KENWOOD TK980
Service Manual

TK-980/981
CIRCUIT DESCRIPTION
Transmitter System
s Outline
The transmitter circuit amplifies the desired frequency. It FM-modulates the carrier signal by means of a varicap diode. line goes low, the KEY line goes high and Q13 turns on. Q12 turns on and a voltage is applied to 8T. The CPU in the TX-RX unit (B/2) monitors the PLL (IC300) LD line directly. When the PLL is unlocked during transmission, the PLL LD line goes low. The CPU detects this signal and makes the KEY line low. When the KEY line goes low, no voltage is applied to 8T, and no signal is transmitted.
8C 8R Q11 SW Q10 SW TR IC300 PLL LD IC511 CPU IC508 SHIFT REG. Q12 SW Q13 SW KEY 8T

s Younger-stage circuit
The signal output from the VCO is amplified by a buffer amplifier (Q7) and goes to the drive block. The youngerstage circuit provides a stable drive output without a need for adjustment. The APC circuit controls voltage in the younger final stage (Q204).

PLL lock : LD �H�

s VCO/PLL Circuit
The TK-980/981 has a common VCO for the transmitter and the receiver in a sub-unit (A1). It is housed in a solid shielded case and connected to the TX-RX unit (A/2) through CN101. A filtered low-noise power supply is used for the VCO and varicap diodes. The VCO is described below. It is designed so that Q101 turns on with a prescribed frequency when a reverse bias is applied to D100 and D101 by using the control voltage (CV) through CN101. The control voltage is changed by turning the trimmer capacitor (TC100). The output from Q103 is applied to the buffer amplifier (Q102) the output from Q102 is applied to the doubler (Q100) to generate a VCO output signal. This signal is used as a drive input signal or a local signal of the first mixer. Since a signal output from Q100 is input to the PLL IC, it passes through CN101 and buffer amplifier (Q300) and goes to the PLL IC (IC300). The modulation signal from CN101 is applied to D102 and passes through C106 and C107 to modulate the carrier. The PLL IC uses a fractional N type synthesizer to improve the C/N ratio and lock-up speed. The VCO output signal input to the pin 5 of the PLL IC is divided to produce a comparison frequency according to a channel step. This signal is compared with the reference frequency which is output from the VCXO (X1). VCXO provides 16.8MHz, 1.5ppm (�30 to +60°C) and guarantees stable performance when the temperature changes. The output signal from the phase comparator passes through a charge pump and an external active LPF (Q301, Q302) in the PLL IC to generate a DC VCO control voltage CV. Serial data (DT, CK, EP) are output from the CPU (IC511) and shift register (IC8) in the TX-RX unit (B/ 2) to control the PLL IC. The PLL lock status is always monitored by the CPU.

TX-RX UNIT (B/2)

Fig. 6

Unlock circuit

s Power Amplifier Circuit/Final
The transmit output signal from the VCO is amplified to a specified level of the power module (IC400) by the drive block (Q202, Q204). The amplified signal goes to a low-pass filter. The low-pass filter removes unwanted high-frequency harmonics. The resulting signal passes through the transmission/reception selection diode (D208), then goes to the antenna terminal.

s APC Circuit
The direct current that flows through the final module (IC400) produces a voltage across resistors R127. This voltage is applied to pin 6 of IC13 (2/2), and is input as the reference voltage difference of pin 5 and amplified.
IC400 Q204 8T POWER AMP Q19 APC DRIVER Q17 PRI DRIVER B R127 LPF D20 ANT SW ANT

IC13 Q20 Q21 DC SW KEY IC508 CURR. DET DC SW

s Unlock Circuit
During reception, the TR line goes high, the KEY line goes low, and Q10 turns on. Q11 turns on and a voltage is applied to the collector (8R). During transmission, the TR
MIC IC711 AF AMP TA75S01F IC504 AF AMP, IDC, LPF TC35453F IC511 CPU 30622M8A -4F9GP IC3 SUM AMP TA75W558FU Q101 VCO 2SK508NV (K52) IC300 PLL SA7025DK Q102 RF AMP 2SC4226 (R24) Q300 BUFFER 2SC4226 (R24)

IC6 PC (1/2)

DC AMP

Fig. 7 APC circuit
ANT Q7 BUFFER 2SC4226 (R24) Q202 RF AMP 2SC3356 (R24) Q204 RF AMP 2SK2596 IC400 POWER AMP M67760LC : TK-980 M67760HC : TK-981

MIC KEY INPUT

X1 VCXO 16.8MHz

26

Fig. 5 Transmitter system