Service manual

19
The second function of the module is the control for one of the eight VCA Groups. The
reference voltage from the Grand Master 2 board is sauced via the VCA control buss connector
ST1 pin 9. It is buffered and inverted by IC1a and fed to the 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 previous details on master/slave linking) summed with it before buffering by IC1b
ready for distribution to the channel control cards.
A set of splash links allow this output control voltage to be assigned as one of the eight VCA
Group control lines. Links will be inserted in the appropriate location (VCA-CTL-1 to VCA-
CTL-8 on the diagram) on the eight VCA Group boards.
Channel VCA control [Diagram XL302_VC.DGM].
All eight Group VCA control lines are available on the channel control module. These enter via
the VCA Control Buss connector ST1 pins 1 to 8 and are selected by switches SW1 thro SW8
for groups 1 to 8 respectively.
The reference voltage on ST1 pin 9 is buffered and inverted by IC1b and fed to the channel vca
level control RV1. The wiper of this level control is summed with the combined group level
control voltage (as selected by SW1 thro SW8) by IC1a. The Group control voltage is only
effective when the VCA ON switch (SW9) is in the on position. The output of IC1a is fed to
the channel module vca input via ST2.
LED Powering [All diagrams].
The status leds are powered by constant current sources, one on each vca control board. These
are based round transistor Q1. The order for the chain starting from Q1 can be clearly seen
on the appropriate diagram. Each chain ends at the -v rail.