Icom IC-R7O Maintenance Manual page 16

After tuning to 9 MHz by L15, it is fed to the notch filter (composed of L16, L17, D34 and X2).
The range of variation is +1.3 kHz.
2. AF stage
The SSB, CW or RTTY signal fed to IC3 by C91 is product-detected with the BFO signal.
The AM signal is input by C92 to D74 and detected.
IC5 is an IC for the electronic attenuation element; the pin 1 input side is for receiver and the pin 8 input
side is for transmission monitoring. The various operations are selected by 034 and Q35. Tone control is
also accomplished by using the terminal (pin 2) for frequency compensation.
Squelch is also applied by 037 to these two types of AF output.
IC5 output is amplified at C6 and is fed to the speaker.
3. Other circuits
(a) Noise-blanker circuit
The signal converted to the 2nd IF at the 2nd mixer passes through C3, is amplified at FET Q1 (in the first
noise-amplifier stage), and is amplified to a sufficient level by the high-gain IC, IC1.
After the noise signal tuned at L8 is subjected to voltage-doubling rectification at C10, D9 and D10, it is
divided in 2, one part of which passes through R17 and is fed to the Q4 ~ Q6 AGC circuit.
The AGC in this stage is obtained by increasing the voltage applied to pin 3 of IC1. This provides a wide
dynamic range.
For woodpecker noise, etc., the AGC voltage rise time-constant is switched by the NB N/W select switch
on the front panel, thus providing excellent blanking.
Noise signals rectified at D9 and D10 are applied to the Q3 base and are current-amplified; bias current
from R7 to D5 ~ D8 is bypassed at R8. As a result, D5 ~ D8 are turned off and the received signal is not
fed to the following circuit.
(b) Filter-select circuit
This circuit selects the appropriate filter for the operating mode by turning input/output switching
diodes of D39 ~ D46, with mode signals input from J10.
At this time, RTTY is caused, by S2 in the main unit, to pass through FI1 (500 Hz/6 dB) during narrow
shift or Fl2 (2.2 kHz/6 dB) during wide shift. During the FM mode, it does not pass through a filter, but
passes through a by-pass circuit consisting of D19 and D20.
FI3 is 6 kHz/6 dB, and is for the AM mode.
(c) AGC circuit
Signals output from the secondary side of L24 are amplified at Q18 after detection at D73, and then an
AGC voltage is obtained from the collector.
When there are no signals, the AGC voltage is offset, through D72, to the voltage (approx. 4V) set by
R125 and R123.
When there are signals, Q18 is ON, collector voltage (i.e., AGC voltage) is decreased, the 2nd gate
voltage of each FET connected to the AGC line is also decreased, and the gain of each amplifier stage
falls.