Icom ID-800H Service Manual Download Page 15

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When setting to 1200 bps mode, the signals pass through
the analog switch (IC509, pins 3, 4), and then pass through
the switch again (pins 10, 11). The switched signals are
applied to the IDC amplifier (IC3A, pins 1, 3) and then
passed through the low-pass filter (IC3D, pins 13, 14) to
suppress unwanted components. The filtered signals from
the low-pass filter (IC3D, pin 14) are amplified at the buffer
amplifier (IC3C, pins 8, 9) and then passed through the D/A
converter (IC4, pins 13, 14) .

The signals from the D/A convertor (IC4, pin 13) are applied
to the VHF or UHF modulation circuits separately via the
analog switch (IC517, pins 1, 6).

4-2-2 CODEC CIRCUIT (CODEC BOARD)

The analog AF signals “DAFIN” from the buffer amplifier
(MAIN unit; IC3, pin 8) via the J301 (pin 4) are amplified at
the buffer amplifier (IC251, pins 6, 7). The amplified signals
are applied to the linear CODEC IC (IC50, pin 5) to convert
into 32 bits digital voice data at the A/D converter section as
the “ADIN” signals. The converted digital signals are applied
to the AMBE voice CODEC IC (IC151) to process code
compression, and are then applied to the CODEC CPU
(IC204). The digital signals from the CODEC CPU convert to
the GMSK base band signal at the GMSK modem (IC252),
and are then amplified at the buffer amplifier (IC253, pin 6, 7).
The amplified signals are applied to the amplifier (MAIN unit;
IC3, pin 15) via the J301 (pin 3).

The amplified signals from the CODEC board are applied
to the D/A converter (IC4, pins 15, 16) to adjust modulation
level and are then applied to the VHF or UHF modulation
circuits separately via the analog switch (IC517, pins 1, 7).

4-2-3 VHF MODULATION CIRCUIT (VCO BOARD)

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

The analog or digital audio signals from the analog switch
(MAIN unit: IC517, pin 1) change the reactance of D1 to
modulate the oscillated signal at the V-VCO circuit (Q3, D1
–D3). The modulated signal is amplified at the buffer ampli-
fiers (Q4, Q5), and then passed through the Tx/Rx switch
(D5) and low-pass filter (L6, C32, C33). The filtered signal is
applied to the drive amplifier circuit on the MAIN unit.

4-2-4 VHF DRIVE AMPLIFIER CIRCUIT (MAIN UNIT)

The drive amplifier circuit amplifies the VCO oscillating sig-
nal to a level needed the power amplifier.

The output signal from VCO board pass through the
attenuator (R133, R134, R137), and is then amplified at the
pre-drive (Q30) and drive (Q33) amplifiers. The amplified
signal is then applied to the VHF power amplifier (IC13).

4-2-5 VHF POWER AMPLIFIER CIRCUIT (MAIN UNIT)

The power amplifier circuit amplifies the driver signal to an
output power level.

IC13 is a power module which has amplification output
capabilities of about 55 W. The RF signal from the drive
amplifier (Q33) is applied to IC13 (pin 1).

The amplified signal from the power amplifier (IC13, pin
4) passes through the low-pass filter (L22, C183, C189)
and power detector (D27, D33), antenna switch (D45) and
another low-pass filter (L45, L46, L49, C282, C285, C289).

4 - 5

Analog

Amp

MIC

Antenna

IC506,
Q505

IC509

IC3A

D45

D40

D28, D34

D27, D33