Icom IC-R20 Service Manual - Page 14

(1) A-BAND CIRCUIT A portion of the AF signals from the FM IF IC (RF unit; IC2, pin 9) passes through the low-pass filter (LOGIC unit; C161, C164, R169 and R175) via the "A_DET" line to remove AF (voice) signals and is applied to the CTCSS decoder inside the CPU (LOGIC unit; IC3, pin 89) via the "A_CTONE" line to control the AF mute switch. (2) B-BAND CIRCUIT A portion of the AF signals from the FM IF IC (RF unit; IC17, pin 9) passes through the low-pass filter (LOGIC unit; C160, C163, R168 and R174) via the "B_DET" line to remove AF (voice) signals and is applied to the CTCSS decoder inside the CPU (LOGIC unit; IC3, pin 90) via the "B_CTONE" line to control the AF mute switch. 4-1-9 AGC CIRCUIT (RF UNIT) The AGC (Automatic Gain Control) circuit reduces IF and RF amplifiers gain to keep the audio output at a constant level. The receiver gain is determined by the voltage on the AGC line (A-BAND circuit is Q4's collector, B-BAND circuit is Q545's collector). • A-BAND CIRCUIT A part of the 3rd IF signal from the mode switch (D111-D113) is amplified at the IF amplifier (Q1), and is then applied to the AM detector. (Q4, pin 5). The signal is output from pin 1, and is then applied to the AGC circuit of the same one (pin 3). The signal is output from pin 4 as "A-AGC" signal, and is then applied to the 3rd IF amplifier (Q1), 2nd IF amplifier (Q5) and RF amplifiers (Q14, Q35, Q36 and Q505). • B-BAND CIRCUIT A part of the 3rd IF signal from the mode switch (D136 and D137) is amplified at the IF amplifier (Q544), and is then applied to the AM detector. (Q545, pin 5). The signal is output from pin 1, and is then applied to the AGC circuit of the same one (pin 3). The signal is output from pin 4 as "B-AGC" signal, and is then applied to the 3rd IF amplifier (Q544), 2nd IF amplifier (Q542) and RF amplifiers (Q24, Q25, Q548 and Q549). The A-AGC or B-AGC voltage is used for the the bias voltage of the IF and RF amplifiers. When receiving strong signals, the detected voltage increases and the AGC voltage decreases. Therefore, the receiver obtain stable receiver gain. 4-1-10 IC RECORDER CIRCUIT (LOGIC UNIT) The IC RECORDER circuit is composed of RECORDER IC (IC7), sub CPU (IC2), flash memory (IC6), USB interface (IC3), etc. (1) IN CASE OF RECORDING • A-BAND CIRCUIT The AF signals from the AF amplifier (Q23) are applied to the REC board via the HJ8, pin 2 as "A_AIN" signal. The signals are applied to the RECORDING IC (IC7, pin 25) to compress the voice signals, and are then converted to digital signals. The signals are applied to the sub CPU (IC2), and are then stored in the flash memory (IC6). • B-BAND CIRCUIT The AF signals from the AF amplifier (Q22) are applied to the REC board via the HJ8, pin 3 as "B_AIN" signal. The signals pass through the AF mute switch (IC8, pins 1 and 2), and are then applied to the RECORDING IC (IC7, pin 20) to compress the voice signals, and are then converted to digital signals. The signals are applied to the sub CPU (IC2), and are then stored in the flash memory (IC6). (2) IN CASE OF PLAYBACK The stored RECORDING data in the flash memory (IC6) are applied to the sub CPU (IC2), and are then applied to the RECORDING IC (IC7). The signals are converted to analog signals, and are then expanded into the AF voice signals. The AF signals output from RECORDING IC (IC7, pin 25). The signals are applied to the LOGIC unit via the J6, pin 3 as "AOUT" signal, and are then applied to the electronics volume (IC7, pin 11) to control volume level. 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). • AF AMPLIFIER AND IC RECORDER CIRCUITS EXT. SP. J3 AF amp. Pre. amp. Q12 INT. SP. J8 IC5 VR Pre. amp. IC7 Q16 SW Q19 Playing line Sub CPU IC2 Memory IC6 Rec. IC IC7 AF Filter Amplifier A_DET Q23, Q25, Q26 AF Filter Amplifier B_DET Q22, Q24, Q27 Recoding line USB Interface IC3 REC UNIT 4 - 6