Service manual
9
Mute control [Diagram.XL301_4.DGM].
The channel mute circuit can be activated from either the channels own MUTE switch SW13
or any of the 8 automute circuits. The eight automute lines enter the module via connector 'B'
(ST2) pins 29 to 36. They are isolated by diodes and fed to the eight automute selection switches.
SW9 to SW12 for automute 1 to 4 and SW1 to SW4 on pcb no. B2715 [Diagram
XL301_M.DGM], linked via ST8, for automutes 5 to 8.
The common output from these nine switches is the main mute line. A second mute line is also
required for operation of the mute control circuit. This is derived from the collector of Q9 whose
purpose is to invert the mute signal and also to provide a current sink for D35 the mute
indicator. The two signals mute and mute-inv are fed to the quad analogue switch IC19
[XL301-2.DGM].
IC19 is used to selectively (see earlier) mute the vca input and/or output. Under normal (unmuted)
operation, mute will be low (0V) and mute-inv will be high (+VC). IC19a and IC19c will
therefore be closed with IC19b and IC19d open. When the mute circuit is operational mute will
be high and mute-inv will be low, IC19a and IC19c will be open and IC19b and IC19d will be
closed, holding the vca input and the muted output (post-sig) from the vca at 0V.
Equaliser [Diagram XL301_3.DGM].
The Treble and Bass controls each provide 15dB of boost and/or cut by varying the feedback
parameters at selected frequencies between the output and the inverting input of IC9a and IC10b
respectively. The frequency can be swept over a 10:1 range by RV5a/R94 and RV5b/R95 in
conjunction with C77, C78 and IC11 for Bass and RV13a/R122 and RV13b/R123 in conjunction
with C98, C99 and IC15 for Treble. These sections operate at a fixed selectivity of 12dB per
octave.
The two mid controls are configured to provide continuously variable boost and/or cut of 15dB
with sweepable frequencies and adjustable selectivity. Taking the low mid as the example, the
boost and cut are provided at IC9b by the same mixing of output and inverting input signals as
was used on the Treble and Bass controls: the frequency setting is determined by RV8a/R103
and RV8b/R105 working in conjunction with C82 and C83. The bandwidth is adjustable by RV7
in conjunction with IC12a between 0.1 and 2 octaves.
Channel level meter [Diagram XL3_M10.DGM].
The pre insert and the post insert/post eq signals are both fed via ST3 to their own full wave
rectifiers on pcb B2706. The rectifier outputs (IC1a for pre insert and IC1b for post eq) are
summed by IC1c and fed to the main led drive IC2 as well as the -25dB signal present circuit
IC1d via level trim preset RV1.
Transistor Q2 and its associated components form a constant current source for the LED chain
D2 to D10. Transistor Q1 is used to provide a stabilised voltage for the LM3915. The
calibration preset for the meter RV1 should be adjusted with a signal of 0dB [monitored at the
insert] so that D5 (0dB) is just activate. IC1d is used as a comparitor to activate D1 when the
level equals or exceeded -25dB.