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Updated
08.08.08
Tune your Theremin using an AM Radio
The RS Illusion
Theremin - A Gift from
Lev Sergeyevich Termen
Design by Christopher - www.oldtemecula.com
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Initial Setup & Tuning
Use battery power!
Begin tuning “without” an antenna or ground connected.
Insert both brass screws in their Tee-Nuts with about 1” sticking out of the metal plate side. I have the head of one screw on the plate side so I can slip on the ferrite rod
over the screw threads eventually, but not yet! The other brass screw is inserted backwards so the threads stick out the Tee-Nut, then I can put a knob on it for precision octave width setting.
The next steps will verify if both oscillators are working properly.
Mount your theremin circuit board on a cardboard box to keep it away from the table top and at least 4’ from any conductive objects in your room.
In the following writing the L1 or Q1 refers to the antenna side oscillator and L2 or Q2 refers the fixed side oscillator which is also the L2 coil you made.
With an AM Radio placed within 18” of the theremin board you will find a quiet spot created by your theremin oscillators on the radio dial by tuning between 700 kHz to1100 kHz, it is very obvious.
Place an alligator clip across "any" of the Q1 transistor pins on the antenna side of the L1 oscillator to short and disable
it. This will not harm anything on the RS Illusion theremin board.
Tune your AM Radio for a quiet spot on the dial created by the L2 fixed oscillator. Put a piece of Tape on the radio dial and mark this
location and write L2. My L2 fixed oscillator tuned in at about 875 kHz on the radio dial.
Tap the A4 side of the L2 coil with your finger and the quiet spot should swoosh if the oscillator “without” the alligator clipped transistor is working.
Remove the Q1 alligator clip and place it across "any"
of the Q2 transistor pins to short and disable the fixed side L2
oscillator; which is the side of the coil you made.
Tune your AM Radio for a quiet spot on the dial created by the L1 antenna side oscillator and then mark this spot on the
tape and write L1. My L1 antenna side oscillator tuned in at about 850 kHz on the AM Radio.
Remove the alligator clip. Hopefully your L2 oscillator which is the
side of the coil you made is found on the slightly higher side of the AM Radio dial.
With the radio still tuned to the lower frequency of the Q1
& L1 oscillator insert the ferrite choke into the L2 coil
holding the choke wire lead with your fingers. If L2 is slightly higher in frequency
(to the right of the L1 mark on your tape) then the ferrite can drop the L2 frequency
over 50 kHz fully inserted. Manually do this and you should hear the L2 fixed oscillator frequency
sweep down and overlap the L1 oscillator frequency. It is at this point you hear your first theremin chirp or your new baby cry!
Remove all the wiring from one of the ferrite choke cores and mount the
ferrite rod on one of your inserted brass screws. You can see
this screw/ferrite combo in this photo.
Use a plastic tube to mount them together. Now adjust it back into the coil to where your theremin wants to sing.
The other brass screw moves the frequency very slowly for fine
tuning.
Trouble Shooting:
“If you could not find the quiet spot on the AM Radio dial from the above procedure for one of the oscillators it is probably not oscillating.
Oscillation is very easy to achieve and very obvious to hear as
a quiet spot on your AM Radio. Check your solder connections for bad soldering and verify the transistor
EBC pin orientation along and the proper component values.”
*Last resort: If the L2 fixed oscillator is the lower frequency
determined by the marks you made on the on the AM Radio dial with both brass screws in the coil form, then pull one wire wrap at a time off the open end of your L2 coil to raise its operating frequency. This should require removing fewer than 10 wire wraps. Pull wraps until you hear the theremin tone over the radio which should still be tuned to the L1 quiet spot on the radio dial.
Balancing theremin oscillators is like a kid’s teeter-totter, adding inductance or load to either coil/side is like placing bricks on either side of the teeter-totter until you achieve a reasonably level
position and sensitivity. This would be the Null Point.
Two Types of Antennas
- "Tune
your AM Radio to your theremin whistle!"
Have the screw "without the ferrite" set with the brass screw tip
1.5" outside the metal plate.
If you add the Antenna Tuner in series with a basic 3’ antenna it will give you excellent linearity and a better sound from an improve wave shape. At first
set the antenna tuners variable capacitor about half way until doing
the final antenna tuning. The antenna tuner must connect directly into your pitch oscillator with no other interfering chokes or capacitors in series.
The connection is to the collector of the Q1 transistor of the pitch/antenna side oscillator.
Use a good earth ground when using
an antenna.
