Service manual
16
COMMUNICATIONS MODULE
Centred in this module are the controls for the intercom, talk and pink noise generator
functions, the automute system, and the signal monitoring facilities, as well as the headphone
amplifier.
Intercom [Diagram XL330_1.DGM].
The intercom shares much of its circuitry with the monitoring and talkback facilities; the intercom
switch [S3] selects the routing and function of the microphone amplifier and headphone signal.
In intercom mode, microphone signals arrive through either the headset or microphone
connectors [both mounted on the modules front panel], pass through the a.g.c. amplifier IC5
and IC6a and to the remote station via the isolating transformer T1 via block d of the intercom
switch S3. VR3 adjusts the amount of microphone signal fed to the headphones by varying
the common mode rejection ratio of the microphone input to IC6b without varying the gain
structure for signals arriving from the remote station via T1. Overall intercom level is adjusted
by RV3 before it passes through blocks a and e of the intercom switch, the headphone level
controls RV1 and the headphone mute switch to the headphone amplifier [see diagram
XL330_5.DGM] and the headphone jack socket on the module front panel. IC7 and IC8 are
isolating networks which ensure that ground loops are avoided when the intercom is used with
an actively powered station. When the intercom is switched on, a negative voltage is applied
via block f of the intercom switch S3 to the anode of the LED in IC7, causing a positive
voltage to be applied to pin 14 of connector 'A' (ST1) via D18 and R52; when a call is
received, the positive voltage on pin 14 energises the photo-transistor within IC8 and turns on
Q1, which illuminates the intercom call light [LP1]. In the event that the console is to be used
as the slave station, the intercom can be decoupled completely from the consoles power
supply by applying the master stations power rail via the unused pin on the intercom XLR
connector and edge connector A pin 15 to RLY1, which disconnects the intercom ground
completely from the console.
Talk Facility [Diagram XL330_1.DGM].
Microphone and headset signals arrive at block d of the intercom switch S3 as previously
described for the intercom: however, with the switch in its normal [non-intercom] position, talk
signals pass through block c to block b of the pink noise generator switch S6 [change to
diagram XL330_2.DGM]. From the wiper of this switch the signal passes both via buffer IC10b
to connector 'A' pin 12 [and thus to the talk external XLR connector on the console back panel]
and to the talk master switch S13. When this switch is on, signal is fed to the three talk level
pots [RV4-6]; a further 10dB of gain is provided by IC10a, IC12a, or IC12b before the
individual talk switches [S7-S12] enable signals to pass to their respective busses. The LED
indicators for these switches are wired in parallel between +VA and ground via limiting
resistors R58-R63, with +VA appearing on each wiper of the LED switching blocks; all the
normally open contacts are joined and fed to the second block of the talk master switch S13.
This means that whenever a talk route is open [i.e. the talk master switch and any individual
talk switch both on] a gate voltage is fed from the talk master switch to Q2 and Q3, which dims
the local outputs to avoid the feedback which would otherwise result if the talkback
microphone and a loudspeaker driven from a local output are close together.