The Moog Minimoog, also known as the “Model D”, has maintained it’s place as one of the most iconic synthesisers in history and is highly sought after to this day for it’s unmistakable Moog sound and it’s classic industrial design. It began with it’s “Model A” genesis as several Moog modular system modules mounted in a case with an integrated keyboard, which was itself a world first and then it evolved through several design changes to become the Model D that began the revolution in compact and (relatively) affordable synthesisers. 

with it’s innovative flip-up control panel, coloured rocker switches, it’s classic knobs from the Moog modular systems, it’s timber case, aluminium panels, dual bend and modulation wheels and distinctive textured front panel the Model D has an instantly recognisable profile. The modern re-boot of the Model D, the powerful Minimoog Voyager shares many of the trade-mark visual and design cues and the ergonomics of the original Minimoog. The Model D also set standards that were imitated by most other manufacturers that came after – The left-to-right layout of oscillator, filter and envelope controls, the idea of pitch bend and modulation control wheels to the left of the keyboard and the basic signal and control paths along with some of the front panel terminology. Throughout it’s production life the Minimoog went through some minor cosmetic changes and a revision of the oscillator board but it basically maintained it’s looks and features from beginning to end. If there is one impression that describes the physical experience of the Minimoog it is that it is solid and substantial. Wood, metal, large switches, glowing filament lamps, no LED’s, smooth knobs that don’t wobble, metal jacks and ancient looking multi pin connectors.

The knowledge and experience that Moog had accumulated from building their modular systems translated very well into the Minimoog. They would have had to take some difficult choices in deciding what functionality to put into the synth and what to leave out –  a whole new design process for people who were accustomed to the unfettered flexibility of modulars that could be patched up in myriad ways. With the Minimoog they had to create a performance instrument that any gigging keyboard player could learn to use confidently within a few hours if they were to be able to sell these new instruments successfully. It was the machine that was intended to take the synthesiser out of the studio and onto the stage and Moog did this very well indeed, the Minimoog has an impressive sonic palette without being too complicated or too limited with some clever features – One of the oscillators is set up to double as a low frequency oscillator, turning it from being a sound source to a control source. A jack on the top allows any audio signal to be fed into the Minimoog’s filter and there was even an A440 tuning source provided. Let’s take a look beneath the surface…

Construction and layout

The Minimoog is quite simple internally, easy to access and service. It’s is clearly hand built with a lot of point-to-point wiring between the circuit boards, controls and jacks. The front panel controls are large, solid and of good quality. The circuit boards are tightly plugged into long socket strips and it’s quite easy to access any part of the synth for servicing. There are five circuit boards inside the Minimoog – A small board on the far left containing the power supply rectifier diodes and filter capacitors, to the right of that are four boards that contain all of the synthesizer circuitry. Some machines also have a tiny extra board for octave switching buffer IC’s.

The power supply boards

Minimoog mains transformer

Power supply/noise/mod board

The mains transformer and secondary power supply boards can be seen in the images above. Below the black transformer on the LHS you can see the small circuit board that contains the power rectifier diodes and filtering capacitors. To the right of the image is the power supply board. Note the two large gold heat sink fins for removing heat from the two transistors that are bolted to the corners of the fins – One transistor provided the positive 10 Volt supply, the other the negative 10 Volt supply. Together they produce the symmetrical 10V-0V-10V power that is needed throughout the synth. In the centre of the image you can see pairs of little black transistors that are connected by a white thermal paste. This is common in analog synths, particularly in oscillators when it is desirable to keep the two transistors that are wired as a symmetrical circuit as close to the same temperature as possible so that they drift with temperature change at the same rate.

Along with the power circuits it also contains the headphone amplifier, the modulation wheel mixing amplifier and the noise generator. The noise generator uses a transistor junction wired in such a way as to produce noise which then passes through three filter stages to produce white, pink and red noise.

This board has been re-capped at some point – the cylindrical electrolytic capacitors on the board are a newer type. With the age of the Minimoog these capacitors have almost invariably dried out and drift in value which alters the sound of the synthesiser and can allow mains hum to increase. They can change the timing of certain circuits too. There is something of a craze in synth circles these days for re-capping, we don’t advocate wholesale replacement of capacitors without testing first. Having said that the Minimoog in particular does benefit from replacing it’s electrolytic caps if they are original and the sound is noticeably improved, particularly if the synth has been in storage for many years unused and the capacitors have degraded.

