Moog Music minimoog D Service Manual
The
CONTROLLERS
OSCILLATOR BANK MIXER
MODIFIERS
OUTPUT
minimoog
MODEL D
TECHNICAL SERVICE MANUAL
by R. J. Folkman
the first sound in synthesizers
moog
MUSIC INC.
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MINI-MOOG
Field Service Manual
CONTENTS
Section Title Page
I
Precautionary Measures
II Sub-assembly Descriptions
III
IV
V
VI
VII
VIII
IX
X
XI
Block Diagram
Trouble Analysis Guide
Sub-assembly Locations
Cover Removal and Sub-assembly Replacement
Adjustment Procedures and Locations
Keyboard Maintenance
Schematic Diagrams
Component Location Charts
Replacement Parts
2
3
5
6
8
9
10
14
15
16
17
7/73 RJF
XII
XIII
Modifications
Ordering and Shipping Instructions
Service Department Policy
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18
19
20
Section I: Precautionary Measures
In the Mini-Moog, as in any electrical device, shock hazards exist. All lethal voltages are
confined to the area around the power switch and the A.C. fuseholder. Always unplug the
instrument while servicing the electronics. Keyboard cleaning and adjusting may be done with
the unit turned on.
When a fuse blows, replace it only with the proper size as indicated on the label directly below
the fuseholder. If the fuse continues to blow, it is an indication that a definite electrical problem
has developed. Never try a larger fuse; this will only result in further internal damage.
Avoid making adjustments at random. Each adjustment is critical and should be made in
accordance with the procedures outlined herein. If a sub-assembly is found to be defective, we
recommend exchanging it rather than attempting to repair it. Unless a qualified technician is
available, with special test fixtures, more harm than good may result and possibly void any
remaining warranty.
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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.
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The external pre-amp is a three transistor high-gain amplifier. The output of the pre-amp is
monitored by an overload indicator. The overload driver circuit is a Schmitt trigger which turns
the overload light on when external signal levels become excessive.
The first and second voltage controlled amplifiers are two nearly identical circuits. Their gain is
controlled by varying the voltage which supplies the current source transistors. A buffer or
output amplifier follows the second VCA stage.
The A-440 reference source is a highly stable Wein Bridge oscillator circuit. It produces a
somewhat imperfect sinewave which increases its harmonic content. These harmonics or
overtones assist in tuning to the reference, especially when the source being tuned is one or two
octaves away from the reference frequency. The output of this circuit is coupled to the VCA
buffer amplifier.
93-118 Rectifier Board
This assembly contains the input bridge rectifier diodes and the unregulated D.C. filter
capacitors.
93-060 Left Hand Controller
This assembly contains the PITCH wheel, MODULATION wheel, GLIDE and DECAY
switches, and two jacks for remote switching of glide and decay. The PITCH wheel has a
mechanical detent which allows it to be easily returned to a pre-set tune point.
To service this assembly, remove the two mounting screws and lift the unit out. All connections
to this assembly are through a 12 prong plug to simplify servicing and replacement.
93-061 Keyboard
The Mini-Moog keyboard is a standard three and one-half octave removable sub-assembly. All
electrical contacts are gold plated for high reliability. The keyboard is held in place by four (4)
mounting screws which are accessible when the bottom cover is removed. All connections to
this assembly are through a 6 prong plug to simplify servicing and replacement.
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Section III: Block Diagram
2/31/3
Noise
Source
OSC
Control
Tune
Vol
Wave
Filter Contour
A440
VCA 1 VCA 2
Loudness Contour
Rise Rise Sus FallSus Fall
8ve
8ve
8ve
Tune
Tune Wave
Wave
Headphone
Amplifier
Volume
Portamento
Keyboard
OSC 3
OSC 2
VCF
Mic Preamp
White
Pink
Modulation
Mix
Mix
Glide Switch
Glide Jack
Freq
OSC 1
Emp Amt
Mod
Roller
Pitch Roller
S-Trig
Decay Switch
Main Out
Pedal
Pedal
Mic Input
High-Z
Pedal
Volume
Decay Jack
(This block diagram is a simplified version of the one from the User’s Manual. The diagram
from the service manual fits on a single 11"x17" sheet.)
