Icom IC-551D Instruction Manual page 38

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APC/ALC CIRCUIT
fluctuating, and sets the output power between 1 and 80 watts. The variation in the collector
C41 and R22, and D6 shortens the attack time. The APC/ALC signals are applied to the accessory
socket on the rear panel through D46, D48 and C17.
L3 of the Low-Pass Filter circuit is an SWR pick up coil, and D1 and D2 are the detection diodes.
The voltage of a traveling wave is detected by ОТ аз а positive DC voltage, and a reflected wave is
detected as a positive DC voltage by D2. They аге. fed to ІС2 and compared there voltage ratio.
When VSWR is over 3:1, the ALC voltage is put out at Pin 1 of 1С2. The ALC voltage is fed to the
2nd gate of 05 through 046 and D48, and the driving power to the final PA stage becomes low
and protects the PA transistors.
VOX CIRCUIT
A new time delay device is used in the VOX circuit. It is a low-noise, no-loss BBD (Bucket Brigade
Device) and its maximum time delay is about 50 milliseconds.
The amplified signal is fed to the base of VOX detector transistor Q1. The output of the emitter is
supplied through R15 to one of the bases of a dual transistor ОЗ, the collector of which is con-
nected to the DC switching circuit Q4 and ОБ.
. When you speak to the microphone, Q5 turns ON and the set is turned in the transmit mode.
C6 and R12, and R140 VOX DELAY control on the MAIN unit compose a circuit to determine
the time constant to the VOX circuit.
The VOX circuit provides the ANTI- VOX circuit to prevent the set turns in the transmit mode
with sounds from the speaker.
A part of the output signals of the AF power amplifier is fed to the amplifier 1/21C3 through
R138 ANTI-VOX GAIN control and ЈА in the main unit and P1. The amplified signal is fed to the