 

The Oscillator board

Model D oscillator board

The oscillator board, located on the right hand side of the synth when looking from the rear. This board contains the three VCO’s, the control voltage mixing circuits for the oscillators, the logarithmic converters to produce the 1V/octave scaling of the VCO’s, the oscillator sawtooth wave cores and the wave shaping circuits that convert the natural sawtooth wave from the VCO cores into the additional triangle and pulse waveforms that the Minimoog offers. The two larger IC’s in the white sockets toward the centre of the image are of special interest – They are transistor arrays that are used to convert the linear voltage from the keyboard into the  logarithmic current that gives the oscillator core the classic 1 Volt/per octave tuning response.

Model D tuning trim pots

Oscillator CV summing

Oscillator CV summing resistors

Oscillator board close up

A closer view of the CV mixing section of the oscillator board. Note the large metal trimmer pots used to set the scaling and offset of the VCO’s in order to tune them to a musical scale. The chips that you can see are the op-amps that are a part of the CV mixing circuits that combine the control voltages from the keyboard, mod wheel, external CV jack and front panel tuning knobs which are fed to each of the three VCO’s. This board would not tune up, at some point these chips had been replaced and the 5534 IC’s that you can see in the picture (probably in an attempt to “improve” the machine) completely messed up the VCO scaling. Once they  were replaced with the original 741 type chips the board was restored to good working order.

The three silver cylinders are the polystyrene integrating capacitors that constantly charge and discharge like tipping water buckets to create the oscillator sawtooth waves from the 3 VCO’s. beside them to the left are the three dual field-effect transistors that form part of the oscillator cores. They are the round white components with the black caps. Further to the left are the components that make up the wave shapers that produce the pulse and triangle waves.

The sparse dual envelope generator board. When the machine is assembled it’s hidden behind the oscillator board.

Synths this age often have odd modifications added over the years. This one is on the envelope generator board.

The Minimoog filter

The filter is usually what first comes to mind when conversation turns to the sound of the Minimoog, or indeed the sound of any Moog synthesiser. In the case of the Minimoog it is the sound of the voltage controlled, low-pass “ladder” filter. It’s called a ladder filter due to the ladder like appearance of the schematic and of the physical appearance of the components when they are placed on the circuit board and because of the ladder analogy of the behaviour of current in the circuit itself. In the picture above you can see the ladder on it’s side across the bottom of the picture.

Despite these transistors and capacitors looking every one of their forty-plus years they are still working perfectly.

Another close-up view of the components in the classic Moog ladder filter. The coloured paint blobs on the tops of the transistors indicate that they were electrically tested at the factory to sort into matched sets for each machine.

Final test and quality control sign off at the factory?

A reliable icon

What other pieces of electrical equipment are still in use after 40 years? Did Robert Moog and his associates ever imagine that the Minimoogs that they were building would still be on stages and in studios in working order in 2016? Therein lies the beauty of the simplicity of the Minimoog and indeed many other vintage synths – There are very few components on the circuit boards of the Model D that cannot be replaced by a new component that is functionally identical and indeed all of the original electronic components can still be found as witnessed by Moog Music having re-started the production line for their modular systems in 2015 and producing the same synth with the same components 50 years on. In fact in 2016 Moog have re-released the Model D and 40 years on it is as popular as ever!

This is one of the cool things about many vintage synths – if you look after them well and don’t damage the cosmetic components, switches and panels and keys then they will generally be repairable for the foreseeable future. Even rare components like the dual FET transistors on the oscillator board could be functionally replaced by modern parts once any remaining stocks of the originals finally run out.

Model D buying guide

If you are looking for a Model D to purchase you may have to take whatever you can find, they are quite expensive now as befits a classic instrument. The 2016 re-release of the Minimoog would undoubtably be the choice in terms of stability, playability and features but if you are seeking an original Mini then the main issues to look out for are cosmetics and physical condition. The electronics are far easier to restore than the panels and switches and wheels. Clunky keys can be re-bushed, capacitors are easily replaced as are the poor quality IC sockets holding the transistor array IC’s. As always electrical safety is paramount, if you buy a used machine it is wise to have a competent person inspect the mains wiring and power cable condition. It’s also worth checking that each of the fuses are of the correct value and that they haven’t been replaced with incorrect parts over the years. The key contact springs can be problematic if abused. It’s not uncommon for tarnished contact rails to produce dropped notes or sliding pitches but if the keys are really bad it’s worth checking to ensure that there is not damage to the springs.

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