static electricity spark now etherwave is silent

I've had my used Moog Etherwave standard for a only few weeks now and was enjoying it greatly but last night when I touched the volume antenna a static electricity spark occurred and it seems to have died as a result. Is that common or possible? Is there any part --a transistor say? --that is the most likely to have become zapped? Are the components testable? I have some familiarity with electronics and soldering. Any advice? Thanks from Columbus, Ohio, USA. 

Fascinating!  (And sorry for your loss.)  Electrostatic Discharge (ESD) is something that doesn't get near the attention it should in Theremin design IMO.

I don't think a spark has enough energy to open one of the series inductors, but you could certainly test them with a DMM in resistance mode (though this wouldn't find shorts).  Check D1 for proper conduction too with the "diode test" DMM function (it should have 0.7V one way and over range the other).

I think it's most likely Q7, the 2N3904 transistor with it's base tied directly to the other end of the series coils.  Check the Q7 bias points, the emitter should be somewhere around ground, the base s/b ~0.7V above this, and the collector voltage s/b even higher.

It could be the side of LM13600 which has pin 13 connected to a similar coil point, but through a 1M resistor. 

Here's an EWS schematic: http://www.theremin.info/-/viewpub/tid/10/pid/58

Good luck, and let us know about any progress!

ps : You might want to put some heat shrink tubing on your antennas to reduce ESD.  A neon bulb to ground might also help but I've never tried this.  While you're in there you might also want to change the tank transistors (basically all of the transistors, but Q1, Q2, Q6, and Q7 in particular) to something a bit beefier.  2N3904 are considered to be "small signal" types, but in the EWS they are being used on the high end of their current capability.

Thank you, dewster, for the theory. In practice, that means in about 90% of cases when this happens, it is the diode D1, a 1N4148 which is easy to replace. Sometimes the diode seems still good when tested with a simple multimeter, but the oscilloscope in the circuit shows a reverse breakdown at about -20V which will mute the volume circuit. Thus it is the first component to replace.

Then, the "VCA half" of the LM13700 (U3A) can be easily checked by temporarily shorting C28. This should give maximum volume less 6dB.

A quick systematic check of the whole volume circuit is as follows:

Everything (hands, cables, etc.) away from the volume loop => -4V DC at the node D1/R14/C12 (first replace D1, if still not, check the volume oscillator with an oscilloscope at the collector of Q6 24Vpp centered around 12V, and a frequency counter at the base of Q7 ~500kHz, tune L11 for maximum negative voltage at the circuit node mentioned above)  => +12V at Pin12 of the LM137000 (if not, replace the LM13700).

Thanks so very much Dewster and Thierry! This is most encouraging. I see the components you suggest may be the issue are almost no-cost, and I look forward to making the Theremin work again. I will keep you posted (and probably have more questions). So much appreciated! You are awesome!  

Bob 

"Electrostatic Discharge (ESD) is something that doesn't get near the attention it should in Theremin design IMO." - Dewster

I agree completely!

IMO, the major problem is not so much the instant failure situation as happened here (in this case, a discharge was noted, and the instrument died.. So the cause of failure was obvious) but when discharge damages a component which fails later.

And statistically, the vast majority of ESD caused failures are not immediate - they are ticking time bombs waiting to fail at the worst possible moment! (nah - theres no real reason why they should fail at the worst moment - thats unscientific! ;-)

It astounds me that theremins are produced without even rudimentary protection - such protection only costs pennies! - I think this may be due to thinking that the large inductance between antenna and circuit will block any ESD - but this is folly, as there is also the capacitance of this inductor across which a HV charge can pass, and other routes from the antenna - Protection needs to be between the antenna and ground with minimum inductance - a discharge tube or solid state equivalent right on the antenna connector.

IMO, your attempt at predicting the failed component was brave - I have done the same at times - but foolhardy.. ESD is horribly unpredictable and failure will depend on all sorts of unseen factors such as clearance distances etc - its not only normally Hi-Z bits like FET gates which are subject to failure (although they are often the most likely) - A reverse biase diode behaves as a Hi-Z capacitor, and if its VR is exceeded, will be damaged, as noted by Thierry.. Even passive components like capacitors and resistors can suffer damage, as can inductors if their insulation breaks down.

Fred.

** Ps .. I missed the word "used" - Presumably the instrument isnt under warranty.. So what follows is not applicable. Sorry.

"I've had my used Moog Etherwave standard for a only few weeks now and was enjoying it greatly " - Bob

Surely the instrument is still under warranty? - If I was you (even being an electronics engineer) I would demand a replacement! Cite the fact that even if you or they replace the faulty component, they CANNOT assure you that no other component has been compromised! - they would need to replace all components to insure this.

I think its a scandal that suck lack of care is taken in the design / manufacture of an expensive (for what it is) instrument, and that Moog should get feedback to this effect!

Fred.

If ESD is such a hidden threat, should we all be doing a Sandy Shaw and play in out bare feet?

Aye, I know this comment dates me...

"In practice, that means in about 90% of cases when this happens, it is the diode D1, a 1N4148 which is easy to replace. Sometimes the diode seems still good when tested with a simple multimeter, but the oscilloscope in the circuit shows a reverse breakdown at about -20V which will mute the volume circuit. Thus it is the first component to replace."  - Thierry

Thanks Thierry!  This seems to happen a lot for you to have this level of detail and a statistic on it?  I'd be quite interested in knowing how many of this particular EWS failure you've encountered / fixed (rough estimate).

Also: Have you seen any obvious ESD failures of the pitch side?  If so, what failed?

ESD is a real threat - and on equipment like a theremin, designers / manufacturers ignoring the issue is just inexcusable - Sorry - But its a fact and even Bob Moog failed on this.. I may well have also been ignorant of the realities had I not spent most of my life working on safety critical applications - So perhaps the blame goes back to engineering education.

If it was something difficult or expensive to protect, then one could argue that cost / risk trade-off justified ignoring it.. But at one-off prices, an extra 50p or £1 maximum per theremin  eliminates most antenna ESD risk - parts like these do the job:

http://www.farnell.com/datasheets/1640723.pdf

For production quantities, one is perhaps looking at an extra 40p total per theremin!

Fred.

Oops - The posting I was replying to seems to have gone.. so it looks like im just banging this drum without provocation! - There WAS a posting, im sure I didnt just imagine it - I think it was someone on my side of the pond too... RoyP ??  ;-)

Oh hell - It just came back! And one by Dewster! ... Some ESD in the system, me thinks! ;-)

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

Roy