1. Connecting a 3’ basic antenna will load down the L1 pitch side oscillator frequency and moves it lower about 15 kHz. If you
connected the antenna while your oscillator was singing over the
radio, the L1 oscillator moves a bit lower in frequency and you now hear quiet static from the L2 oscillator which remains at its originally tuned frequency.
Re-tune the L2 oscillator slightly lower using the brass screw with the
ferrite core mounted, turn it
counterclockwise. Listen on the radio for the lowest theremin tone you
can adjust.
Or
2. Use the Lev Antenna spring with the antenna tuner for precision linearity, it could add one or two extra linear octaves; it has more influence on the L1 pitch side oscillator as it transfers more energy into the pitch playing field.
The Lev Antenna maximum stretch is about 1/2" or a first to
last coil length of 16.5” for best octave linearity. 17” stretch
can make the outside octaves slightly narrower than the inside
octaves. Attaching the Lev Antenna spring to the antenna tuner moves the L1 pitch oscillator about 30 kHz lower.
You will hear the quiet spot from the L2 oscillator. It remains at it's originally tuned spot as the L1 oscillator frequency moves
lower from antenna loading.
Re-tune the L2 oscillator slightly lower using the brass screw with the
ferrite core mounted, turn it
counterclockwise. Listen on the radio for the lowest theremin tone you
can adjust.
Precision "Antenna Tuning" for the RS Illusion theremin
After initial oscillator setup with your antenna using the Antenna
Tuner and a good earth ground,
do the following.
Have your AM Radio within 18" and tune it to hear your theremin tone over the radio.
Disable the L2 oscillator by placing an alligator clip across the Q2 transistor leads.
On the RS Illusion theremin this shorting of Q2 will not harm anything.
Tune the AM Radio to improve the center on the tuned quiet spot created by the L1 oscillator.
If you could mount a 2' wooden or plastic stick to control
turning the C4 antenna tuning capacitor this would be much
better than disrupting the tuning field with your hand reaching
in.
Sweep the antenna tuner capacitor back and forth while listening
on the radio. The antenna
tuner is set to the left side at the bottom of the dip you hear
while sweeping the tuning range.
If you are using your hand instead of a stick, try and set it to
where you are at the bottom of the dip after you move your hand
away. What you hear is very similar to setting a quiet Null
Point yet your L2 oscillator must be disabled doing this
adjustment. It is cause from your theremin oscillator and radio
tuning beating together.
Mark this spot for future reference on the variable capacitor
and circuit board.
Any type of antenna change will require this tuning procedure to
be done over.
Note: Counter-clockwise is the maximum capacitance of the
C4 variable antenna capacitor.
Mark Barton writes an excellent email on why perfect pitch linearity occurs.
Note: The antenna starts at the wire
solder point on the
circuit board to any distance above the board and is
mirror imaged using the ground at the board downward toward the
floor or earth. Keep the antenna wire away from the opposite side
L2 oscillator coil or it will desensitize the theremin response.
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This is the 100 uh ferrite choke from Radio Shack Cat# 273-102
$1.29
We want two for L1 and one for it's its ferrite core for
tuning. |
Mount the two L1 ferrite choke coils with
3/4” clearance above the circuit board.
L2 = Coil is the one you made.
A1 = open end of coil
A3 = 33 turns from open end
A4 = Collector hot side
This is the Actual Chirp
(64kb) or ideal tone recorded off of the AM Radio while tapping the coils.
You hear the sweeping theremin tone instead of the swoosh because both oscillators are already
tuned or adjusted to the same exact frequency. The
theremin tone window of control is very narrow.
.
Oscilloscope
(Not needed and not recommended for this project) 4/022/05
View the signal off of the emitter lead of Q1 & Q2 above the
10k resistor.
Make your scope settings 1 volt vertical and 1 us. horizontal. You will see around 8 cycles on the scope screen at about
2.5 volts P-P. The oscillator side with slightly fewer cycles on the screen is the lower frequency and you
will need to remove wire wraps to increase it's frequency to match the other oscillator. |
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Contrary to what most people believe,
heterodyne theremin
control is caused from the effects of your hand
absorbing the emanating
electro-magnetic energy departing the theremin. The Lev
Antenna changes the RF wave polarization
by 90 degrees a
right angle and allows for a more controlled distribution
of the radiated energy.
Interesting fact: Did you know with
good antenna tuning and the Lev Antenna you can make the octaves next to the antenna
play wider for aerial fingering than the octaves in the outside playing
field? Strange, yet remarkable.
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