Service manual
13
Output Meter [Diagram XL20M.DGM].
IC3a and b receive, rectify, and calibrate the signal fed from the group module connector ST5.
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 [the level can be
externally verified at whichever output is driving the meter, selected by the meter change-over
switch S34], 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.
Solo Routing [Diagram XL310_A3.DGM (both groups)].
Each of the four pfl switches, two per group (mix PFL and direct in PFL), provide a gate signal
to select the appropriate fet switches for the source. These four gate signals are summed by
diodes D51 to D54 and are used to turn on fet Q14 routing the pfl signal to the pfl buss via
connector 'B' pin 10. They are also used to route the left and right solo buss feeds from IC15
via fets Q10 and Q13 to connector 'B' pins 11 (left) and 12 (right). In addition to this the signal
is inverted by transistor Q15 in order to drive transistor Q16. Q16 is used to activate the
Solo_Gate line via connector 'B' pin 13.
In the case of channel A, the two group solos are switched by Q8 and Q9 into the summing input
of IC15a via R111 and R112 to give unity gain. Likewise for channel B with Q11 & Q12 with
R123 and R124 into IC15b. Under normal (two separate groups) operation, the SOLO STEREO
SPLIT switch is in the off position. This allows Q17 and Q18 to turn on whichever solo is
selected to ensure that channel A and channel B solo outputs are crosscoupled to each other for
monitoring through both left and right solo monitor channels. If the SOLO STEREO SPLIT
switch is on, this crosscoupling does not occur and so the group solo monitoring is as a stereo
pair.
Switch LED Powering [All diagrams].
The status LEDs are powered by three constant current sources based round Q1, Q2 and Q201
[Diagrams XL310_A1.DGM & XL310_B1.DGM].
The order for the first chain starting from Q1 is: Direct in A PFL, Direct in A MUTE, Direct
in B PFL, Direct in B MUTE, A Matrix 1 MUTE, A matrix 2 MUTE, B Matrix 1 MUTE,
B Matrix 2 MUTE, A Level to Masters ON, A Automute Safe ON.
The order for the second chain starting from Q2 is: A Automutes 1 thro 8, A TALK, A
INST, A PFL, SOLO Stereo Split.
The third chain from Q201 is: B Level to Masters ON, B Automute Safe ON, B Automutes
1 thro 8, B TALK, B INST, B PFL.
All three chains finish at the negative rail.