Moog prodigy service manual

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SECTION 2A
CIRCUIT DESCRIPTION @

2.1 GENERAL (Serial Number 4160 and Below)

The Moog Prodigy Synthesizer, Domestic Model
336A and Export Model 3368K, consists of a 32 note
F to C keyboard, two voltage controlled oscillators,
two envelope generators, one voltage controlled low
pass filter, one voltage controlled amplifier and a reg-
ulated power supply. For ease of description, the cir-
cuits are broken down into the following major
blocks:

Power Supply EnveIOpe Generators
Keyboard Circuit Voltage Controlled Filter

Modulation Oscillator Voltage Controlled Amplifier
Oscillators 1 and 2

2.1.1 POWER SUPPLY

The power SUpply consists of a step-down trans-
former, full wave bridge rectifier, filter capacitors,
and two 3-terminal voltage regulators.

Transformer T1, rectifiers CR3 through CR6,
and filter capacitors C38 and C39 convert the 120
volts from the AC line to 3-; 23 volts unregulated. U24
and U25 are nonadjustable preset regulators that pro-
duce regulated +12 and -- 12 volts respectivelyCapaci-
tors C36 and C37 prevent the regulators from oscil-
lating while capacitors C34 and C35 improve the regu-
lator transient response.

The export version of the Prodigy has a 240 volt
transformer with extra secondary fuses to comply
with stringent IEC safety regulation.

2.1.2 KEYBOARD AND ASSOCIATED CIRCUITRY

The keyboard circuit consists of a 32-note F to C
dual-contact keyboard, constant current source, sam-
pie-hold amplifier with linear glide and a summing
amplifier. Integrated circuit U11B feeds a constant
0.84mA through 31 100 ohm resistors in the key-
board producing 1 volt per octave. The Freq Scale
trim pot R178 adjusts this current for 1 volt per octave
across the keyboard resistor string. When a key is de-
pressed, two things occur: a voltage corresponding to
the depressed note appears first at pin 5 of U12B and

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then +12 volts appears on the trigger buss which
activates the sample-hold circuit and the dual envelope
generator. '

Operational amplifier U128. GLIDE control
R79, integrated circuit [5148 and operational ampli-
fier U12A form a combination sample-hold amplifier
and a linear glide circuit. As long as the trigger voltage
remains on, the output of U12A will track the pitch
voltage from the keyboard. When all keys are released,
the trigger voltage disappears, U148 turns off and C21
and U12A hold" the last pitch voltage. The pitch and
trigger contacts on the keyboard are arranged so the
trigger will disappear before the pitch to ensure that
the sample-hold will remember the correct pitch.
GLIDE control R79 and capacitor C21 form a lag cir-
cuit inside the sample-hold feedback loop which pro-
duces linear glide between notes. At low resistance
values of R79, two successive notes played on the key-
board will appear as discrete pitch jumps, but as the
value of R79 is increased, the pitch will glide betweeng
notes.

The output of U12A is routed to DNA, where
it is summed with the pitch wheel voltage, the tune
control and oscillator modulation voltage selected
from OSCILLATOR MOD switch SW7. Diodes CR1
and CR2 provide a small electrical dead band in the
pitch wheel to compensate for play in the wheels me-
chanical detent. The inverted signal from U11A is rout-
ed to both oscillators.

2.1.3 MODULATION OSCILLATOR

The modulation oscillator is a standard single op-
erational amplifier triangle and square wave generator
followed by a waveform selector switch, buffer and
the modulation wheel.

The voltage on capacitor C24 oscillates between
the : 1.5V trip point set by resistors R101 and R100,
producing a 3 volt peak-to-peak triangle on capacitorg

C24 and a 24 volt peak-to-peak square wave on the
output of U13A. The smaller the value of MOD RATE

control R98, the higher the frequency of oscillation.

