Service manual

The stereo aux signal can be sent to the solo busses via CMOS switch IC5 which is controlled by

NAND gate IC8. Sections A and C of the NAND gate form a bi-stable flip flop which is triggered

by switch SW3 and capacitor C40. When a solo is first enabled capacitor C38 creates a negative

going pulse which is inverted and buffered by section B of IC8. This will clear any other solos but

is prevented from self clearing by the logic low charge held on C40; once this capacitor has

charged to a logic high the solo can be cleared by the solo clear bus via section D of IC8. If the

solo switch is held down capacitor C37 dis-charges and resets the flip flop; the solo is now held

on by diode D12 until the switch is released.

11. 20 led Meters.

Each led meter covers a dynamic range of 60dB and is able to monitor signals from up to six

points within the console (only fully used on the group meters). The response is peak reading and

the value is in dBu i.e. 0dB is 0.775 volts.

Incoming signals are attenuated by resistors R27 to R38 so that they can pass through the CMOS

switch IC4. This is necessary because the switch runs off reduced voltage rails. The CMOS switch

selects one of the six inputs to route to amplifier IC5 which recovers the signal voltage level and

feeds to the rectifier circuit.

The input selection for CMOS switch IC4 is controlled by three lines. The pre meter change over

line routes to control line A which selects between X0/X1, X2/X3, X4/X5 and X6/X7. When the

control line is logic high the CMOS switch routes pre signals only. Control line B selects between

X0+X1/X2+X3 and X4+X5/X6+X7. When the control line is logic high the CMOS switch routes

aux return signals only (providing aux signals are selected by control line C). Control line C

selects between X0+X1+X2+X3/X4+X5+X6+X7. When it is logic high aux signals are selected,

if it is low group signals are selected. Control lines B and C are switched by switch SW1; pro-

gramming resistors select the function of the switch as follows:-

For a left hand group meter (switching group/aux) R42 is fitted and R41 is

removed.

For a right hand group meter (switching aux send/aux return) R41 is fitted and R42 is

removed.

For a matrix meter both R41 and R42 are fitted.

For a master or comms meter switch SW1 is removed or dissabled.

On the group meter lines M1 and M2 are connected in left/right pairs on the group mother board

so as to enable meter function switching in pairs.

The rectifier circuit is calibrated for 0dB by its input pre-set VR2. The rectifier is of a precision

full wave type formed by amplifier IC3. Amplifier off-sets are adjusted out by pre-set VR1, how-

ever practical adjustments are made with a signal present at -30dB. The rectifier output charges a

capacitor C6 via resistor R15 which sets the attack time; the release time is set by resistor R14.

The capacitor charge is buffered and fed to the high level bar graph IC2; it is also amplified by

30dB to feed the low level bar graph IC1. Transistors Q1 and Q2 provide a stable 24 volt supply