Right handed Moog to a left handed - cannot tune properly...

Alright - I am a little frustrated!
In the last week or so, I am having the hardest time tuning my Moog Etherwave for when I am playing (left handed).

Yesterday I was getting ready to record some Theremin onto a song that I am fnishing and it took me 45 mins to tune it. It was so weird cause usually when you go from body to antenna, you go up in pitch, but yesterday, it was going down in pitch... What am I doing wrong?

If you get a left handed Theremin (one that is custom made for that playing) will it be as easy to tune as a normal Theremin? Cause I cannot spend 45 mins each time trying to tune it. By the time it's all set to go, I am pooped out! :(

Argh! lol

Really shouldn't take that long.

The field is symmetrical - you should be able to tune it as if you were right handed, then walk around to the other side of the instrument, stand the same distance away and it will behave the same way.

If the field is inverted - playing high to low as you approach the field then it's way out of tune. If you can't find the right tuning by turning the pitch knob, then you need to open it up and tweak the coils a bit.

The instructions I use for this procedure are here:

http://www.moogmusic.com/forum/viewtopic.php?p=60972#60972 (The posting by Amos.)

THanks Gordon - I'll look into this tonight.. I'll move my Moog somewhere else and see how it handles...
:)

Good idea.

Also try sitting a CD on top of the enclosure next to the pitch rod.

My thinking is that when I say "inverted" I actually mean that the playable part of the pitch field is shrunk down to utterly tiny, and you are playing on the part of the field outside the zero-beatosphere, which is the other way around to the playable part. (if you see what I mean.)

Putting a conductive surface such as the foil inside a CD near the rod will affect the part of the rod inside the enclosure (the rigid wire leading to the circuit board) expanding the size of the playable part of the field. You will notice when you look inside that there is a layer of foil underneath the wire - the CD just adds a bit more.

[i] This is a technical posting - only presented to hopefully illuminate the matter and explain why the "CD on top of the theremin" might work, and the implications. Tuning a theremin is not to be done without awareness that you can easily completely screw up that theremins performance - Dont do anything I suggest below unless you are aware of this.. AND have read the posting by Amos linked above. I will NOT be following this posting up or replying to anything related to it! ;-) [/i]

It works like this:

As the hand approaches the antenna, capacitance 'seen' by the antenna increases, and the frequency of the VFO decreases.

[b]With a correctly tuned theremin:[/b]

The capacitance 'seen' by the antenna at the zero-beat point (say 50cm from the antenna) causes the VFO frequency to be the same as the reference frequency (which does not change) and the beat frequency (difference frequency of Ref <-> VFO) is 0Hz.

As the hand is moved closer to the antenna, capacitance 'seen' by the antenna increases, VFO frequency drops, and the difference frequency ( frequency of Ref <-> VFO) increases, so output pitch increases until maximum capacitance is 'seen' by the antenna with the hand close to the antenna - with the EW, the VFO is frequency is about 3kHz lower than the reference frequency at this point, so 3kHz is heard.

At the null point, (about 50 - 60 cm from the antenna usually) VFO and Reference frequencies are the same.. If the hand is moved further from the antenna, and if the resulting further reduction in capacitance is enough to pull the oscillators out of 'lock', the VFO frequency will become higher than the REF frequency, and a difference (beat frequency) will be produced which rises in pitch the further the hand is from the antenna (the opposite from how the theremin is supposed to perform, and a mode which is extremely non-linear).

[b]With a wrongly tuned theremin: [/b]

The REF frequency is tuned too low, or the VFO frequency is tuned too high - This results in the null point being positioned too close to the antenna. Placing a CD (or other capacitive object) close to the antenna or antenna wiring - as suggested by Gordon - effectively lowers the VFO frequency (by increasing the antenna and/or circuit capacitance). This will "work" - but has many undesirable concequences - The main one being that increasing the antenna capacitance in this way reduces the antenna's sensitivity to player induced capacitance change.

The simplest way to correct the problem is by adjusting the REF frequency is by re-tuning the REF frequency (L6) so that, with the tuning knob in its centre position, a pitch difference of 3kHz is heard when a gloved hand is lightly squeezing the pitch antenna.

One must then check that the pitch decreases as the hand is moved away from the antenna - If it does the opposite, then L6 would need to be radically re-tuned through to the other side of the virtual null-point.. One wants the REF frequency (which one is tuning with L6) to be 3kHz HIGHER than the VFO frequency when the pitch antenna is being 'squeezed' - If the REF frequency is 3kHz lower, one will hear 3kHz, but the theremin will be in 'reverse' mode.. I think it is unlikely one would be able to tune T6 to be this far out - the VFO frequency would need to be out of adjustment - but I do not have any 'real' EW oscillators so I dont know.

[b]Other issues:[/b]

The best tuning involves setting up the VFO frequency so that it is correctly tuned to the antenna - this maximises linearity - and then adjusting the REF frequency so the null position is correctly located away from the antenna.. Then, if required (which only occurs if the theremin is grossly mal-adjusted), the Volume oscillator needs to be adjusted IF there is any problems with beat frequencies from harmonics of this oscillator with the VFO/REF oscillators ("birdies")

To do things right, I think it is worth followin

Fascinating as ever, Fred.

I probably should have mentioned that I was suggesting the CD thing as a quick test before tuning the theremin properly.

