Alinco DR-620, Service Manual, Page: 7 / 56

8) Transmit Signal Path
The speech signal from the microphone passes through the MIC jack CN601 to AF amplifier IC601
(M5218FP) on the FRONT UNT. The amplified speech signal is subjected to amplitude limiting by
IC601 (M5218FP), then passes through the Front interface jacks CN602 and CN2 to MAIN Unit. On
the MAIN UNIT, the speech signal passes through the audio mute switch IC7 (TC4066F), MIC gain
control VR5 and buffer amplifier IC1 (NJM2902V-C) and a low-pass filter network at IC1 (NJM2902V-
A) to deviation control VR3 (for VHF Tx audio) or VR4 (for UHF Tx audio).
9) VHF Transmit Signal Path
The adjusted speech signal from VR3 is delivered to VHF VCO Q9, which frequency modulates the
transmitting VCO D6 (1SV278).
The modulated transmit signal passes through buffer amplifier Q7 (2SC5066), a low-pass filter network,
and another buffer amplifier Q3 (2SC5226) to another low-pass filter network.
The filtered transmit signal is applied to the Pre-Drive amplifier Q2 (2SK3074) and Drive amplifier Q1
(2SK2975), then finally is amplified by Power amplifier Q4 (RD70HV1) up to 50 Watts. This three
stage power amplifier’s gain is controlled by the APC circuit.
The 50-Watt RF signal passes through a low-pass filter network, antenna switch D1 (XB15A407), and
another low-pass filter network, and then is delivered to the ANT jack.
10) UHF Transmit Signal Path
The adjusted speech signal from VR4 is delivered to UHF VCO Q29 which frequency modulates the
transmitting VCO D35 (1SV278).
The modulated transmit signal passes through buffer amplifiers Q28 (2SC5066) and Q7 (2SC5226) to
a high-pas filter network.
The filtered transmit signal is applied to the Pre-Drive amplifier Q2 (2SK3074) and Drive amplifier Q1
(2SK2975), then finally is amplified by Power amplifier Q4 (RD70HV1) up to 35 Watts. This three
stage power amplifier’s gain is controlled by the APC circuit.
The 35-Watt RF signal passes through a high-pass filter network, antenna switch D12 and D13
(UM9401F), low-pass filter and high-pass filter networks, and then is delivered to the ANT jack.
11) VHF Tx APC Circuit
A portion of the power amplifier output is rectified by D8 (MA4S713), D9 (MA4S713) and Q12 (2SC4081),
then delivered to APC IC1 (NJM2902V-D) as a DC voltage which is proportional to the output level of
the power amplifier.
The APC IC1 compares the rectified DC voltage from the power amplifier and the reference voltage
from the main CPU IC19, producing a control voltage for the Automatic Power Controller Q8 (RN2107)
and Q11 (RN1107) which regulates supply voltage to the Pre-Drive amplifier Q2, Drive amplifier Q1,
and Power amplifier Q4, so as to maintain stable output power under varying antenna loading conditions.
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