Icom IC-M604A Service Manual - Page 9

SECTION 4 CIRCUIT DESCRIPTION 4-1 RECEIVER CIRCUITS 4-1-1 ANTENNA SWITCHING CIRCUIT (MAIN UNIT) The antenna switching circuit functions as a low-pass filter while receiving and as resonator circuit while transmitting. The circuit does not allow transmit signals to enter the receiver circuits. Received signals enter the MAIN unit from the antenna connector and pass through the low-pass filter (L1, L2). The signals are then applied to the RF circuit via the antenna switching circuit (D1, L3, L4, C7-C9). 4-1-2 RF CIRCUIT (MAIN UNIT) The RF circuit amplifies signals within the range of frequency coverage and filters out-of-band signals. The signals from the antenna switching circuit pass through the tunable bandpass filter (L11, C23-C25) which the object signals are led to each RF amplifier of channel 70 circuit (Q7) or other channels (except channel 70) circuit (Q1). • CHANNEL 70 CIRCUIT The amplified signals from the RF amplifier (Q7) are applied to the 4-stage bandpass filter (L31-L34, C407, C408, C410 -C423) to suppress unwanted signals and improve the selectivity. The signals are then applied to the 1st mixer circuit for channel 70. • OTHER CHANNELS CIRCUIT The amplified signals from the RF amplifier (Q1) are applied to the 4-stage bandpass filter (L12-L15, C33, C34, C36- C48) to suppress unwanted signals and improve the selectivity. The signals are then applied to the 1st mixer circuit for other channels. 4-1-3 1ST MIXER AND 1ST IF CIRCUITS (MAIN UNIT) The 1st mixer circuit converts the received signal into a fixed frequency of the 1st IF signal with a 1st LO (VCO output) frequency. By changing the 1st LO frequency, only the desired frequency will pass through a pair of crystal filters at the next stage of the mixer. • CHANNEL 70 CIRCUIT The signals from the RF circuit are mixed with the 1st LO signals at the 1st mixer circuit (D21, L48, L49) to produce a 21.7 MHz 1st IF signal. The 1st IF signal is amplified at the 1st IF amplifiers (Q8, Q9), and then passes through the pair of crystal bandpass filters (FI4, FI5) to suppress out-of-band signals. The filtered signal is then amplified at the 2nd IF amplifier (Q10), and is then applied to the 2nd mixer circuit (IC6). • OTHER CHANNELS CIRCUIT The signals from the RF circuit are mixed with the 1st LO signals at the 1st mixer circuit (D11, L18, L19) to produce a 30.15 MHz 1st IF signal. The 1st IF signal is amplified at the 1st IF amplifiers (Q3, Q4), and then passes through the crystal bandpass filter (FI1) to suppress out-of-band signals. The filtered signal is then amplified at the 2nd IF amplifier (Q2), and is then applied to the 2nd mixer circuit (IC1). • 1ST MIXER AND 1ST IF CIRCUITS to the 2nd mixer circuit (IC1, pin 16) IF amp. For other channels BPF 1st LO signal RF amp. to the 2nd mixer circuit (IC6, pin 16) IF amp. For channel 70 only BPF 1st LO signal RF amp. 4 - 1 ANT BPF ANT sw. LPF