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There were four models manufactured in the 600 series during its production years. All of the models consisted of two tone generators per key. One set of generators was tuned at a eight ft. pitch, while the other generator was tuned at a four ft. pitch. Each set of generators had its own wave shaping circuits, and its own output jack. The output level available at each output jack was controlled by a control mounted on the front panel, and a foot operated expression pedal. The idea was to combine the new design of the Electra Piano with the accepted tones of the Rock Si Chord.
This was the first model in the series, with production starting in early 1968. A single foot operated switch activated the sustain circuitry. The keyboard consisted of sixty-one keys, and the unit was housed in the familiar RMI wood case design. The list price of this model was $1795.
The voices available are listed below. The 600A was produced until early 1970, when it was replaced by the 600B.
A Elect. Guitar 8'
A Elect. Guitar 4'
B Elect. Guitar 8'
B Elect. Guitar 4'
Organ Mode 8'
Organ Mode 4'
The 600B was the second model manufactured in this series, with production starting in early 1971. The main difference from the 600A was that the instrument came with individual sustain foot switches for the two tone generators. The wave shaping circuits were also redesigned. The unit was housed in the familiar RMI wood case. The retail cost of this instrument was $1895. This instrument was produced until late 1972, when it was replaced by the Model 668.
The 668 was the third model manufactured in this series, with production starting in late 1972. An additional seven keys were added, along with a pre amp, and bass boost controls. The front panel controls were changed to the sliding type. Otherwise, the instrument was identical to the model 600B. This model was manufactured till 1974 when it was replaced by the 668X. The list price was $2045.
The 668X was the forth and last model manufactured in this series. Introduced in late 1974, it was produced into the early 1980s. The instrument was identical to the model 668, with the exception that it was housed in the plastic hard shell case, the same one used for the 368X series. The unit was available only in black. The list price for this model was $2095. This was the last model manufactured using analog circuitry.