Just finished construction of a Keppinger Theremin, need advice.

Phew! That was tricky.

Even with a PCB the oscillator chassis is still a rat's nest of wires.

I was able to power up my recently constructed Keppinger Theremin last night and after some tweaking get a null point about 6" away from the Pitch antenna. So far so good.

Unfortunately the volume antenna is not working at all.

Does anyone here have experience with the Keppinger Theremin, and can they suggest some test points/values in and around the volume circuit.

I'm completely lost when it comes to vacuum tubes.

I have access to a good scope and signal gen.

Thanks,
Steve
For all your laser harp, Tesla Coil, and killer robot needs... (http://www.stephenhobley.com/blog)

Just some hints for understanding the left side of the Keppinger circuit: (http://www.thereminworld.com/files/projects/mktheremin/mkschematic1.pdf)

All three oscillators, both pitch and the volume oscillator are basically Hartley oscillators with the triodes as cathode followers, similar to the FET circuits in the SC/EPE theremins.

In both pitch oscillators the anode(plate) resistor (R4 and R7) is (from RF view) bypassed to ground via C8 and C13 so that they produce a steady amplitude. The pitch antenna is connected via L6 to the tank circuit of the variable oscillator and influences its frequency while the fixed pitch oscillator around V3 remains at its free run frequency which leads after mixing through the buffer left 1/2 V4 and the diode (upper 1/2 V2) to a variable audible beat frequency.

The volume oscillator has a different principle of operation: Its frequency remains almost unchanged (out of internal capacitive effects in the tube) since there are no variable parts in the tank circuit (out of C3 for tuning). You see that there is a coil L3 in the anode circuit of V1 which increases the RF impedance. Now it will be the impedance of L2 in series with the capacitance of the volume antenna towards your hand which will determine the oscillator's amplitude. After rectifying it with the lower 1/2 V2 diode you get a varying DC voltage which controls the output volume by varying the DC working point at the grid of the right 1/2 V4 which acts as VCA.

In order to diagnose the volume oscillator, you'll need a 1:10 oscilloscope probe not only because of the higher or even lethal voltages in tube circuits but also because of its lower input capacitance. Connect it to the cathode (pin 8) of V1 and check if the oscillator works and if you may influence the amplitude with your hand closer to the volume antenna.If there is no oscillation check if there are (i guess) around 75 to 100V DC at the anode (pin 3) and post the value back here.
If there is oscillation but with invariant amplitude, check L2. If all is fine here, check at pin4 of V2 if you get a DC voltage which varies proportionally to the oscillator's amplitude at the cathode.

Instead of letting me write lots of further paragraphs with "if yes" or "if no", please tell us what you found up to here and then I'll give you the next run of instructions.

There are things which make me sad, for example:

Someone has a problem and posts his question(s) here on the forum while his soldering iron is probably already hot.

Someone other, perhaps thousands of miles away, sees that and sacrifices of his rare and previous free time in order to study schematics and find a (hopefully) helpful answer, translates it into (hopefully) understandable English and writes it down here as soon as possible. What happens then? Nothing, no feedback, no follow-up.

Isn't that demotivating?

"Isn't that demotivating?" - Thierry

Yes, it is, but all too common these days. However, we are in the midst of a rather important holiday, and people are often distracted by other things. I'd give him until the beginning of the New Year before getting too upset about it.

Also consider that answers posted online can benefit more than the original enquirer.

Actually, it does seem to me that the answering questions on the forum is not the most efficient approach - forum + search engine is not the most convenient way of locating specific information. The time and effort spent in answering the question could just as easily have been used in making an entry in a theremin engineering wiki, for instance. And then, instead of answering the question directly, refer the enquirer to the relevant wiki page.

OK first things first - Thierry thank you very much for your help - I appreciate it greatly.

I have just returned after travelling for Christmas visiting relatives - the internet was not always available and I have only just got back home and been able to catch up on all my correspondence and postings.

I will try your advice later today and post my findings - I cannot do it immediately as I'm alone looking after my 2 year old daughter. The moment I can get someone to look after her I will have the time to investigate further.

I have the necessery 1/10 probe and will begin work ASAP.

Thanks again,

Steve

Managed to get some testing done tonight.

The Theremin does produce a tone, but it is mixed with static.

All three oscillators are working, but are exhibiting ringing - see the attached picture. The ringing decreases when I touch one of the coils in the circuit.

Frequency is around 177Khz.

I just removed the trimmer caps as I realised they were only rated 50V - I've just ordered some 200V ones.

All three tube plates are registering a nice clean 100V (approx) with minimal ripple.

Here is a link to a movie showing how the Theremin sounds (I made this for Mark Keppinger, but I think it demonstrates part of the problem really well)

Image 1 - Click Here (http://www.stephenhobley.com/Tesla/pitch_coil1.jpg)

Image 2 (hand damped ringing) - Click Here (http://www.stephenhobley.com/Tesla/pitch_coil1_damped.jpg)

Movie Link : Click Here (http://www.youtube.com/watch?v=P21axUNpEi0)

I hope that makes sense.

If anyone knows why I might be getting the distortion/ringing on the oscillators I would be very interested to hear from you.

Steve

I know the electronics wizards don't like it when we amateurs make comments but...

I may be way off base, but that rats nest of wires is frightening. Theremins are very sensitive instruments and that tangle of wires could be causing all kinds of issues. Using wire that's much heavier than necessary can be adding to the problem.

For reference here (http://s776.photobucket.com/albums/yy42/anatak/) is a couple of photos of the Keppinger made chassis wiring. You will note the connections are as short and neat as possible. The routing of some of the wiring may be critical as well.

I think you're aboslutely right.

I never intended it to look like that, and I tried to trim things down whenever I could - but it's just plain wrong.

I'm going to be stripping it all out, and starting again with much thinner wire.

It's tough to wire up without a PCB - and you are constantly running the risk of wires shorting out, but a PCB doesn't seem to help much either.

I actually don't know how Neidlinger did his wiring, there doesn't seem to be enough connections in his photographs.

You should note that the green wires at the top of the oscillator chassis photo are lightly twisted together to minimize interference.

Of course, also keep any AC mains wiring well away from any of the other circuit wiring.

[edit] As Philip pointed out, the twisted green insulated wires are to the tube filaments. [/edit]