Service manual

clipping and it is matched by pre-set VR1 so as to give the maximum CMR when the signals are

de-balanced by amplifier IC2. The differential gain of this stage is adjustable from +36dB to -4dB

by half of VR3.

The signal then passes to a simple shelving EQ formed by amplifier IC2 and potentiometers RV1

and RV2 before entering the VCA IC3. The VCA gain is controlled by the second half of

potentiometer RV3 via amplifier IC5. The gain is adjustable from +14dB to -26dB which when

added to the first stage gain gives the full +50dB to -30dB range. The +50dB gain calibration is

set by pre-set VR3 and the distortion is trimmed to a minimum by pre-set VR2. The VCA can

also be controlled from the master vca fader busses which are connected via switches SW4 to

SW13.

The remaining descriptions apply to both types of group module:-

The signal then passes through amplifier IC4 and FETs Q1 and Q2 which form the mute circuit.

This is controlled by invertor IC8 which forms a bi-stable flip flop which is triggered by switch

SW2 and capacitor C183. Transistor Q3 drives the FETs can be further controlled by the auto-

mation system via R249 or the mute aux return line.

The post mute signal is selected to pass directly to the audio group circuits by SW1 or to the

master busses via the pan potentiometer RV4, amplifier IC7 and FETs Q4 and Q5.

The aux return signal can be sent to the solo busses via FETs Q6, Q7 and Q8. These are control-

led by NAND gate IC9, invertor IC8 and transistor Q11. Sections A and B of the NAND gate

form a bi-stable flip flop which is triggered by switch SW3 and capacitor C39. When a solo is first

enabled capacitor C42 creates a negative going pulse which is inverted and buffered by section D

of IC9. This will clear any other solos but is prevented from self clearing by the logic low charge

held on C39; once this capacitor has charged to a logic high the solo can be cleared by the solo

clear bus via section C of IC9. If the solo switch is held down capacitor C38 dis-charges and

resets the flip flop; the solo is now held on by diode D8 until the switch is released. Aux return

signals can also be soloed as part of a vca solo group; this is selectable via SW4 to SW13.

Group and aux summing amplifiers IC11 and IC15 are referenced to a clean 0VA or noise

reference which is sent from the master module to minimise crosstalk and mains born interference.

Capacitors C74 and C79 provide AC coupling and capacitor/resistor networks C72, C77, R161

and R170 ensure maximum stability and high frequency rejection at all gains.

Bus inject signals are fed via T filters in the frame to differential amplifiers IC10 and IC14. Pre-

sets VR4 and VR5 provide CMR adjustment and capacitors C46, C47, C48, C49, C66, C67, C68

and C69 provide additional high frequency filtering. C50, C51, C70 and C71 provide AC cou-

pling.

The summing amplifier outputs are fed via the change over switch SW16 to the second half of

amplifiers IC11 and IC15 where they are summed with the bus inject signals and talk signals from

the comms module.

The aux send signal passes from amplifier IC11 through level potentiometer RV5 to 10dB ampli-

fier IC27. It then passes via mute switch SW14 to the differential output amplifier IC17. Output

symmetry is adjusted by pre-set VR6 and high frequency stability into any load is assured by filter

capacitors C88, C89, C92 and C94. Further filtering is achieved by T filters in the console frame