Jaycar/Silicon Chip troubleshooting part 2

Hi MarkT,

I was actually using the antenna length to fine tune the pitch range - but I'm fully intending to replace the aerial with a fixed unit and make a nice wooden cabinet housing.

I'm very interested in the 'lev antenna' http://www.oldtemecula.com/theremin/ult_antenna/index.htm which is using a ferrite coil from an AM radio, and a coiled spring (amongst other things) to make the tuning more linear. I note that somewhere in the above website it mentions that it works for Theremins working in a certain frequency range and it cites some Theremin models other than the Jaycar kit - so I'm hoping that the Jaycar design will be suited to this particular mod (actually I'm also interested if it would work on a Moog Etherwave standard as well)).

I guess I can only give it a go although it would be interesting if you have any opinions on it.

Cheers

AndrewH

You already got an answer on http://www.oldtemecula.com/theremin/ult_antenna/index2.htm.

The Lev Antenna is designed to work originally with theremins which operate around 800kHz (in order to use cheap Japanese if transformers with 796kHz).

The Jaycar Kit operates with more conventional 455kHz coils, which is about the half of the intended frequency, from where the need of a bigger coil [b]and[/b] a bigger capacitor.

But... [b]take care!!![/b] As far as I remember the Jaycar Kit has already a linearization coil of about 1mH. Adding still more to that may even decrease linearity or make the theremin quite unplayable. I would rather recommend you trying to modify slightly the original coil in order to increase linearity.

Another important aspect for linearity especially in the lower frequency range which is often neglected is the correct pitch setting. Imagine that you make a standard hand movement for an octave jump in a low range, i.e. from 55 to 110Hz. If the difference between variable and fixed pitch oscillator would be only +/- 30Hz, you could hear from 85 to 140Hz which is smaller than an octave, or from 25 to 80Hz which is much more than an octave. In the higher playing range these +/- 30Hz difference would not be audible or simply corrected by a slight hand variation.

That's why I start tuning my Etherwave first for a correct octave from middle C to the C above. Then I go lower, still checking the octaves and correcting slightly the pitch potentiometer, sometimes only by tenths of millimeters. This allows me to end up with almost two and a half linear octaves below middle C.

Thanks Thierry - your knowledge of the subject is very impressive and I appreciate your time in clearing that up for me + I should have read through to the bit at the bottom of the Lev Antenna page.

Hi all,

I just finished building mine and there's some wonderful howling coming out already :)

Altough, as I build this without buying the kit, I don't have any kind of clue how I should adjust it.

I've built it with these mods http://www.thereminworld.com/
silicon_chip_theremin_modifications.html

Which adjustment, and how, should I do first?

Thanks a lot!

Have you put it in a box with a fixed length pitch control rod and volume?
If you haven't, you will have to go through the same procedure again when you do.

MarkT

I have lots of different antennas, and I'm trying to find out which one works best.

I can't post a new reply?

Well, I deciced to use telescopic antennas for both volume and pitch, it seems to work fine in the box, too. However, the theremin is now fully playable, but is there a "correct" way to adjust the IF-transformers and trimmer?

Hello all,

I've got a Jaycar theremin that I bought broken from eBay, hoping to fix it. I tried Thierry's advice from this thread, and I have oscillation on all of the points mentioned in b), but no sound. The only time I get any noise out of it is when I turn it off. When the theremin goes from on to off, it lets out a little chirp. Bad caps, maybe?

I'll take any advice anyone has to give.

How about step C?

How about step C?

I did that as well.

I discovered that there was actually a tiny bit of noise coming out of the speaker, so I poked around in the amplifier circuit a bit, and discovered that I could get more sound to come out of the speaker if I poked pin one of the LM386 with a pair of metal tweezers.

I think my problem is in the amplifier section, but also after checking it again, I'm getting no oscillation on Q4.