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Section IV: Trouble Analysis Guide
Use this chart along with the block diagram (Section III) to determine which assembly or
component is defective.
MALFUNCTION PROBABLE CAUSE AND CURE
Unit dead (no pilot light) 1. Blown A.C. fuse
2. Defective power switch
3. Defective line cord or plug
Unit dead (pilot light on,
overload light off)
Unit dead (pilot light and
overload light on)
Excessive hum and constant
modulation of all signals
No output from any mixer source
(A-440 output works)
No oscillator output (noise works) 1. Defective oscillator (board 1)
One oscillator dead or malfunctioning,
other two operating normally
Oscillators drift and
exhibit poor tracking
Oscillator 3 modulates oscillator 1 or 2
with modulation switches off
1. Defective power supply (board 3)
2. Defective power transformer
3. Defective output amplifier (change filter board 4)
1. Blown D.C. fuse
2. Defective power supply (board 3)
1. Defective rectifier diode on 93-118 assembly
2. Defective filter cpacitor on 93-118 assembly
3. Defective power supply (board 3)
4. Broken ground wire
1. Defective contour generator (board 2)
2. Defective VCA or filter (replace filter board 4)
1. Defective oscillator (board 1)
Refer to Section XII - Modifications
under Oscillator Assembly
Refer to Section XII - Modifications
under Power Supply Assembly
Improper or missing waveform
(any oscillator)
Noise source dead or producing poor
quality noise
External input dead 1. Defective pre-amp (replace filter board 4)
Filter inoperative or malfunctioning 1. Defective filter board 4
Filter regeneration weak or absent
(EMPHASIS at 10)
1. Defective oscillator (board 1)
2. Defective waveform switch
1. Defective noise transistor (Q15 on board 3)
2. Defective noise generator
(replace power supply board 3)
1. Defective filter (board 4)
2. Regen. Cal. not adjusted properly
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MALFUNCTION PROBABLE CAUSE AND CURE
Excessive drift or pitch change
1. Defective keyboard circuit (replace board 2)
after key is released (greater than 1
semitone per minute)
Loudness and/or filter contour
1. Defective contour generator (board 2)
generator operate improperly
No output at phones headphone jack
(main output normal)
1. Monaural plug in stereo jack
2. Defective headphone amp
(replace power supply board 3)
Overload light fails to operate
when excessive signal is applied
1. Defective bulb
2. Defective lamp driver circuit
(replace filter board 4)
Thumping sound heard
when depressing a key
(AMOUNT OF CONTOUR at 0)
1. First VCA out of balance
Refer to Section VII - Adjustments
2. Defective VCA (replace filter board 4)
3. Refer to Section XII - Modifications,
under contour generator assembly
Keyboard glides when glide is off 1. Defective keyboard circuit
(replace contour generator board 2)
Unit cannot be tuned
(A-440 on frequency, within 1 Hz)
1. Defective oscillator (board 1)
2. Defective keyboard circuit (replace board 2)
Unit cannot be tuned
(A-440 off frequency more than 5 Hz)
A-440 reference oscillator dead
(other outputs normal)
1. Power supply voltages improperly adjusted
2. Defective power supply (board 3)
1. Defective reference oscillator
(replace filter board 4)
No modulation (filter or oscillator) 1. Defective mod. mix amp.
(replace power supply bd. 3)
2. Defective MODULATION wheel control
Range switches have little or no effect 1. Octave Range misadjusted
2. -5 volt source defective
(replace oscillator board 1)
Noise or static when turning a control 1. Control dirty − spray clean
2. Control worn-out; replace
Noisy or intermittent switch 1. Switch dirty − spray clean
2. Switch worn-out; replace
Intermittent operation
or loss of some functions
1. Clean circuit board contacts with a pencil eraser
2. Tighten tension on circuit board sockets
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