Service manual
15
[Diagram XL320_3.DGM], and via JPC to the matrix output fader RV2. The non-inverting
buffer IC13a feeds both the meter [via the meter changeover switch S8] and the mute switch
S7. The inverting input of IC13a is fed via connector B pin 12 [component side only] with talk
signals from the communications module. The mute switch [S7] wiper feeds the output
stage IC10, which is protected from excess voltage by the diode bridge D21-24, and set for
maximum common mode rejection at 100Hz by VR6 [typically 85dB].
Record Output [Diagram XL320_2.DGM].
The record outputs share one level control and one level check switch [which overrides
whichever input has been selected by the meter changeover switch S8 on diagram XL320_3.DGM].
The controls are linked by the ribbon connector between ST2a on the lower (left) PBC and ST2b
on the upper (right) PCB, but in all other respects each output is entirely separate. Signal is
derived from the wiper of the master mute switch S4 [diagram XL320_3.DGM] and fed to the
top of the level control RV3a (left) or b (right), from the wiper of which it passes through a
non-inverting buffer IC14b. From here the signal is fed both to the check level switch S9a
or b via C66, and to the output stage IC11a, which is protected from excess voltage by the
diode bridge D25-28, and set for maximum common mode rejection at 100Hz by VR7
[typically 85dB].
Output Meter [Diagram XL20M.DGM].
IC3a and b receive, rectify, and calibrate the signal fed from the master module via connector
ST1. The signal, now a varying positive voltage, is fed to IC3c and d; IC3c has 30dB of gain,
which enables D1-D10 to turn on 30dB ahead of D11-D20. Transistor Q1 provides a regulated
power source for the LM3915s and transistors Q2 and Q3 are constant current sources for the
LED chains. To calibrate the meter, a signal of -30dB is applied at pin 1 of ST1 and the
cathode of D23 is monitored by an oscilloscope. When a symmetrical trace is obtained by
adjusting VR1, the level is increased to 0dB and VR2 is adjusted so that D13 [the 0dB yellow
LED] just illuminates. Then the signal is reduced to -30dB, when the LED chain should have
only the first three LEDs [D1-D3] illuminated. If the calibration appears to be wrong, the
symmetry should be rechecked.
LED Powering [All diagrams].
The status LEDs are powered by a constant current source based round Q2 [Diagram
XL320_1.DGM]. The order for the chain starting from Q2 is meter changeover, matrix insert,
master insert, matrix PFL, master PFL, matrix mute, and master mute. The chain finishes at the
negative rail.