Apc Circuit; Rf, Alc, Swr Meter Circuits; Frequency Construction; Power Meter - Icom IC-703 Service Manual

When the "FORL" voltage exceeds the "POCV" voltage, ALC
bias voltage from IC2701 (pin 1) controls 1st and 2nd IF
amplifiers (Q831, Q1401). This adjusts the output power to
the level determined by the RF power setting until the
"FORL" and "POCV" voltages are equalized.
In AM mode, IC2701 operates as an averaging ALC amplifi-
er with Q2703. Q2703 turns ON and the "POCV" voltage is
shifted for 4 W AM output power.
The ALC bias voltage from IC2701 is also applied to the
main CPU (IC5901) via the A/D converter (IC5601) as the
"ALCL" voltage for ALC meter indication.
An external ALC input (minus voltage) from the [ACC] sock-
et (pin 6) is shifted to plus voltage at D2781 and is applied
to the buffer amplifier (Q2781). External ALC operation is
identical to that of the internal ALC.
3-2-8 APC CIRCUIT (MAIN UNIT)
The APC (Automatic Power Control) circuit protects the
power amplifiers on the PA unit from high SWR and exces-
sive current for the HF/50 MHz band.

• FREQUENCY CONSTRUCTION

(0.03—60 MHz)
1st IF signal
(64.455 MHz)
Antenna
1st mixer
D701
1st LO signal
(64.4850—124.4550 MHz)
3 dB ATT
LPF
Q811
IF
* 1 Oscillates 64.4850—94.454999 MHz
* 2
Oscillates 94.4550—124.454999 MHz
LO
HPF
Q751
Amp.
Ref Oscillator
IC9521
X9501
PLL IC
The reflected wave signal appears and increases on the
antenna connector when the antenna is mismatched. The
HF/50 MHz reflected signal level is detected at D561 (PA
unit), and is amplified at the APC amplifier (IC2701) and
applied to the ALC circuit as the reference voltage.
For the current APC, the driving current at the power ampli-
fier is detected in the voltages ("IDH" and "IDL") which
appear at both terminals of a 0.012 Ω resistor (R216) on the
PA unit. The detected voltages are applied to the differential
amplifier (IC2701, pins 9, 10). When the current of the power
amplifier exceeds allowed current, IC2701 controls the ALC
line via IC2701 to prevent excessive current flow.

3-2-9 RF, ALC, SWR METER CIRCUITS

(MAIN UNIT)
While transmitting, RF, ALC or SWR meter readings are
available and can be selected with the [MET] switch.

(1) Power meter

The "FORL" voltage is applied to the main CPU (IC5901) via
the A/D converter (IC5601, pin 12) for indicating the output
power.
69.0115 MHz
Q881
(WFM: 70.7 MHz)
Crystal
IF
BPF
LPF
DDS
IC9001 IC9101
Amp.
IC9601 Q9621 Q9651
D9621
(Oscillates 64.0 MHz)
SINGLE LOOP CIRCUIT
3 - 6
2nd IF signal
(455 kHz)
2nd mixer
D901
to each detector circuits
2nd LO signal
(64.0 MHz)
Q9201
D9201
* 1
( for HF bands)
Amp.
Q9301
Q9621
D9621
2
( for 50 MHz band) *
PLL IC
MAIN LOOP CIRCUIT
LPF
ATT
3 dB