Icom IC-R20 Service Manual - Page 13

(3) AM MODE The 3rd IF signal is output from the FM IF IC (IC17, pin 3), and passes through the mode switch (D134) and ceramic bandpass filter (FI8). The filtered signal passes through the mode switch (D136), and is then applied to the IF amplifier (Q544). The amplified signal is demodulated at the AM detector (Q545). The AF signals pass through the ANL circuit (D139), and are then applied to the AF switch (IC21, pin 4) via the "AM_DET" line, and are then applied to the AF circuit (LOGIC unit) as "B_DET" signal. 4-1-7 AF AMPLIFIER CIRCUIT (LOGIC UNIT) The AF amplifier circuit amplifies the demodulated AF signals to drive a speaker. • A-BAND CIRCUIT While in FM mode, AF signals from the demodulator circuit (RF unit) pass through the AF filter (Q25), and are then amplified at the AF amplifier (Q23). While in WFM/AM/SSB/CW modes, AF signals from the demodulator circuit (RF unit) bypass the AF filter via the AF filter bypass switch (Q26), and are then amplified at the AF amplifier (Q23). The signals are applied to the electronics volume (IC7, pin 6) to control volume level. The signals output from pin 5, and are applied to the pre-amplifier (Q12). The signals are amplified at the AF amplifier (IC5, pin 1), and then output from pin 6. The signals are applied to the internal speaker which is connected with J8 via the external speaker jack (J3). • B-BAND CIRCUIT While in FM mode, AF signals from the demodulator circuit (RF unit) pass through the AF filter (Q24), and are then amplified at the AF amplifier (Q22). While in WFM/AM modes, AF signals from the demodulator circuit (RF unit) bypass the AF filter via the AF filter bypass switch (Q27), and are then amplified at the AF amplifier (Q22). The signals are applied to the electronics volume (IC7, pin 11) to control volume level after passing through the mute switch (Q19). The signals output from pin 12, and are applied to the pre-amplifier (Q16). The signals are amplified at the AF amplifier (IC5, pin 1), and then output from pin 6. The signals are applied to the internal speaker which is connected with J8 via the external speaker jack (J3). The electronic volume control circuit controls AF gain, therefore, the AF output level is according to the [VOL] setting and also the squelch conditions. 4-1-8 SQUELCH CIRCUIT (LOGIC AND RF UNITS) • NOISE SQUELCH The noise squelch circuit cuts out AF signals when no RF signals are received. By detecting noise components in the AF signals, the squelch circuit switches the AF mute switch. A portion of the "A_NOISE" signals from the FM IF IC (RF unit; IC2, pin 13) are applied to the CPU (LOGIC unit; IC3, pin 5). The CPU analyzes the noise condition and outputs the "AFON" signal (from pin 17) to the AF amplifier's controller (LOGIC unit; Q7). • TONE SQUELCH The tone squelch circuit detects AF signals and opens the squelch only when receiving a signal containing a matching subaudible tone (CTCSS). When tone squelch is in use, and a signal with a mismatched or no subaudible tone is received, the tone squelch circuit mutes the AF signals even when noise squelch is open. • B-BAND 3RD MIXER AND DEMODULATOR CIRCUITS "B_AGC" signal to RF amplifiers D139 ANL "AM_DET" signal to the AF switch AGC Q545 AM DET Q545 IF Q544 C768 C769 R439 8 R438 7 5 R440 C770 Active filter FM detector Noise detector Limiter amp. RSSI 9 10 11 "DET_OUT" signal to the AF switch R443 C772 C765 R437 L105 C880 C764 R436 AM/NFM FI8 WFM LPF 3 Q555 LO 2 REF OSC REF CONT. Q556 X1 D171 19.2 MHz 3rd Noise Mixer comp. IC2 TA31136F 12 13 16 2nd IF (19.65 MHz) from Q5 "B_NOISE" signal to the CPU (LOGIC unit; IC3, pin 4) "B_RSSI" signal to the CPU "B_WFMC" (LOGIC unit; IC3, pin 87) Q543 C767 4 - 5