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emitter of transistor Q40. which is called the sustain

voltage. The voltage at this point varies between 0 and

Svolts depending on the setting of the SL'STAIN con-
trol R162. As long as any key on the keyboard is

depressed. capacitor C44 will hold at the sustain
voltage.

When all the keys on the keyboard are released,

the trigger generator returns to normal and dual timer
U23 is reset back to zero. What happens next depends
,on the setting of the FINAL DECAY RELEASE
switch SWlO. In the OFF position, capacitor C44 is
discharged very quickly to ground through resistor
R156. When switch SW10 is O.\', capacitor C44 is

discharged through LOUDNESS CONTOUR DECAY?
RELEASE control R158 to ground. Therefore, the
discharge rate is the same as the initial decay rate.

The output of the top envelope generator is
routed to the VGA to control loudness. The bottom

envelope generator is romed to the VCF to control
cutoff frequency.

2.2.6 VOLTAGE CONTROLLED FILTER lVCFl

The Prodigy uses the patented Moog 24dB per
octave low pass filter with exponential control of
cutoff frequency. A positive feedback loop is provided
which puts a variable height resonance peak at the
cutoff frequency of the filter. The filter can also be
used as a sine wave oscillator.

The outputs of both oscillators are summed and
attenuated by resistors R104, R105 and R137 and
then applied to the base of Q29. Transistors Q29 and
Q30 convert the input signal into a differential signal
current in the ladder. The collectors, capacitors and
the next emitters form a 1-pole current-controlled low
pass filter. The cutoff frequency of the filter is directly
proportional to the standing current in each leg of the

ladder. The four stages in the ladder add up to a four-
pole filter.

The differential signal voltage is buffered by an
ac differential Darlington stage consisting of transis-
tors Q32, Q33, Q34 and Q35. The signal is routed to
the VGA, U17 and to the emphasis amplifier U16B.
This converts the differential voltage into a single

ended output which is applied to the FILTER EM-
PHASIS control R136.

Since the total phase shift equals 180 decrees at
the cutoff frequency of the filter and the feedback
signal is injeCted into the minus input of the filter, we
get positive feedback. This puts a resonance peak in
the response of the filter at the cutoff frequency. The
height of the peak depends on the setting of the
FILTER EMPHASIS control R140. At full emphasis,
the filter will oscillate, producing a sine wave. Em-
phasis trimpot R135 sets the oscillation threshold.

As previously stated, the cutoff frequency of the
filter is controlled by the standing current in the fil-
ter. The standing current is generated by transistors
Q37 and Q38. a poor man's exponential voltage to
current converter. For every 18mV increase in base
voltage on transistor Q37, the current through Q38
doubles, resulting in an exponential current response.

The control signals for cutoff frequency are
generated in many circuits. The FILTER CUTOFF
FREQUENCY control R141 allows manual setting
of the cutoff frequency from the front panel. The
FILTER KYBD TRACKING switch SW6 allows the
filter to track the keyboard in oneohalf scale (2 0c.
taves of keyboard movement per octave of filter
movement) or full scaling with scale set by Filter
Scale trimpot R152. The FILTER MOD switch SW8
routes the output of the modulation wheel to the
filter. Lastly, the \"CF envelope generator is routed
through the FILTER CONTOUR AMOUNT control
R150, allowing the enveIOpe waveform to sweep the
filter.

2.2.7 VOLTAGE CONTROLLED AMPLIFIER (VCA)

The VCA uses an operational transconductance
amplifier as a control element. Integrated circuit U17
takes the differential signal voltage from the VCF and
amplifies it. The gain of U17 is proportional to the
current flowing into pin 5. This current is supplied by
transistor Q39, which is controlled by the VCA enve~

.lope generator.

The amplified voltage appears across resistor
R119, where it is buffered by integrated circuit U16A.
The output of U16A is routed to the MASTER VOL-
UME control R122 and then to output jack J3. VCA
Balance trimpot R120 compensates for the input
offset of U17 to reduce control signal bleed-through.

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