Jaycar theremin kit - pitch antenna problem

Posted: 1/5/2010 4:36:45 PM

From: Southern NJ

Joined: 1/5/2010

I've successfully assembled a Jaycar theremin kit V2.

I'm having a problem with the pitch antenna. It doesn't seem to have ANY affect on the sound, whereas the volume plate works as expected.

If I actually TOUCH the pitch antenna, the sound spikes, but other than that it does nothing.

I'm not sure if the problem is:
1) working as designed
2) an assembly error
3) a damaged component(s), possibly during construction
4) a tuning/adjustment problem

Has anyone had this problem and, if so, do you know where I should look first?
Posted: 1/5/2010 5:41:45 PM

From: Escondido, CA

Joined: 2/6/2008

email Thierry ... he's fixed all sorts of problems with these theremins.
Posted: 1/6/2010 5:41:33 AM

From: Colmar, France

Joined: 12/31/2007

I'm currently at work - will answer later
Posted: 1/6/2010 7:12:44 AM

From: Colmar, France

Joined: 12/31/2007

It's most probably an environment problem. The Jaycars are very sensitive to parasitic capacitances. The described effect may appear if the pitch antenna is less than 4ft away from an armored concrete wall.

Put the theremin on a pure wooden table in the middle of your room, the most far away from walls and pull the 4 thickest segments of the 6-segment or the 2 thickest segments of the 4-segment telescopic antenna out. Then pull the innermost (thinnest) segment still half-way out. Connect the audio output to an external (hifi- or instrumental) amplifier in order to obtain a good grounding. Pull the internal speaker as far away from the pitch antenna as you can or, still better, remove it totally since its sound is only crappy.

It may be that you have first to re-tune the volume circuit under these new conditions in order to hear something.

Turn the variable oscillator coil T2 (white) fully counterclockwise and then 1/2 turn back.

Now adjust T1 (white) carefully with a plastic RF coil adjustment tool so that you hear a very high, almost inaudible tone when your hand is at 1cm (3/8") from the pitch antenna and goes down when your hand moves away until you reach the zero beat point (where the frequency doesn't get lower but starts to rise again) at about 40cm (16") from the antenna.

This may be a question of 1/1000 of a turn, so be careful and make sure that your paunch is more than 30" away from the pitch antenna when trying it out.

If that works, your theremin is ok. If it doesn't work, describe exactly at which point your theremin behaves different from the procedure described above. In that case it would be interesting to hear if the audible frequency changes when you modify the length of the pitch antenna by adding 1/2" to the initial 4 and a half segments.

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