Kenwood TH-D74E Service Manual page 9

2.1.4.3 Thermal protection circuit
The sub MPU (IC661) monitors the detected voltage (FTH) of the thermistor (TH1) arranged for the prevention of the thermal
breakdown of the final amplifier (Q2) in use. An APC voltage (OAGCA/APC) change will occur when the temperature exceeds the
preset temperature, at which time the TX power will be controlled to prevent excessive heat generation.
D36
RF SW
430T
Q361
From
VCO B
144T
RF SW
VHF LPF
D35
IC702 IC671
AGCA/APC
D/A
ch1,ch6
MAIN
IC725
DET1
MPU
A/D
CURR
+B1
IC3,Q12
+
R55,R57
-
To
FINAL AMP
DRAIN
2.1.5 VCO-PLL Circuit
2.1.5.1 Oscillator circuit
A standard reference frequency of 19.2 MHz is generated from the TCXO (X362) of the reference oscillation circuit is divided in the
PLL IC (IC364) to provide a comparison frequency of 800 kHz. The reference frequency tripled to 57.600 MHz in the RF amplifier block
(Q382 and Q378) is divided in the PLL IC (IC284) to provide a comparison frequency of 480 kHz.
Furthermore, a reference frequency of 57.600 MHz is used as the second local oscillation frequency of both A-band and B-band.
The A-band VCO signal is generated and amplified by the oscillation amplifier (Q386) and sent to the F-in amplifier (Q384) and F-out
amplifier (Q381) through the buffer amplifier (Q383). The B-band VCO signal is generated and amplified by the two oscillation
amplifiers (Q368 and Q369) and sent to the F-in amplifier (Q364) and F-out amplifier (Q365) through the buffer amplifier (Q389).
The signal will be branched directly to the transmission side and the reception side if the signal is not divided after the signal passes
through the F-out amplifier. If the signal is split, the signal will be branched to the transmission side and reception side through the
frequency divider (IC361) and amplifier (Q363). The signal on the reception side is provided to the mixer (IC283). The signal on the
transmission side is provided to the pre-drive (IC1) after the signal passes through the amplifier (Q361). The reference oscillator
modulates the signal at the time of transmission.
2.1.5.2 Phase comparator
These PLL ICs of fractional-N type divide the input oscillation frequency and reference frequency according to PLL data from the sub
MPU (IC661) and make phase comparison to achieve the PLL synthesizer according to the desired step frequency.
2.1.5.3 Lock voltage
Pulse signals obtained by phase comparison in the respective PLL ICs (IC284 and IC364) are swept output from the built-in charge
pumps and used as the control voltages of the respective VCOs for oscillation frequency control after the loop filters eliminate the
ripples in the signals.
D33
RF SW
PRE DRV AMP DRIVE AMP
IC1
Q6
RF SW
D32
IC287
OAGCA/APC
Q8
APCSW
DCSW
Q7
DCSW 45TXS
Fig.6 Transmitter circuit
FINAL AMP
D31
Q2
RF SW MATCH
430T
144T
RF SW
RF SW MATCH
D26
D22
DCSW Q5
FTH
SUB
TH
MPU
IC661
TH1
430T D2,D3
UHF UHF
D18,D19
LPF HPF
D42,D43
MATCH RF SW
RX
D11,D12 D8,D9
RF SW RF SW
ATT
RF SW
MATCH
D4,D5 VHF 220
RX
D13,D14
LPF LPF
D45 144T
220
HPF
RX
D10,D17
DET1
Voltage
shift
(No.RA044<Rev.001>)1-9
ANT
UHF
LPF