AUSTRALIAN MONITOR AMIS120 Series, Руководство по обслуживанию, Страница: 7 / 26

muting move the jumper (labelled JP2, located just to the left and behind the level pot for channel 1) to the
middle and left pins. (Factory setting is with VOX enabled with the jumper on the middle and right pins).
Channels 3 & 4 are summed through IC1B (LM1458 or equivalent) to the VCA IC4 (M5222P) which does
the muting. The VCA is held on (i.e. no attenuation) by R55 (10k) pulled to the reference voltage and the
reverse biasing of diode D3 (BAV21 or equivalent) by R22 and R21. This is to eliminate any attenuation by
the op amp IC1A (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 IC1A. 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 R22 (100k) and the
release by the discharge if C13 through R37 (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 AMIS60, 230 VAC: 2 Amperes Slow Blow
Mains AMIS120, 230 VAC: 3.15 Amperes Slow Blow
(115V version)
Mains AMIS60, 115 VAC: 3.15 Amperes Slow Blow
Mains AMIS120, 115 VAC: 4 Amperes Slow Blow
The DC fuse is located on the circuit board. This is a feature of the AMIS series amplifiers, which are
equipped with a current limiting circuit preventing excessive DC currents, thus eliminating the risk of
blowing high tension fuses. In the unlikely event that the DC fuse actuates, the output transistors should be
checked, as it is probable that the amplifier has been subjected to very extreme conditions. The DC fuses
are 10A time lag fuses.