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Updated
05.08.07
Tune
your Theremin using an AM
Radio
The RS Ultimate 2b - A Gift from
Lev Sergeyevich Termen
Design by Christopher - www.oldtemecula.com
Tuning the Oscillators
Do the tuning below with nothing connected except the batteries
and antenna.
Disconnect all test leads or cables hooked up. I would even leave the TLO82 and TLC555 out
of their sockets. You need to hear the perfect theremin response
through an AM Radio before it passes through the detector diode
and the amplifier output.
Do
not hook up your antenna for this test, it just gets
in the way!
Do not install the Brass Screws until tuning is done
Do not use a power supply, rather use 6 volts
from batteries.
Do not have any instruments connected to the
circuit.
We are trying to "isolate" the oscillators from
the antenna
and ground just for this initial tuning!
Have your circuit board mounted on something so the coils are
away from the table surface, any metal or conductive objects.
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Let's
Validate
that both Oscillators are working
Digital
AM Radios can be more difficult to use for the following test,
use a analog AM Radio.
L1 oscillator - short out the EB (emitter/base)
with an alligator clip to stop it from oscillating.
Turn on an AM radio
within 24" (60cm) of your circuit board. With 6
volts of power applied to your
board begin tapping the coil lightly
between the area of A1 & A2 wire loop with your finger or a 1"
square piece of aluminum foil. Tune across the AM Band from about
700 KHz to 1100 KHz. You will hear a distinct
swoosh on
the radio while tapping or rubbing a 1"x 1" piece of
aluminum foil if your oscillator is working.
Do
the same on the other oscillator. The AM Radio
swoosh
is very obvious.
On the operating oscillator opposite the
alligator clip, tune and mark the radio dial with
a piece of tape to indicate its operating frequency, it
will be an obvious quiet place on the tuning dial.
Now remove the alligator clip
L2 oscillator - short out the EB (emitter/base)
with an alligator clip to stop it from oscillating.
If one oscillator is not
working go back and check the soldering of this oscillators seven components.
The coil connections are the weakest link, check that
the enamel was removed properly and the solder grabbed. The transistors must be
NPN and the pins must be EBC and not ECB, some
brands are different, verification should be on the
package they came in.
A theremins true voice is
generated from
the heterodyning principle.
This when both oscillators match one another in
frequency, a perfect match would be the Null Point where no
sound is heard. One oscillator pulled slightly off of Null is the difference that creates
a tone.
440 hertz difference from Null creates the A4 tone.
Pitch
Tuning for a
RS Ultimate, PAiA Theremax, Theremaniac, SWTP
142
Listen and verify that both pitch oscillators
are generating a
quiet spot on a standard (not digital) AM Radio
while sweeping the radio tuning across its mid range. Adjust your L1 or L2 oscillator coil
slugs so the two verified oscillations found on
the radio tuning dial move to the same location/frequency.
(Not
one oscillator at 850kz and the other 900kz).
You will hear
a strong chirp/whistle when the oscillator
frequencies begin to overlap.
Be cautious when a you
think your theremin is tuned properly because you hear a weak theremin tone against
background static or a weak radio station.
This is a good working oscillator but not tuned
to the other pitch/reference
oscillator.
When both oscillators are tuned to the same frequency
they will block out "all" background
noise including radio stations.
Oscillation is easy but matching their
frequencies can be a challenge! |
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You could get lucky
and hear the theremin whistle immediately and not require major
tuning or wire removal. Accomplished this by measuring the C2
& C8 capacitors and use two that match
in value very closely!.
This is the 100 uh ferrite choke from Radio Shack Cat# 273-102
$1.29,
we want it for its ferrite core.
You can view this choke on the Antenna
Page
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 first because both oscillators are already
tuned, then your hear a distinct chirp.
After you hear the
test swoosh/chirp from
both oscillators, determine which
one operates at the lower frequency
by tuning the AM Radio to each swoosh, then tune to the lower one on the
dial.
Here are some methods of moving the oscillator
frequencies together using an AM Radio:
Note:
The "ferrite choke" can be secured in
the coil permanently with tape, when done.
1.
To determine which oscillator is operating
at the higher frequency slide the ferrite choke inside of
each coil tube one at a time.
In one coil you will hear a distinct
chirp over the radio as the choke enters slowly. The
coil operating at the higher frequency will give you
the nice theremin chirp because the ferrite lowers that
oscillator's frequency. Secure
the choke with tape. The brass
screws will now be used for all
your future fine tuning.
Or you could
2.
