Service manual

17
Pink Noise Generator [Diagram XL330_3.DGM].
IC9 is the noise source; R92/C62, R93/C63, and R94/C64 provide the necessary colouration and
IC11a corrects the gain structure and provides a buffer stage before the pink noise is fed to its
switch S6 [Diagram XL330_2.DGM], from which point it follows the same path as the talk
signals.
Automute Master Controls [Diagram XL330_3.DGM].
The eight illuminated MUTE GROUP switches [S14-21] feed positive rail voltage current-limited
by R111 and decoupled by C69-768 and R103-110 to DC buffers IC13 to IC16. The outputs of
these feed the automute busses on connector 'B' (ST2) [pins 15-22].
Signal Monitoring [Diagram XL330_1.DGM].
PFL signals arrive at connector B pin 1 where they are summed by the virtual earth stage IC1b
and reinverted by IC1a. The solo listen switch S2 blocks c and d select either PFL signals from
C3 or solo signals from the master module via connector B pins 13 and 14 and feed them through
the intercom switch S3 blocks a and e to the headphone level control RV1 and headphone mute
switch S4, and thus to the headphone amplifier [see diagram XL330_5.DGM]. In addition to
driving the headphone level control, the wipers of S3 blocks a and e feed the local output level
control RV2 [change to diagram XL330_2.DGM]: signals pass through the local output mute
switch S5 and buffers IC2a and b to the balanced local output stages IC3 and IC4, which are
protected from excess voltage by the diode bridges D6-9 and 10-13, and set for maximum
common mode rejection at 100Hz by VR1 and VR2 [typically 85dB].
Headphone Amplifier [Diagram XL330_5.DGM].
R19/C17 and R20/C18 are the power supply decoupling networks; RF protection is afforded by
R2/C1 and R4/C2. Connections to the module are hard-wired.
Signal Metering [Diagram XL330_1.DGM].
The two meters are controlled by changeover switch S1. In solo mode [switch on], blocks c and
d are fed with solo left and right signals respectively from connector B pins 13 and 14 and pass
them to the meters via ST3 and ST4. In PFL mode [switch S1 off], block c is fed with PFL
signals from C3 and passes them to the left meter via ST3; block d is fed with left local output
post-fader signals from the junction of R17 and C6 [diagram XL330_2.DGM] and passes them
to the right meter via ST4.
LED Powering [All diagrams].
The status LEDs [with the exception of all the talk switches] are powered by a constant current
source based round Q4 [Diagram XL330_3.DGM]. The order for the chain starting from Q4
is meter changeover, intercom, pink noise, mute group masters 1-8, solo listen, local output mute,
and headphone mute. The chain finishes at the negative rail.