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4 - 4

The signal from the AM detector circuit (Q13) is detected at
the AGC detector circuit (D19–D21). When receiving strong
signals, the detected voltage increases and the AGC voltage
decreases via the AGC circuit. The AGC voltage is used for
the bias voltage of the receive switching PIN diodes to atten-
uate the received signals. Therefore, this transceiver keeps
the audio output at a constant level.

4-1-11 S-METER CIRCUIT (MAIN UNIT)

The S-meter circuit indicates the relative received signal
strength while receiving by utilizing the AGC voltage which
changes depending on the received signal strength.

The FM S-meter signal from the FM IF IC (IC8, pin 12) is
applied to the main CPU (IC505, pin 40) as an S-meter sig-
nal via the “RSSI” signal line.

The S-meter signal is applied to the sub CPU and is then
displayed on the LCD.

4-2 TRANSMITTER CIRCUITS

4-2-1 MICROPHONE AMPLIFIER CIRCUIT

(MAIN UNIT)

The microphone amplifier circuit amplifies audio signals
from the microphone to a level needed at the modulation cir-
cuit. The microphone amplifier circuit is commonly used for
both the VHF and UHF bands.

• AUDIO SIGNALS

The AF signals from the microphone are amplified at the
microphone amplifier (Q505, IC506), and are then applied to
the analog switch (IC509, pin 2). The signals from analog
switch (pin 1) are applied to the IDC amplifier (IC3A, pin 3)
via the “MODIN” line, and then pass through the low-pass fil-
ter (IC3D, pins 13, 14) to suppress unwanted components.
IC3A composes limiter amplifier and pre-emphasis circuits.

The signals are amplified at the buffer amplifier (IC3C, pins
9, 8), and are then applied to the D/A converter IC (IC4, pin
13) to adjust modulation level. The signals from the D/A con-
vertor (IC4, pin 23) pass through the reference controller
(D1, X1), and are then applied to the VHF or UHF modula-
tion circuits separately.

• DATA SIGNALS

The data signals from the 6-pins jack (J504, pin 1) pass
through the limiter circuit (D524) to limit the level.
When setting to 9600 bps mode, The signals pass through
the analog switch (IC509, pins 4, 3), and then pass through
the switch again (pins 9, 8). The signals are amplified at the
buffer amplifier (IC3c, pins 9, 8) via the “DATAMOD” line,
and are then applied to the same line as audio signals
(applied to the D/A converter IC’s pin 13).

When setting to 1200 bps mode, The signals pass through
the analog switch (IC509, pins 4, 3), and then pass through
the switch again (pins 10, 11). The signals are then applied
to the same line as “MODIN” (applied to the IDC amplifier;
IC3A, pin 3).

4-2-2 VHF MODULATION CIRCUIT

(MAIN UNIT AND VCO BOARD)

The modulation circuit modulates the VCO oscillating signal
(RF signal) using the microphone audio signals.

The audio or data signals from the reference controller (D1,
X1) are amplified at the buffer amplifier (Q2), and then pass
through the VHF PLL IC (IC1, pins 1,15) and VHF loop filter
(IC6, Q9, Q10, D4). The filtered signals are applied to the V-
VCO circuit (VCO board).

The signals change the reactance of D1 (VCO board) to
modulate the oscillated signal at the V-VCO circuit (VCO
board; Q3, D1, D2). The modulated signals are amplified at
the buffer amplifiers (VCO board; Q4, Q5), and then passed
through the Tx/Rx switching circuit (VCO board; D5) and
low-pass filter (L6, C32, C33). The filtered signals are
applied to the drive amplifier circuit on the MAIN unit.

Analog

Amp

MIC

Antenna

IC506,
Q505

IC509

IC3A

IC3D

IC3C

IC4

Q102,
Q103

IC1