Moog prodigy service manual rvgm

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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

2

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.