AUSTRALIAN MONITOR AMIS250 Series, Руководство по обслуживанию

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Four separate tones are available from the in-built tone generator board. All four tones can be activated
individually via a contact closure connected to the screw terminals on the rear of the amplifier. When any
tone is activated, all inputs (except for inputs 1 and 2) will automatically mute. The level of the tone
generator is controlled by the pot labeled R6 (located behind the Bass adjustment pot). This pot adjusts the
level for all 4 tones.
Tones available on the AMIS250 include:
- Evacuation Tone
- Alert Tone
- Bell Tone
- Pre-Announce Chime
Theses inputs are pulled up to 5VDC internally through 1k resistors.
The tones are generated through a combination of digital frequency modulation and analogue amplitude
modulation. The output of the microcontroller IC7 (PIC16C55A) on pin 6 is a 0-5V square wave of varying
frequency (depending on the tone selected). This signal is amplitude modulated using a VCA IC8
(M5222P). The envelope is controlled by the charging and discharging of the electrolytic capacitor C40
with the sink and sourcing of current on the microcontroller outputs pins 8, 9 & 17.
VOX Muting
This feature provides automatic muting channels 3 & 4 when signal is applied to either channels 1 or 2. It is
normally used so that a paging microphone can have priority (by muting) over background music. The
muted channels will automatically ramp back up to normal volume when the signal on channels 1 and/or 2
is no longer active. The amplifier ships with the VOX muting function enabled. To disable the VOX
muting move the jumper (labeled JP2, located just to the left and behind the level pot for channel 1) to the
middle and rear pins. (Factory setting is with VOX enabled with the jumper on the middle and front pins).
Channels 3, 4, 5 & 6 are summed through IC1A (LM1458 or equivalent) to the VCA IC4 (M5222P) which
does the muting. The VCA is held on (i.e. no attenuation) by R22 (10k) pulled to the reference voltage and
the reverse biasing of diode D2 (BAV21 or equivalent) by R20 and R21. This is to eliminate any
attenuation by the op amp IC1B (LM1458 or equivalent) and its ½ supply which may be different to the
internal reference voltage of the VCA.
The control pin of the VCA is pulled down by op amp IC1B. This op amp runs at very high gain to activate
on even very small signal. The jumper JP2 shorts out the feedback resistor thus removing the sensitivity.
The attack of the muting circuit is controlled by the charging of C13 (47uF) through R20 (100k) and the
release by the discharge if C13 through R16 (2k2). Note that VCA will mute at 250mV (-80dB) below the
reference voltage so the muting will occur only over the range of approximately (depends on VCA
production batch) 7.2V to 7.5V. The charging of the capacitor occurs from approximately 1V to 8.1V.
VCA M5222P pin out
1 – Output1
2 – Input1
3 – Reference voltage (½ Vcc)
4 – Gnd
5 – Control
6 – Input2
7 – Output2
Note: The M5222P is a current in, current out device. Voltage conversion is done through resistors. The
maximum input current is 50uA rms. Current gain is 0.5.
Fuse Sizes
(240V version)
Mains: 230 VAC 6.3 Amperes Slow Blow