Furuno FS-1503, Service Manual

Page: 25 / 136

2-18
ALC (From PA/FIL PCB)
ALC
FL1
U7/8
ALC
BLANK
TP3
CR5
U6
54.455 MHz
2Lo (54 MHz)
CR11
-12
NB
CR24
F + 1.5 kHz
455 kHz
CR10
Q7
30M L.P.F
1Lo (F + 54.455 MHz + offset:1.5 kHz)
TP1
60M H.P.F
Q26 Q25
ALC REF (From CPU: Power data)
(MC1350D)
CHK EXC
(To PA/FIL PCB)
(RF DET)
(Power CONT)
Figure 2-10 455 kHz Circuit thru Exciter
The power of this transmitting signal is adjusted in a gain-control IF amplifier U6
(MC1350D), which is controlled by the ALC circuit that consists of U7 and U8.
The input signals to the ALC circuit are the power data (ALC REF) at CPU board and a
feedback signal of transmitting power detected at the output of the FIL section on PA/FIL
board. That is, the transmitter output level of TX/RX board is automatically adjusted so
as to derive the transmitting power set by the power data.
This transmitting signal outputs to PA/FIL board after passing through amplifiers Q26
and Q25.
The transmitting power signal of TX/RX board is detected by CR24, C103, and R112
(self-check signal) and input to CPU as CHK EXC signal.
The transmitting power is self-checked in a condition; 3 MHz, J2B, 1700-Hz tone signal
and a power data of 255.
Reference)
When a transmitting power is 140 W to 150 W, CHK EXC level is 2.4 to 2.6 V.
PA/FIL section
The transmitting output signal from TX/RX board (EXC OUT) inputs to PA/FIL board. It
is passed through a
π
-type attenuater (-1.5 dB) that consists of R14 to R16, amplified by
a two-stage push-pull amplifier, and sent to FIL section. The gain in the PA section is
about 25 dB, and the loss in the FIL section is less than 0.5 dB.
In transmitting, the gain of the PA circuit is controlled by switching ON/OFF the bias
voltage of the two-stage push-pull amplifier in accordance with PA ON signal from CPU.
The bias voltage for Q4 and Q5 is 40 to 60 mV across R69.
For Q6 and Q7, the current flowing through FL3 is adjusted to 500
±
50 mA by R39
[BIAS].
To protect the PA circuit, a thermister RT2 detects the temperature of PA transistors. If it
detects a temperature of 90 to 100
°
C, the power data is automatically decreased to a
lower set value (LOW) and prevents PA transistors from being damaged. When it detects
45
°
C, fan automatically air-cools the PA section.
The voltage at J1-7 (TEMP) is 1 V
±
0.1 V at the normal temperature of 25
°
C, 1.85 V at
45
°
C (FAN ON), and 4.1 V at 90 to 100
°
C (LOW).