Icom ID-800H Service Manual page 16

The filtered signal is passed through the low-pass filter (L51,
L52, L56, C295, C299) to suppress unwanted signals, and
is then applied to the antenna connector (CHASSIS unit J2).
4-2-6 VHF APC CIRCUIT (MAIN UNIT)
The APC circuit stabilizes transmit output power and selects
output power of HIGH, MIDDLE and LOW.
The power detector circuit (D27, D33, L29) detects transmit
power output level and converts it into DC voltage.
The detected voltage is applied to the APC amplifier (IC15,
pin 6) and is compared with the reference voltage which is
supplied from the main CPU (IC505) via the D/A converter
(IC11, pin 8) as the "PCON_V" signal.
The output voltage from the APC amplifier (IC15, pin 7) con-
trols the bias voltage of the power amplifier (IC13) for stabi-
lize the transmit output power.
4-2-7 UHF 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 D100 to
modulate the oscillated signal at the U-VCO circuit (Q101,
D101, D102). The modulated signal is amplified at the buf-
fer amplifiers (Q102, Q103), and then passed through the
Tx/Rx switch (D103) and low-pass filter (L104, C122, C124).
The filtered signal is applied to the drive amplifier circuit on
the MAIN unit.
• APC CIRCUIT
VHF power
From the VHF
drive amplifier (Q33)
From the UHF
drive amplifier (Q34)
UHF power
IC13
amplifier
HV
APC amplifier
7
1
HV
amplifier
IC14
4-2-8 UHF DRIVE AMPLIFIER CIRCUIT (MAIN UNIT)
The output signal from VCO board passes through the high-
pass filter (L69, C119) and attenuator (R135, R136, R138),
and is then amplified at the pre-drive (Q31) and drive (Q34,
D14) amplifiers. The amplified signal is then applied to the
RF power amplifier (IC14).
4-2-9 UHF POWER AMPLIFIER CIRCUIT
(MAIN UNIT)
IC14 is a power module which has amplification output
capabilities of about 50 W. The RF signal from the drive
amplifier (Q34) is applied to the power amplifier (IC14 pin 1).
The amplified signal from the power amplifier (IC14, pin 4)
passes through the low-pass filter (L23, C184, C190) and
power detector (D28, D34), antenna switch (D40) and high-
pass filter (L47, L50, C284, C288, C292).
The filtered signal is passed through the low-pass filter (L51,
L52, L56, C295, C299) to suppress unwanted signals, and
is then applied to the antenna connector (CHASSIS unit J2).
4-2-10 UHF APC CIRCUIT (MAIN UNIT)
The power detector circuit (D28, D34, L30) detects transmit
power output level and converts it into DC voltage.
The detected voltage is applied to the APC amplifier (IC15,
pin 2) and is compared with the reference voltage which is
supplied from the main CPU (IC505) via the D/A converter
(IC11, pin 8) as the "PCON_U" signal.
The output voltage from the APC amplifier (IC15, pin 1) con-
trols the bias voltage of the power amplifier (IC14) for stabi-
lize the transmit output power.
VHF power detector
L29
LPF
D27
(IC15)
6
5
3
2
D28
LPF
L30
UHF power detector
4 - 6
to VHF TX/RX switch
D33
"PCON_V" signal from
D/A convertor (IC11, pin 8)
"PCON_U" signal from
D/A convertor (IC11, pin 9)
D34
to UHF TX/RX switch