Service manual

8
Diagram XL301_2.DGM
The PHASE REVERSE switch SW4 controls the inversion mode of IC2a,and passes the signal
to the 12dB/octave high pass filter around IC2b and IC3a controlled by SW5 and RV2a/RV2b.
From the wiper of SW5 the signal is fed to three different points:-
[a] The INSERT switch SW6 to provide the insert return by-pass.
[b] The insert send balancing network. This is built around IC4 as a unity-gain, phase-
contrasting amplifier whose common mode re-jection ratio is set by VR3 [typically
85dB at 100Hz]. Diodes D9, D10, D11 and D12 are provided to protect the insert
send circuitry from damage by excess voltage.
[c] The input channel meter via connector ST3 pin 1.
The insert return appears at connector A pins 15 [hot] and 16 [cold]; it is protected from R.F.
contamination by R48/C41 and R53/C44, and from over-voltage by diode bridge D13-16. IC5 is
a differential amplifier whose common mode rejection is factory set by VR3 to maximum at
100Hz, typically 85dB. The insert return is fed via C47 to SW6 (INSERT) and from here to the
EQ in/out switch SW7. After returning from the equaliser section, a signal is fed to the meter
via ST3, the SOLO switch SW8 via buffer IC6b and the VCA section via either Jumper ST4/pre
or IC19a.
VCA and Mute
The mute control is detailed later. Two mutes are associated with the vca stage, pre-fade and
post-fade. These can be selected as required by jumpers in ST4 and ST5. The options are shown
on diagram XL301_LK.DGM.
The vca is based around IC6a, IC7 and IC8a in the audio path and IC8b in the control path. The
control voltage from either the internal module level control RV3 (if fitted) and the fader tray
level control are summed with an offset voltage by IC8b. This offset voltage adjusted by VR6
is used to define the available vca control range. It has been preset in the factory and should not
require adjusting unless the vca has been replaced. The output voltage of IC8b, and therefore the
control range of the vca, is further clamped by D58 and D59 to prevent overshoot.
The VCA output is routed either pre or post mute (dependent upon ST5) to the pan control
RV14a/RV14b which offers 3dB of attenuation at pan centre [Diagram XL301_4.DGM]. The pan
control is buffered by IC16a and IC16b and then fed to the left and right master busses via the
fet switches Q27-30 activated by SW5 (MASTER ON) [Diagram. XL301_M.DGM]. The outputs
from IC16a and IC16b are also fed to the SOLO switch SW8 for routing to the solo busses.
The feeds for the mixes are derived from the VCA input (Pin 1 of IC6) and the muted vca output.
These are buffered by IC17a & b for Mix 1 to 8 [Diagram XL301_5.DGM] and IC18a & b for
Mix 9 to 16 [Diagram XL301_6.DGM]. Each Mix can be selected pre or post fade (vca). The
mixes are turned on and off by a combination of mechanically breaking the feed to the level
control and effectively isolating the level controls output from the mix buss with the fets (Q11
to 26). The fets control voltage is arranged to ramp up or down to provide a click free transition.