Moog System 55 Restoration

I got the opportunity to restore a near-original Moog System 55.  Most of the modules were revisions from the earlier three cabinet Moog so I documented them in these pages.  Modules such as the 903-A, 960, and 962 were the same so I just refer to those pages.  The owner also posted this restoration on Matrixsynth.

I strived to keep the repairs as original as possible and this Moog was near-original.  There was not much value drift as most resistors are metal film.  Many of the modules needed significant calibration and I only found a few minor modifications which I restored to factory configuration.  Some of the Moog modules such as the 960 Sequential Controller, 961 Sequencer Interface, and the 962 Sequential Switch use 700 Series RTL (resistor-transistor logic) logic.  These were the first commercially available low and medium scale integrated logic introduced in the early 1960's and operate on a +3.6V supply so some of the V-Trigger outputs are limited to 3.6V.   There were some electrical problems but mostly electro-mechanical problems - switches and jacks no longer made contact and some trimmers were broken.  In particular, there were a number of open trimmers where the wiper contact is simply held in by two bent over tabs.  The tabs fatigue and the wiper falls off.  Bourns 3319-9mm trimmers are almost a perfect fit and the pins on a Bourns 3386-3/8" trimmer can be bent to fit.



These systems are rare.  The date on the modules is 1975 although some of the ICs have date codes back to 1969.  The purchase price of this synthesizer in 1976 was $11,900.  In 1975 an average car cost $3,800 and the average wage was $12,686 so this synthesizer cost nearly a year's wages.

1976 Moog Module Price List  Note that this price list is in Canadian dollars but the exchange rate in 1976 averaged 1.0144 CD/USD



Moog Keyboard

The 951 uses a Pratt-Read keyboard and has Scale, Range, and Glide controls.

951 Keyboard



Moog Bottom Cabinet

The bottom cabinet contains the trunk and controller bus wiring, power supply, and 22 MU module space with 14 modules and 6 console panels.

Moog System 55 Cabinet Wiring Diagram

High resolution main cabinet photo

High resolution main cabinet inside photo



930 Power Supply

921A Oscillator Driver

921B Oscillator

921 Oscillator

995 Attenuators

903-A Random (Three cabinet Moog)

994 Multiples

960 Sequential Controller (Three cabinet Moog)

962 Sequential Switch (Three cabinet Moog)

Console Panels

961CP Sequencer Interface



Moog Top Cabinet

The top cabinet contains bus wiring and 22 MU module space with 16 modules and a blank panel.

High resolution top cabinet photo

High resolution top cabinet inside photo



914 Fixed Filter Bank

904B High Pass Filter

904A Low Pass Filter

992 Control Voltages

902 Voltage Controlled Amplifier

993 Trigger and Envelope Voltages

911 Envelope Generator

911-A Dual Trigger Delay



Internal Wiring, Rear Panel, and Auxiliary PCBs

Internal Wiring, Rear Panel, and Auxiliary PCBs photos