Icom IC-M604 Service Manual

Also see for IC-M604: Instruction manual Instruction manual Instruction manual Service manual Instruction manual

Contents

4-1 RECEIVER CIRCUITS4-1-1 ANTENNA SWITCHING CIRCUIT (MAIN UNIT)The antenna switching circuit functions as a low-pass filterwhile receiving and as resonator circuit while transmitting.The circuit does not allow transmit signals to enter thereceiver circuits.Received signals enter the MAIN unit from the antenna con-nector and pass through the low-pass filter (L1, L2). Thesignals are then applied to the RF circuit via the antennaswitching circuit (D1, L3, L4, C7–C9).4-1-2 RF CIRCUIT (MAIN UNIT)The RF circuit amplifies signals within the range of frequen-cy coverage and filters out-of-band signals.The signals from the antenna switching circuit pass throughthe tunable bandpass filter (L11, C23–C25) which the objectsignals are led to each RF amplifier of channel 70 circuit (Q7)or other channels (except channel 70) circuit (Q1).• CHANNEL 70 CIRCUITThe amplified signals from the RF amplifier (Q7) are appliedto the 4-stage bandpass filter (L31–L34, C407, C408, C410–C423) to suppress unwanted signals and improve theselectivity. The signals are then applied to the 1st mixer cir-cuit for channel 70.• OTHER CHANNELS CIRCUITThe amplified signals from the RF amplifier (Q1) are appliedto the 4-stage bandpass filter (L12–L15, C33, C34, C36–C48) to suppress unwanted signals and improve the selec-tivity. The signals are then applied to the 1st mixer circuit forother channels.IFamp.For other channelsto the 2nd mixer circuit(IC1, pin 16) BPFRFamp.IFamp.RFamp.BPF LPFANTANTsw.BPF1st LO signal1st LO signalto the 2nd mixer circuit(IC6, pin 16)For channel 70 only• 1ST MIXER AND 1ST IF CIRCUITSSECTION 4 CIRCUIT DESCRIPTION4-1-3 1ST MIXER AND 1ST IF CIRCUITS(MAIN UNIT)The 1st mixer circuit converts the received signal into afixed frequency of the 1st IF signal with a 1st LO (VCO out-put) frequency. By changing the 1st LO frequency, only thedesired frequency will pass through a pair of crystal filters atthe next stage of the mixer.• CHANNEL 70 CIRCUITThe signals from the RF circuit are mixed with the 1st LOsignals at the 1st mixer circuit (D21, L48, L49) to produce a21.7 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 bandpassfilters (FI4, FI5) to suppress out-of-band signals. The filteredsignal is then amplified at the 2nd IF amplifier (Q10), and isthen applied to the 2nd mixer circuit (IC6).• OTHER CHANNELS CIRCUITThe signals from the RF circuit are mixed with the 1st LOsignals at the 1st mixer circuit (D11, L18, L19) to produce a30.15 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 signalis then amplified at the 2nd IF amplifier (Q2), and is thenapplied to the 2nd mixer circuit (IC1).4 - 1