Service manual

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Section II: Sub-assembly Description
93-113 Oscillator (Board 1)
This assembly contains three tone oscillators, summing amplifiers, exponential current
generators, waveshaping circuits and a -5 volt reference source. The three oscillators are
independent and non-synchronous. Each oscillator is voltage controlled from external sources
such as the keyboard. Linear input voltages are converted to exponential currents which are used
to control the frequency or pitch of the tone produced. The basic sawtooth signal is processed to
derive triangle and variable width rectangular waveforms. Oscillator #3 has a wide range
capability which enables it to be used for modulation purposes. The -5 volt reference source is
used to establish the precise voltage steps on the octave range switches.
93-114 Contour Generator (Board 2)
This assembly contains two identical contour or envelope generators, and the keyboard circuitry.
Upon triggering, the contour generators produce a voltage envelope with variable attack, decay,
and sustain. The output is used to control the gain of the first voltage controlled amplifier or the
cutoff frequency of the voltage controlled filter.
The keyboard circuit contains a constant current source which feeds the keyboard resistor string.
This insures the proper voltage drop across each of the 43 keyboard resistors. Also contained on
this assembly is the sample-hold circuit which measures the voltage from the keyboard and
maintains it until a different key is depressed. The output of this circuit is the control voltage for
the oscillators and the filter. The glide feature is a function of the sample-hold circuit.
93-115 Power Supply (Board 3)
This assembly contains the plus and minus 10 volt regulators, the headphone amplifier, the noise
generator and the modulation mix amplifier. The plus supply uses a temperature compensated
zener diode for a voltage reference. The minus supply is referenced to the regulated plus voltage
causing the two supplies to "track". Both supplies employ remote voltage sensing for optimum
stability.
The headphone amplifier is an A.C. coupled emitter follower with a push-pull output. The noise
generator uses a reverse biased transistor junction for a noise source. The output of this
transistor is processed through three filter circuits to derive the white, pink, and red noise
outputs. The modulation mix amplifier takes signals from Oscillator #3 and the noise generator,
sums them and supplies an output which is used for modulation.
93-116 Filter (Board 4)
This assembly contains the voltage controlled filter, the external input pre-amplifier, the overload
lamp driver, the first and second voltage controlled amplifiers and the A-440 reference oscillator.
The filter is a voltage controlled low pass type which employs the Moog patented ladder
network. It is capable of regenerating (oscillating) over a wide frequency range. The scale is
adjustable so the filter can be set to track the keyboard.