Service manual
18
VCA CONTROL
The internal mixes vca control voltages are derived either on the individual modules or more
commonly from the fader tray circuitry.
Grand Master VCA Control [Diagram XL321.DGM].
DC power for the fader tray pcbs are linked to the Grand Master 2 board via connector ST4.
From here it is distributed to all other master and channel control cards via the VCA Control Buss
ribbon cable linked to ST1. A stable vca reference voltage is also derived on this board from D2
via IC2b. It is adjusted with VR2 to give the required control range and distributed via the VCA
Control Buss.
This reference voltage is buffered and inverted by IC1a and fed to the Grand Master fader RV1.
Dependent upon the status of SW1 (VCA MUTE), either the attenuated voltage from RV1 or
the inverted reference voltage is summed with an offset voltage derived from the reference
voltage via VR1. Provided SW2 (VCA ON) is in the on state, this summated voltage will have
the output voltage from IC3a (see later) summed with it before buffering by IC1b ready for
distribution to the group and channel control cards.
A set of splash links allow this output control voltage to be assigned as one of four grand master
or one of eight group control lines. Links will be inserted in Grand Master 1 and 2 (GDM-CTL-
1 & GDM-CTL-2 on the diagram) respectively for the two Grand Master boards.
Master/Slave buffering.
Facilities are fitted to allow this console to act as either a master or slave console to another for
vca control.
When used as a master, the modules control voltage, as well as being distributed to the fader
tray boards, is buffered by IC3b and, with the appropriate splash link made, connected to the
DC CONTROL OUT connector on the mixer rear panel via ST2.
For situations where then console is to be used as a slave, the DC control voltage from the master
console is connected to the DC CONTROL IN connector on the mixer rear panel. From here it
connects into the board(s) via ST2 and via the appropriate splash link to IC3a. IC3a is con-
nected as a ground compensated differential buffer and provides a dc voltage which is summed
into IC1b with the internally derived control voltages.
Group (Master) VCA Control [Diagram XL311.DGM].
The VCA Group control boards combine two internal desk control functions as well as interfac-
ing to external master/slave desks.
The two associated group channels (A & B), can each have there vcas controlled not only by
the internal channel fader but also a combination of one or both the Grand Master controls.
Considering channel A, The two Grand Master inputs from ST1 pins 11 and 12 are selected
by switches SW3 and SW4 for Grand Masters 1 and 2 respectively. The signals are summed and
inverted by IC2a and connected to the group module via ST2. Channel B is similar using SW5,
SW6 and IC2b.