Gordon said [i]"I probably should have mentioned that I was suggesting the CD thing as a quick test before tuning the theremin properly.."[/i]

I dont think you needed to .. it is a good test, and a possible quick fix.

There is an interesting aspect in having an extra deliberate 'load' capacitance.. If one tunes the equalizing coil and oscillator to cater for this increased capacitance, one can deliberately change the sensitivity of the theremin.

Register switching has been implemented (E-Pro and E-Vox) but I have not seen "Range" switching (as in, a switch which allows selection of say 3,4,5 or 6 linear octaves within the same playing field / distance) implemented.. But have seen frequent mention of the fact that the ClaraMin only had 3 octaves, and how this could be desirable ("less is more").

As all the elements (oscillators, Equalizers etc) of my forthcoming theremin have a voltage control input, I am hoping to implement range switching..

But, perhaps just having different sized CD's (or different placement positions marked on the top of the theremin) would do something quite similar.. LOL!

;-)

Fred.

Normally an Etherwave doesn't get out of tune from one day to another.

If it happens suddenly that you can't no longer tune it correctly, there may be another problem on the circuit board. The circuitry around Q9 creates a delayed helper voltage of about -11.2V for the tuning pots (the black cable which goes to the front panel). Q9 gets sometimes defective due to a too high leak current through C33 (I've seen this most times on the 11-211H board revision). In that case, the helper voltage floats around -5 and -10V depending on the tuning potentiometer settings, temperature and the moon phase, which makes correct and stable tuning impossible. Fiddling around with L5 or L6 will not really help, because the tuning even if newly "centered" will not remain stable.

So the first thing in such a case is to let the fingers away from the adjustable coils, but to take a multimeter and check the DC voltage between the black and the green cable while turning the pitch and volume tuning knobs. If it is not rock stable between -11.1 and -11.3V, replace C33 (100uF 25V) and Q9 (2N3906). This will re-stabilize the tuning voltage and you are done without playing unnecessarily on the oscillators.

@Fred: You are a genius on the theoretical field. But sometimes, a more pragmatic view paired with some practical experience, may be more helpful.

[i]"@Fred: You are a genius on the theoretical field. But sometimes, a more pragmatic view paired with some practical experience, may be more helpful." - Thierry[/i]

LOL ;-) - Thank you for the compliment, Thierry!

I wholeheartedly agree with your "a more pragmatic view.." The object of the game "should" be for me to get my bloody theremin on the market, and start earning some money - rather than stretching the process out in my search for perfection..

But as for "practical experience" I am certainly not lacking.. Oh, there was a time about 3 years ago when my ideas did lack practical theremin expierience, and I ignorantly spouted quite a lot of BS.. But I have now spent a full 3 years actually inventing/designing/building/testing/evaluating almost every possible theremin circuit and configuration.

I have not gained much of my expierience the way you have - I have worked on / modified / repaired reletively few "standard" theremins like the EW - And therefore, unlike you, the finer points of particular theremins (the recent modification to the EW tuning circuit, for example) are not known to me.. So you are, without doubt, the most knowledgable person to consult regarding these issues.. I go by the EW "Hotrodding" schematic when discussing the EW - You have more up-to-date knowledge.

Fred.

(P.S. - It would be really usefull to the theremin community if up-to-date EW schematics were available.. Do you know where these can be obtained, or could you perhaps post a schematic to TW, so that everyone can benefit ?)

[b] Added ->[/b]

It seems from what you say in this, and prior posting (http://www.thereminworld.com/forum.asp?cmd=p&T=4741&F=557), that Moog have modified the EW tuning circuit - and that this modification is prone to failure.

If this is true, then a theoretical evaluation of the failure mechanism needs to be done, and changes need to be made to eliminate this failure mechanism.

To me, there seems little point in replacing components which have failed, if the causitive mechanism for the failure of the previous components is not rectified first - one is just asking for the same fault to re-appear at some later date if one simply replaces these components without first fixing the cause of the problem.

If Q9 is operating as a voltage follower, and blows up, there are only a few possible causes [i](without seeing the actual circuit, this is a guess)[/i] - Either Ic (current through the collector -> emitter) is excessive, or the dissipation is excessive, or there is some mechanism wherebye base->emitter current/voltage is exceeded.. most likely is probably that Ic (Ie) is being exceeded if it is dumping excessive current into a discharged capacitor (this mode of failure would likely occur as the theremin is turned on, and not after that - a bit like how the surge current into a fillament bulb usually causes the fillament to blow when it is first turned on), and there is inadequate current limiting (emitter series resistor) protecting against this scenario.. but any of the above mechanisms could be to blame - and all should be easy to rectify - probably with no more than adding a single resistor.

It is, IMHO, extremely unlikely that leakage current through a modern capacitor could be great enough to exceed the 200mA current rating of a 2n3906 - even if this capacitor does not have a decent voltage over-rating..

The problem is that for only a part of the production (H board revision) they used 100uF electrolytic capacitors which tend to draw lots of DC current after some time (it transforms into a ca. 200Ohm resistor...). I've not seen this problem on other board revisions where capacitors from another manufacturer were used.

So the problem is not by design. I have a much older C board revision and it works still fine with the original capacitor/transistor.

And finally... I'm not too unhappy with such situations... On one side I may make people happy by repairing their boards rapidly (I have a big stock of 2N3906 and good capacitors) and on the other side I may make some extra money with such actions which helps me feeding my four daughters.