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
The 951 uses a Pratt-Read keyboard and has Scale, Range, and Glide controls.
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
903-A Random (Three cabinet Moog)
960 Sequential Controller (Three cabinet Moog)
962 Sequential Switch (Three cabinet Moog)
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