The EM Theremin - Pitch and Volume Antenna: not working.

A coworker of mine would say "it's always something stupid" when people (myself included) would begin to over-speculate as to the cause of the most recent bug.  Words to live by.

A coworker of mine would say "it's always something stupid" when people (myself included) would begin to over-speculate as to the cause of the most recent bug.  Words to live by.

Indeed they are, at least when we jump into these things and assume the worst only later to find out its something stupid we usually feel stupid ourselves. Should help prevent future ones but we know it wont.

I recently finished building the EW Theremin, from the schematic shown in "Hot-Rodding your EW Theremin" but there are quite a few problems. I tried to figure things out from other threads, but i couldn't solve any of them.

So, the main problem is that the volume antenna doesn't work (first time you heard that, no? :p ). Thierry has posted several times a debugging method for this.

1) Coils L7 to L9 are ok as far as the resistance is concerned.

2) I don't have an oscilloscope here at home, but last time I checked the volume oscillator with an oscilloscope it was producing a waveform of about 450kHz (I don't remember if by messing with L11 it went up to 500kHz)

3) When I tune the L11’s slug, the voltage at the anode of D1 varies significantly: from -0,4V to -18V, then -1,25V to -17V. That changes when I put my hand near the antenna.

4) Voltage at pin12 of U3 is -11,23 and it doesn’t change neither when I put my hand near the antenna, nor when I mess with L11.

5) Pin13 is constantly at 8,12V (should be ~0 as Thierry points out) and L11 has no effect on it.

6) Voltage at emmiter of Q7 varies from 0,16 to -0,03, depending where L11’s slug is.

I should mention that I have changed D1 two times. The first time I changed it, when I was messing with L11 there was a point where the volume went up, though the sound was really distorted and the volume antenna was acting as pitch antenna. But after a while (20-30 seconds if I recall) the sound stopped. When I changed D1 for the second time, nothing similar happened. Also, when I turn the volume of my amp all the way up I hear sound. I guess that is because of the constant -11,23V ot U3’s pin12. I also changed LM13600 to LM13700, but nothing different happened. Do you suggest any other part other than D1/U3 that may be broken?

The second problem is on the pitch side. When I short circuit C28, there is sound, but when I keep my hand at a steady distance from the antenna the pitch doesn’t stay the same, but it changes instead (sometimes faster, other times slower, but smoothly in general. Non periodically.). I haven’t installed any aluminum foil, but from what I read this only improves linearity, right?

PS: This week (maybe on Tuesday) I will test my board with an oscilloscope.

PS2: Thanks in advance!

Will leave these specifics for EW competent's to answer..

"I haven’t installed any aluminum foil, but from what I read this only improves linearity, right?"

Well, not exactly... As I understand it, this foil adds capacitance to the antenna in an adjustable way..

There is no way to tune the equalizing (antenna)  coils, and the correct resonant operating point of the antenna circuit is critical for operation of the pitch oscillator and for obtaining linearity improvement.. this foil adds the possibility to separately tune the antenna circuit and get it optimum for the specified variable oscillator frequency.

Coil inductance can vary a lot - +/- 5% for those in the EW (?) (dont have my data on hand) and my understanding is (and it could be wrong) that the primary purpose of the foil is to allow tuning to compensate for errors here and allow adjustment of the antenna circuits resonant frequency.. I do not know how much capacitance it adds, but suspect that for a EW built to spec this foil is quite important, and probably important for more than just optimum operation.

ANY increase or decrease in capacitance "seen" by the antenna is EXTREMELY important - important enough to warrant threads here on TW discussing the effect of paint! - Mess with this only if you really know what you are doing! - Foil, screws, wire length, antenna length and width - all are critical - in fact, everything near the antenna coils or wire on the antenna side of these coils is critical.

I am not saying this is the cause of your problem/s.. Probably this post is more because there's not a lot going on at TW right now! ;-)

Fred.

Thank you for the answer Fred!

I will install the foil tomorrow if i have time, and i will update if anything improves on the pitch side, but I don't expect it to :\
Could the non steady pitch be due to wrong topology? (Maybe the connections at the power supply are not short enough or something like that?)

I forgot to mention that i have a supply of +12,05V/-11,83V, but i think that is enough, isn't it?

Also, (another) newbie question: when I measure the AC voltage with a voltmeter (a cheap one), what is the voltage that I read? Peak to peak, RMS or peak voltage? (I read ~20V at the collector of Q6 when the slug of L11 is all the way in)

3) When I tune the L11’s slug, the voltage at the anode of D1 varies significantly: from -0,4V to -18V, then -1,25V to -17V. That changes when I put my hand near the antenna.

If that is DC, measured with a good multimeter (cheap ones may show wrong values because of spurious RF) this variation is ways too high! It should only vary between 0 and -4V. Are you sure that you connected L7 well to the base of Q6 and NOT to the collector of Q7 as in the pitch circuit?