You could remove one of the 100 pf
capacitors "C2 or C8" and measure for another
one that is higher or lower in value (5 pf) to shift the
frequencies closer together then go back and do step #1
above. If an oscillator needs to go higher you would
want to use a 95 pf in place of a 100 pf if you have
one. The ceramic capacitors I use have a 5% tolerance so
they are manufactured slight up or down in value.
Or you could
3. Tap each coil lightly between
the area of A1 & A2 wire loops, alternate tapping
each coil and tuning the AM Radio higher until one
chirps and the other doesn't. The
side of silence is the lower frequency and this is the coil you need to remove wraps of
wire from. This will
raise the
operating frequency of the lower oscillator to match the
opposite
oscillators frequency.
Wire removal should
be a last resort when you have both oscillators working.
Only remove wire when the two swoosh frequencies
are
to far apart as detected on the AM Radio dial and
you cannot achieve
a Chirp & Null Point
slowly sliding the
ferrite choke in and out of either of the coils.
Remove
the ferrite choke and any brass tuning screws from both
coils.
Remove one
wire wrap at a time from the oscillator operating at the
lower frequency until you hear the theremin
tone on the radio as you move into range of the opposite coils
tuning. The wire wraps should be
removed from the outside A4 side of the coil
and this is suppose to be the coil operating at the lower frequency. A4
is opposite the tuning screw side
and is the wire that connects to +V
side of the circuit board. Only
a few wraps would need removal, less than 10 probably.
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Now go back and review the ferrite choke procedure
1. above.
Fine Tune by screwing in
both brass screws centered inside the wire coil of both
coil forms.
Now adjust until the
"Pitch" Nulls. Use the ferrite choke for major
tuning differences and secure it with tape.
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You should now have the AM Radio Ideal
Tone which means
a true theremin whistle.
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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After the Ideal Tone
has been
tuned, remove excess unwrapped Coil
Wire if you have any.
On L1 leave enough
wire so you can have coil rotation. Cut the wire about
1" from the bottom of the board, then remove the
enamel with a butane lighter, clean and re-solder.
Connecting
an Antenna & Ground drags the frequency of
the L1 oscillator lower. You will have to re-adjust your ferrite
choke
position slightly.
(It is probably in coil L2)
Screwing the brass screw further into the antenna side oscillator coil raises the
frequency of that oscillator. Doing this on the L2
fixed coil pitch adjuster will raise the pitch tone in the proper direction.
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.
Contrary to what most people believe, theremin
control is caused from your hands influence on the emanating
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 the Lev
Antenna you can make the octaves next to the antenna
play wider for fingering than the octaves in the outside playing
field? Strange, yet remarkable.
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Preliminary
Coil & Potentiometer Adjustments
Potentiometers
Pot 1 - Pre-Audio Gain set mid-way (Use
shielded cable to this pot off the board or instability occurs.)
Pot 2 - Line out down to a microphone level out adjustment. Full
clockwise when using the LED output.
Pot 3 - Turn full counter-clockwise for
sine wave on audio output or when using the sine wave LED.
Turn clockwise slightly to increase the timbre square
edge using the
pitch only Audio Output.
You will need to reposition the "Ferrite Choke"
slightly when doing any of the adjustments below.
You
now want a good earth ground & antenna connected to
the board!
Rotating the L1
coil clockwise
"90 degrees" toward the L2
coil will drive the final audio tone harder, shaping the sine wave into a soft square wave. Turning it counter-clockwise will
create a nicer low pitch wave shape, but
will allow external noise to sneak in and it also dampens the overall audio signal output strength.
On some wave shapes I keep Pot 1 turned down just below
triggering U4 TLC555 for a clean
sound.
Find
your own personal preferences for the above adjustments with
lots of experimenting.
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Wire leads from the batteries or pitch only audio output must
leave the board immediately away from the coils
The Antenna connecting wire must leave
the board on its own coil side!
Troubleshooting:
Think of
the Ultimate 2b Theremin circuit board in two sections of parts.
If you can hear good theremin response through an AM Radio
then both oscillators and everything to the left side of Pot
1 in the schematic is working fine. If it's not working then go to the top of
this page and tune your oscillators over the AM Radio, this
must work first.
If the above works and you don't have sound at the audio output
off of Pot 2 then the problem rests between Pot 1 & Pot 2.
The U4 TLC 555 is "not needed" in the circuit
to get strong output at the audio out. Check your U2
TLO82 direction and all solder connections for solder bridges
between IC pads and bad solder joints.
Often
when sound is not present at the Audio Out, it's because the
builder did not do the above first step of Tuning the
Oscillators. The tuning window of theremin response is "very very
narrow".
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