Service manual

The mute output from the vca is summed with any talkback signals from the communications

module by IC12a and then fed to the solo switching circuit (see later) as well as the group output

stage. This is built around IC13 as a unity-gain, phase-contrasting amplifier whose common mode

rejection ratio at 100Hz is set by VR6 [typically 85dB at 100Hz]. Diodes D16-19 are provided

to protect the circuitry from damage by excess voltage. The group output appears on connector

'A' pins 10 [Hot] and 11 [Cold], from where it connects to the rear panel xlr group output

connector. Two talkback lines are available from the comms. module on connector B21. Talk_All

on the solder side of the connector and Talk_Select on the component side of the connector.

Talk_All is always summed into IC12a, Talk_Select only if S11 the talk switch is on.

Master and Matrix [Diagram XL310_A2.DGM].

The master buss signal level is controlled by RV2 and selected to be either pre or post vca by

S5. If the signal is pre-vca, the option to mute or not with the main group mute is determined

by the link in ST3. If post-vca, the location of the link in ST6 will determine if the signal is pre

or post mute (see diagram XL310_LK.DGM for link locations). The signal is routed via the non-

inverting buffer IC212b to the pan control RV3a/RV3b which offers 3dB of attenuation at pan

centre. The pan control is buffered by IC14a and IC14b and then fed to the master busses left

and right via the master on circuit Q5/Q19 and Q6/Q20 activated by S6 (Master on).

The matrix buss signal levels are controlled by RV4 and RV5 and selected to be either pre or

post vca by S7 and S9. If the signals are pre-vca, the option to mute or not with the main group

mute is determined by the link in ST4. If post-vca, the location of the link in ST7 will determine

if the signals are pre or post mute (see diagram XL310_LK.DGM for link locations). The signals

are routed via R103 and R105 to the two matrix mix busses. The feeds can be muted with S8

and S10.

Mute control [Diagram XL310_A2.DGM & XL310_A3.DGM].

The groups mute circuit can be activated from either the groups own mute switch S12 or any

of the 8 automute circuits. The eight automute lines enter the module via connector B pins 29

to 36. They are isolated by diodes and fed to the eight automute selection switches. S14 to S21

for module group A and via ST8 to module group B. The common output from the eight

automutes are connected to S12 and the main mute line via the Mute Safe switch S13. A second

mute line is also required for operation of the mute control circuit. This is derived from the

collector of transistor Q7 whose purpose is to invert the mute signal and also to provide a

current sink for D40 the mute indicator. The two signals mute and mute-inv are fed to the

quad analogue switch IC9. IC9 is used to selectively (see earlier) mute the pre and post vca

signals. Under normal (unmuted) operation, mute will be low (0V) and mute-inv will be high

(+VC). IC9b, IC9c and IC9d will therefore be closed with IC9a open. When the mute circuit is

operational mute will be high and mute-inv will be low, IC9b, IC9c and IC9d will be open and

I9a will be closed, breaking the signal feed to the master and matrix pre/post switches (depending

upon the links in ST3 and 4) and holding the output from the vca at 0V.