AUSTRALIAN MONITOR AMIS120XL Series, Service Manual

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The switched contacts are used to break the signal internally.
When an external processor is used via the insert point, it only affects the power amplifier section and line
output of the AMIS120XL. The tape output remains unprocessed.
VOX Relay Output
The terminal strip to the immediate right of the AC inlet features a relay output. This relay is deactivated
when signal becomes present at any of the priority inputs. This would normally be inputs 1, 2 or 3, however
these channels can be removed from the priority bus via internal links. The unit is shipped from the factory
with inputs 1, 2 and 3 having priority. The emergency tones also deactivate the relay. The relay can also be
removed from the priority bus allowing muting to occur without operation of the relay.
The relay outputs are C (Common), NO (Normally Open) and NC (Normally Closed). The NO or NC
selection provides the installer with the option of the relay either opening or closing a contact. This feature
is normally used in conjunction with relay override attenuators (volume controls). In this application, the
relay output could trigger an accessory power supply which in turn bypasses the remote attenuators. The
result is that priority inputs will always be heard irrespective of the attenuator setting. The relay contacts
are 3 amps at 125VAC/30VDC.
The relay outputs are marked in the fail safe position, i.e. power off. When power is applied and no signal
is present the relay energizes through transistor V3 (BC546). This transistor is turned on by IC7A (LM1458
or equivalent). This output is normally high. Switch on and switch off times are controlled by the time
constants of the RC network of C19 and R33. The switch off time is determined by the current sinking
capabilities of the LM358 through diode D4 (BAV21 or equivalent). This time can be considered instant.
The on time is determined by the charge time of C19 (22uF) through R33 (22k). This is approximately 2
seconds. It must be remember that the relay circuit is designed to be fail safe so that when power is on the
relay is on and therefore the relay must turn off quickly (when priority is on) and on slowly (when priority
is off). This is to avoid chatter of the relay.
VCA Control
An external pot (500K) can be connected to the AMIS120XL for remote control of the master level. The
external pot is governed by the master level of the amplifier allowing the installer to set the volume, then
lock the amplifier in a rack, leaving the user with just a master volume control that cannot go beyond the
level set on the master (front panel) control. Connection is via a 2 wire terminal strip on the rear panel of
the amplifier. Shorting this input will mute the amp.
The VCA IC has a Voltage/Gain ratio of approximately -3mV/dB. That is a voltage difference of
approximately 300mV below the reference voltage (pin3, ~7.5V) will provide full mute (-80dB). The need
to have the remote pot on two wire connection means that there is a small amount of attenuation (approx
1dB) when the pot is connected. For the best audio control a log pot should be used. The control circuitry is
a DC attenuator and an inverter. When a pot is connected, the current through R16 (10k) and the pot
resistance generated from the 1/2Vcc bias voltage flows through R21 (330R). This generates a voltage
increase on the output if op amp IC16B (LM1458 or equivalent) above the reference voltage (1/2Vcc). The
second op amp IC16A (LM1458 or equivalent) inverts this voltage to a voltage difference below the
reference voltage. Thus if the pot is turned to S/C the voltage difference below the reference voltage is:
7.5(V) / 10(kohm) * 330(ohm) = 248(mV) = 248(mV) / -3(mV/dB) = -82(dB)
From this you can see that when a 500k pot is attached the initial attenuation is:
7.5(V) / 510(kohm) * 330(ohm) = 4.85(mV) = 4.85(mV) / -3(mV/dB) = -1.62(dB)
These calculations are only approximate and are used to show operation of the circuit.