4) Voltage at pin12 of U3 is -11,23 and it doesn’t change neither when I put my hand near the antenna, nor when I mess with L11.

That's because of

5) Pin13 is constantly at 8,12V (should be ~0 as Thierry points out) and L11 has no effect on it.

So, there is most probably no connection from the anode of D1 via R14 (1Meg), if there is a variation on the anode of D1, it must also be on both sides of R14 (0 to -4V on the diode side, ca. +2V to -2V on the IC side) or U3 is defective again. If Pin14 is really connected to ground, the difference voltage between pin13 and 14 may not exceed +/- 5V, if it's more it will kill U3. You should take U3 off, make sure that the voltages are correct (Pin13 varying from -2V to +2V, Pin14 = 0V, Pin15 ca. +0.6V) before inserting a new one!

Today it appears very frustrating to me that I have to write the same simple things again and again. It's all about basic knowledge in electronics (RF oscillators, rectifiers, operational transconduction amplifiers), reading schematics which one should have BEFORE deciding to build a theremin by oneself.

" It's all about basic knowledge in electronics (RF oscillators, rectifiers, operational transconduction amplifiers), reading schematics which one should have BEFORE deciding to build a theremin by oneself." - Thierry

I agree with you on this, Thierry -

Its true even if one is building a simple kit like the Jaycar / Silicon Chip theremins (although with these there is a good chance of it working without needing debugging) - But when one is building something from a "service manual" like the EW "hot-rodding" manual (which is not a kit-building guide) and wiring up or designing your own board, doing so without full understanding is pure folly.

Playing theremins is hard - Perhaps we should be warning people more that building (and particularly debugging  them) is also hard.. Even an electronics engineer picking up the "hot-rodding" manual and building an "EW" from it could easily have problems if they dont pay due care and attention to details which may not be obvious to someone unused to high sensitivity HF analogue circuits.

And yes, it is extremely frustrating - and takes a lot of time to go over given voltages and try to deduce (almost telepathically ;-) what the problems might be - particularly with a "home brew" where it could be almost anything - misplaced or wrong value component, poor solder joint, even reverse image PCB! ;-)

I admire your patience.. I can rarely be bothered to even look at the schematic anymore!

Fred.

For those who dont know, you can buy a ready-built new EW board for ~$100 .. You still need box, power supply, antennas, potentiometers, wire - but you have a hell of a lot better chance of building a good theremin if you buy one of these boards than if you try to do it yourself from the "hot-Rodding" manual!

And if you did have problems, at least we would know the layout of your board (and that it had stood the test of time), that the components were correct, and all that stuff.. Even if you show us your layout, it takes a lot of effort to go over such a layout to see if anything is wrong or a possible problem.

"For those who dont know, you can buy a ready-built new EW board for ~$100"  - FredM

Wow, I wish I'd known about that before I bought the EW kit, could have saved me a wad of dough.

The worst is that it takes a lot of time to analyse the problem, to find possible causes of the described problem, and to write everything down, especially if you are not an English native speaker. All that within less than an hour after the question was asked.

And then you get no reaction or feedback for more than 24 hours. I wonder if people are really interested in getting an answer and if they work seriously on their projects.

I should perhaps act in the same way as Moog Music Inc and ask for a $50 bench fee before I start wasting my time...

Thierry, first of all, thanks for bothering to answer (Fred as well. You two have the most vital role in this forum from what i read). I really can see your frustration with people making the same questions over and over.
You and Fred are obviously right when you say that one should have basic knowledge in electronics before attempting to build a theremin. I do have some, but only theoretical knowledge, never tried to built anything else before. Now i see that this was a mistake, but i can't (don't want to) give up now, that i have come so far (finding all the necesarry parts, having the board built, building the box and the antennas, etc.) I tried to resolve my probems by reading other posts before posting my questions as i said above, but I still couldn't figure what was wrong.

Anyway, these days i will study some more about RF oscillators and try to get full grasp of what is going on with the circuit in general.

L7 is well connected to the base of Q7 (not Q6), as shown in the schematic.
I changed L11, but i get the same results. Voltage at R14 on the diode side changes signifficantly again, but on the IC side the change is about 0,1V (~11,95V now instead of 8,12V since last time). R24 is 1M as stated in the schematic. I will change that in case it is defective (how possible is that?). Pin15 is at 10,95V and pin14 is well-grounded (0V). I have double and triple checked my connections, but i found nothing wrong. I will check again, and then, if i find nothing again, i will replace all the parts on the volume oscillator. All these measurements were done with a cheap voltmeter, so maybe these are not actual. Next week I might have access to a good voltmeter and an oscilloscope and make all the measurments again. (hoped that this would happen today, but unfortunately it didn't)

PS: Sorry for not answering sooner, but I have a lot of things going on these days. I hope that in a week or so I will have time to deal with my (failed) theremin full-time.

Again, thanks for devoting time to answer. (I am really disappointed, didn't expect to face so many problems with the project. Or at least i hoped i would face problems i could solve by reading this forum)