Icom IC-V8000 service manual Transmitter Circuits

The AF signals are applied to the AF power amplifier circuit (IC9, pin 1) to obtain the specified audio level. The amplified AF signals, output from pin 4, are applied to the internal speaker (CHASSIS UNIT; SP1) via the speaker jack (J6) when no plug is connected to the external speaker jack (J1).

4-1-6 SQUELCH CIRCUIT (MAIN AND LOGIC UNITS)

A squelch circuit cuts out AF signals when no RF signals are received. By detecting noise components in the AF signals, the squelch switches the analog swtich.

A portion of the AF signals from the FM IF IC (IC4, pin 9) are applied to the D/A converter (IC5, pin 13) as the DETO sig- nal. The signals from the D/A converter (IC5, pin 14) are applied to the FM IF IC active filter section (IC4, pin 8) where noise components are amplified and detected with an inter- nal noise detector via the SQLIN line.

The trigger circuit converts the detected signals to a HIGH or LOW signal and applies this (from pin 13) to the CPU (LOGIC UNIT; IC7, pin 19) as the NOIS signal.The CPU controls the analog swtich IC (LOGIC UNIT; IC6) via the expander IC (LOGIC UNIT; IC8). When the CPU receives a HIGH level NOIS signal, the CPU controls the RMUT line to cut the AF signals at the analog swtich IC (LOGIC UNIT; IC6). At the same time, the AFON line controls the AF mute circuit (Q29) to cut out the VOLOUT signal for the AF power amplifier (IC9).

4-2 TRANSMITTER CIRCUITS

4-2-1 MICROPHONE AMPLIFIER CIRCUIT (LOGIC AND MAIN UNITS)

The microphone amplifier circuit amplifies audio signals with +6 dB/octave pre-emphasis from the microphone to a level needed for the modulation circuit.

The AF signals from the microphone pass through the MIC switch (IC2, D4) and high-pass filter (IC3a, pin 2), and are then applied to the microphone amplifier circuit (IC3d, pin

12)via the R39 and C47 for +6 dB/octave pre-emphasis. The amplified AF signals are applied to the analog swtich (IC6, pin 4), and are then applied to the D/A converter (MAIN UNIT; IC5, pin 1) via the MODIN signal. The AF signals are applied to the modulator circuit via the MOD signal.

4-2-2 MODULATION CIRCUIT (MAIN UNIT)

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

The audio signals (SHIFT) change the reactance of D2 to modulate an oscillated signal at the VCO (Q6, D4). The oscillated signal is amplified at the LO (Q9) and buffer (Q11) amplifiers, then applied to the TX/RX switch circuit (D6, D7).

4-2-3 DRIVE/POWER AMPLIFIER CIRCUITS (MAIN UNIT)

The signal from the VCO circuit passes through the TX/RX swtiching circuit (D6) and is amplified at the pre-drive (Q17), drive (Q18, Q20) and power (Q23) amplifiers to obtain 75 W of RF power (at 13.8 V DC/typical). The amplified signal passes through the low-pass filter (L37, L39, C151, C156, C159, C167, C176, C227–C229), and then applied to the antenna swtiching circuit (D12). The signal is applied to the antenna connector (CHASSIS UNIT; J1) after being passed through the low-pass filter (L44, L47, L48, L51, C190, C197, C203, C208, C210, C217, C218).

The bias current of the drive (Q18, Q20) and power (Q23) amplifiers is controlled by the APC circuit to stabilize the out- put power.

4-2-4 APC CIRCUIT (MAIN UNIT)

The APC (Automatic Power Control) circuit (IC6a, Q26) pro- tects drive and power amplifiers from excessive currents and selects HIGH or LOW output power.

The output voltage from the power detector circuit (D14, D17) is applied to the differential amplifier (IC6a, pin 2), and the “T3” signal from the D/A converter (IC5, pin 23) is applied to the other input for reference.

When the driving current increases, the input voltage of the differential amplifier (IC6a, pin 2) will be increased. In such cases, the differential amplifier output voltage (pin 1) is decreased to reduce the drive current.

Q26 is controlled by the TXC signal from the expander IC (IC2, pin 14) to select HIGH or LOW output power.

• APC CIRCUIT

HV
SWHV
RF signal from	Q18	Q20	Q23	ANT
	Driver	Driver	Power
PLL IC (IC1)				SW
	amp.	amp.	amp.

+5V
TXC	2
		1
Q26	IC6a
T1	3
	+

APC control circuit

L44, C190, C191, C196, C197

LPF to antenna

D14 D12

Power detector circuit (D12, D14)

4 - 2