Kenwood TM-221A Service Manual page 12

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14
TM-221A/E/ES
DESCRIPTION OF ELEMENTS
Element Function
High-pass filter
Limiting
Description
Improves AF frequency characteristics in the receive mode.
Limits the first IF signal.
Reversal current protection
Reversal current protection
Discharge
Turns on the SO circuit and O4 for AL, in transmit mode for muting of the AF fine.
Prevents flow of RF meter current to the microphone check circuit and rectifies the
microphone check output.
For discharging any residual charge on the 8T line.
D5 AVR Zener diode for setting the AVR circuit reference voltage.
D6 Switching characteristic
' D7 VCO output switch
D8 { VCO output switch
Diode to provide rise and fall hysteresis on the LD line.
Reduces the drive circuit load in receive mode.
Reduces the oscillator load in transmit mode.
Dg Temperature compensation
D110 Temperature compensation
Temperature compensation for Q19 (driver).
Temperature compensation for APC circuit.
D11~D13 | Wideband RF amplifier
Varicap-diode of the RF tuning circuit.
S-meter circuit protection
VCO (X58-3090- 00)
Element Function
Protect for S-meter circuit when TX to RX mode.
Description
Ql Amplifier
Operates in all modes to amplify the VCO output to the required level.
Transmit VCO Operates in transmit mode as the PLL VCO (144MHz band).
Transmit VCO switch
Turns on the transmit VCO.
Receive VCO Operates in receive mode as the PLL VCO.
OR circuit
Transmit modulation varactor
Transmit frequency control varactor
D4 Receive frequency contro! varactor
| os Band switch
DRIVE (X59-3120-00)
Element
Function
ORs 8T and 8R to operate Q1 at normal temperature.
Adds FM modulation to TX VCO.
Expands the receive band' by switching for 138 to 152.995MHz (low) and
153 to 173.995MHz (high) signals.
Description
| Switching
Supplies 8V to the drive circuit; switched by Q3.
Switching Supplies 8V to the local oscillator amplifier; switched by Q3/
Switching
ON when the PLL is locked.
High-frequency amplifier
Operates in transmit mode. When checking levels near these transistors, be careful
of the probe ground points. :
Reversal current protection
APC (X59-3130-00)
Element Function
Separates Q1 and Q2.
Description
Q1 Differential amplifier
Q2(1/2) Protection control
Q2(2/2) | APC control
Adjustable with VR5.
Adjustable with VR6.
Q3 Drive stage + B AVR
IF (X59-3140-00)
Function
Second focal oscillator, mixer, 1F ampli-
Description
@ S-meter output. @ Noise amplifier output (first stage).
fier, quadrature detector, noise amplifier
@® Detector output @®) First IF signal input.