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The Becker Electrodeum is a parallel tuned electrode
or Pitch Antenna for perfect Pitch Field Linearity.
The Pitch Oscillator board uses L3 - 3.3 mh
choke to pick up the inductive field from coils L1 & L2 and L4 - 3.3 mh the antenna base coil. (This needs more study)
Also a short tickle wire
2" (50 mm) is mounted at Terminal B, a direct connection to the diode mixer to allow mild adjustments in RF signal
mixing.
The area of
the terminal block
off the Pitch Oscillator side allows for the shaping of your own
personal sound. Dial in the very female like vocal sound and others. You
could short out all the terminals at the same time and not harm anything
here so experiment.
A small
metal shield is place
over the L2 fixed oscillator section to keep the Electrodeum capacitive
feedback from interfering or distorting the audio wave shape. It also
blocks 50/60hz from being picked up.
The IF
Transformer Shield/Cans of L1 & L2 are ungrounded so they can be used
for adjustable capacitance coupling using inline resistance. This allows for an adjustable wave shaping at the detector diode. Pot-5 is connected to
the L2 shield/can to ground through resistance, the RF amplitude
escaping from this
L2 coil is controlled to have more or less influence on the detector
diode circuit, this creates the Timbre control or how strong the detected audio signal drives the LM358 which feeds the transformer
circuit.
L1 is
ungrounded on the antenna side, to get the more vocal sound in the
future place a high resistance from Terminal-A to Terminal-C ground, then
L1 IF shield/can is partially grounded. (This needs experimentation)
On the Pitch board Pot-1
is between the LM358 a & b sides which determines the drive amplitude into the transformer circuit. This is set about
1/3 the way at first and works in conjunction with Pot-5 the Timbre signal strength from the
oscillators. This combination in series drives the transformer circuit. (Pot-5, parallels
another pot, the total should be about ground at 0 to 2k for the timbre
adjust) Using one 2k pot here would be Ok.
At test point TP2 next to the transformer the amplitude is kept low and should be around .5 volts p-p for the proper sound to develop. The ideal wave shape will be viewed here.
The ideal wave shape level can be raise to 1v
p-p on the volume board using the U1b side of the LM358.
The Opto chip U3 V0618a works in conjunction with the
600 ohm 1:1 audio transformer
to control the skew at the top of the wave shape.
This is where the even harmonics are generated. Using the
higher voltage transistors and the sluggishness detector circuit creates the
lower more throaty side of the vocal sound.
An oscilloscope should view the
audio signal at TP1 or pin 7 of the LM358. This will be a rough sine wave with
up to a 100x gain over the diode detector
output amplitude. Then view TP2 after the transformer for the real and
desired .5 volt p-p wave shape.
On the Volume board the
RF signal for detection comes from the L1 variable oscillator or pitch
antenna side, off of the emitter of Q1 through R23- 220k resistor. A short 2” (50mm) wire is used to pick
up enough of the other the RF signal for mixing. (Tickle wire)
Unlike the
Pitch board the LM358 on the Volume board is use for two different amplification purposes.
The U1a side amplifies the detector signal which then drives the Q3
transistor which then drives the 555. The U1b side is used to raise the
.5 volt pitch audio to line level or 1 volt p-p and drive the pitch
preview jack to feed an ear bud before the volume control has effect on
the sound.
The
main purpose of the 555 is to drive the LED's which one is inside the
Vactrol for the pulse width modulation PWM volume control. The C9- 470uf capacitor
averages this control voltage from the 555 which varies the resistance
of the Vactrol output cell which the audio passes through, controlling
the volume.
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