Is my Big Briar (Moog) Theremin faulty?

Hi,
I recently bought a Big Briar (Moog) Theremin in the UK from Turnkey. It's the £250 black box model.

Since the first time I used the theremin, it makes a constant tone even when noone (or no object) is near the machine. Is this normal or should the machine be silent in this state?

This does not affect usage of the machine as when I am playing it I can use the other controller to mute and unmute the tone as necessary, however when watching this machine in use on Bill Bailey's stand up show 'Part Troll', the machine appears to be silent until his hands go near the pitch aerial. Could it be that my mute aerial is working the opposite way round to how it should be (i.e. muting when it should be unmuting and vice versa)?

Pretty sure the machine was silent when left idle when I tried it in the shop. Can a theremin be affected by interference? I live near some fairly mean looking cell phone masts....

Thanks to anyone who can assist with this.

I'm not really sure what you mean. It sounds like you need to alter the pitch control though.

The volume control is operated in the same way as the conductor controls the orchestra. When your hand goes up, the instrument gets louder, and when your hand goes down, the sound diminishes (as though you are decreasing an imaginary volume slider). The instrument will only be loud when something inteferes with the field around the vertical pitch antenna though- when there's nothing in the field then the oscillators are both the same and thus no sound is produced at all in the heterodyning process. You need to set zero beat (the point when the theremin's pitch gets so low that no sound is produced) so that it's closer to the theremin- otherwise you'll get a constant growl.

Fiddle with the 'pitch' knob a bit until the octaves are compressed enough that no sound is produced when you are far away from it. The shape of the pitch field changes as the instrument warms up (it may begin silent and then start growling after 10 minutes) so it's best to leave the theremin on with the amplifier turned down for about 15 minutes before you start playing. Then you can turn up the amp and tune the theremin so that when your hands are your side it makes no sound at all.

I hope that makes sense- I'm getting my theremin from turnkey in a week. Hopefully I can get b-stock! Might I ask why you got the instrument? Are you in Britain?