R6 GATE (VCA)/RING MODULATOR
From the catalog: The RING MODULATOR/GATE (MOD) is an AC or DC coupled Ring Modulator and VCA featuring superior audio processing capabilities. The Ring Modulator offers two VC inputs in addition to the two signal inputs which may be used to perform voltage controlled transitions between full ring modulation and amplitude modulation. The VCA can be used as a standard voltage controlled amplifier with either log or linear control voltage characteristics. The DC coupled input is useful for using the VCA as a DC multiplier or gate.These modules share a PCB design, and were usually sold in pairs, or sometimes as groups of four. The catalog the above text was taken from has them for sale as dual module, containing one RM and one GATE. The ex- La Trobe Serge from 1975 has these modules paired as like types, i.e. two GATEs together or two RM together. The PCBs had originally been assembled as 'one of each' and were modified at the time the boards were installed behind the panels.
The stabilizing caps across the op-amps were fitted or omitted as required. Valued varied from one installation to another, in at least one case being as high as 470pF. Different transistors were fitted at different times of manufacture. The power connections on the PCB marked X and Y are incorrect. They are swapped.
above, the schematic for the GATE/VCA version.
above, the schematic for the Ring Modulator version.
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