a high transmittance reading where it should have been "0F" but after replacing the diode the Theremin is still silent. Sigh. While I was at the store I picked up the transistor which was suggested as a suspect also. It looks a little tight in there to replace. It seems odd that more than one part would fail. Maybe I can test it with the DMM as suggested above. Thanks!

Bob 

...the voltage at the node D1/R14/C12, where Thierry above indicated that we expect -4V is -2.1 V, and is the voltage is the same whether or not C28 is shorted. The steps involving an oscilloscope may be above my capabilities. I do have one here, but I think it pre-dates Theremin himself, and I don't exactly know how to operate it, or if it works properly. The DMM has a frequency function, however. 

...Temporarily shorting C28 does result in sound...

Theremin repair progress report, of sorts, from Bob in Ohio...

I thought I had it licked! I purchased a DMM from local electronics store ("Radio Shack") and used it to test the the diode D1 and it indeed tested bad, with a high transmittance reading where it should have been "0F" but after replacing the diode the Theremin is still silent. Sigh. While I was at the store I picked up the transistor which was suggested as a suspect also. It looks a little tight in there to replace. It seems odd that more than one part would fail. Maybe I can test it with the DMM as suggested above. Thanks!

Bob 

...(without any tuning or the use of a oscilloscope) the voltage at pin 12 of the LM13700 is -11.6. When I short C28 (get a sound) the voltage at pin 12 drops to 0 (that's expected I guess). 

...the voltage at the node D1/R14/C12, where Thierry above indicated that we expect -4V is -2.1 V, and the voltage is the same whether or not C28 is shorted. The steps involving an oscilloscope may be above my capabilities. I do have one here, but I think it pre-dates Theremin himself, and I don't exactly know how to operate it, or if it works properly. The DMM has a frequency function, however. 

...Temporarily shorting C28 does result in sound...

Theremin repair progress report, of sorts, from Bob in Ohio...

I thought I had it licked! I purchased a DMM from local electronics store ("Radio Shack") and used it to test the the diode D1 and it indeed tested bad, with a high transmittance reading where it should have been "0F" but after replacing the diode the Theremin is still silent. Sigh. While I was at the store I picked up the transistor which was suggested as a suspect also. It looks a little tight in there to replace. It seems odd that more than one part would fail. Maybe I can test it with the DMM as suggested above. Thanks!

Bob 

"LEDs hanging out of the metal faceplate were often the worst, sending the processor into la la land (call in the lightpipes)." - Dewster

Yeah! I had this problem with a safety critical fuel monitor - it was epoxy encapsulated in an iron box, but had a couple of LED's on short light pipes - they got zapped during testing... The absurd thing was that extreme HV was used to apply the maximum industrial levels to meet the spec.. Such a discharge would have blown up the train whether my box survived or not! - but I had to redesign the whole damn thing.. Leds are a pain generally at all levels including EMC, even with light pipes.

" hard clamping when there is LC resonance involved can sometimes cause trouble of it's own. "

Oh yes, it certainly can.. but thats no surprise really, the energy must go somewhere, and clamping voltages must be specified so that the currents are within spec.. clamp too low, and one can certainly do as much or more damage than clamping too high.

"Those are some interesting Bourns parts Fred!  First time I've seen them. "

I only discovered them as a result of (illegally) modifying my phone lines wiring.. Found these parts on the board wired across the incoming line, wanted to know exactly what they were, and discovered them - but they hadnt turned up in any searches I had done for this kind of part - I think because I had been searching with the keyword "neon" and "ESD" - When I searched Element-14 with "discharge tube" I found them.

Fred.

...final update until I have any new ideas. This thing doesn't seem to be on the road to recovery...using the diode tester function of the DMM on various transistors including Q7 shows them all "behaving" similarly to one another (tested in place....is that valid?) so it doesn't seem like any one transistor is "off." The 3 coils on the volume side show (again, in place) resistances of 8 ohms in two instances and 12 ohms in a third; this contrasts slightly with the ones on the pitch side which are all 24 ohms. I'm kind of at a loss here. Any suggestions would be most appreciated. 

...(without any tuning or the use of a oscilloscope) the voltage at pin 12 of the LM13700 is -11.6. When I short C28 (get a sound) the voltage at pin 12 drops to 0 (that's expected I guess). 

...the voltage at the node D1/R14/C12, where Thierry above indicated that we expect -4V is -2.1 V, and the voltage is the same whether or not C28 is shorted. The steps involving an oscilloscope may be above my capabilities. I do have one here, but I think it pre-dates Theremin himself, and I don't exactly know how to operate it, or if it works properly. The DMM has a frequency function, however. 

...Temporarily shorting C28 does result in sound...

Theremin repair progress report, of sorts, from Bob in Ohio...

I thought I had it licked! I purchased a DMM from local electronics store ("Radio Shack") and used it to test the the diode D1 and it indeed tested bad, with a high transmittance reading where it should have been "0F" but after replacing the diode the Theremin is still silent. Sigh. While I was at the store I picked up the transistor which was suggested as a suspect also. It looks a little tight in there to replace. It seems odd that more than one part would fail. Maybe I can test it with the DMM as suggested above. Thanks!

Bob 

"(tested in place....is that valid?)" - Bob

Probably not!

I think you need to wait for Thierry - You wouldnt want to hear my prognosis, even if it was correct! .. I would get my vaccum soldering iron out and remove and replace all the active and probably all the passive components related to the volume circuit.. Oh, I probably wouldnt replace the inductors, unless I had reason to think they were compromised.

I think, at this stage, Thierrys advice is your only hope!

Fred.

The first question: Your single resistance values (2 x 8R and 1 x 12R) are basically ok. But is everything interconnected? Check the overall resistance from the base of Q7 to the volume loop: Do you have a reading of about 30 Ohms?

The second question: are these -2.1V constant, even if you put your hand very, very close to the volume loop, but without touching it?
- If the voltage goes down to near -0.3V, that means that the volume oscillator is working and you'll have to replace the LM13700.
- If the voltage remains constantly at -2.1V, that means that the volume oscillator is not working and you'll have to replace Q6 and Q7 together.