KENWOOD TK285N
Service Manual

TK-285/(N)
CIRCUIT DESCRIPTION
3-3. Audio amplifier circuit
The demodulated signal from IC12 goes through the mute switch (Q15) and is amplified by IC4 (2/2), high-pass filtered, low-pass filtered, high-pass filtered, band-eliminate filtered, and de-emphasized by IC13. The signal then goes through an AF amplifier IC7 (2/2), an electronic volume control (IC8), and an AF switch (Q310 is on), and is routed to audio power amplifier (IC300), where it is amplified and output to the internal speaker. The audio mute signal (AM) from the shift register becomes Low in the standby and Q304, Q305 which are power supply circuit for IC300 turn off. Also, IC13 is set to the power down mode according to data from microprocessor, and the AF signal is muted. When the audio is output, AM becomes High to turn Q304, Q305 ON, and voltage is supplied to power terminal VP of IC300. Also, IC13 is canceled out of the power down mode. The speaker is switched by the logic of speaker switching terminal SSW on the universal connector. When SP-MIC is not attached, the logic of SSW becomes High and SW (Q310) is turned ON, and the AF signal is input to both amplifiers of IC300. When SP-MIC is attached, SSW is connected to GND at inside of SP-MIC. For this reason, Q310 is turned OFF, and the AF signal is input only to amplifier for EXT SP of IC300. Change of INT/EXT SP refer to Fig. 3. AM H H L L
SB Q305

3-4. Squelch circuit
The output from IC12 enters FM IC again, then passed through a band-pass filter. The noise component output from IC12 is amplified by Q4 and rectified by D4 to produce a DC voltage corresponding to the noise level. The DC voltage is sent to the analog port of the CPU (IC19). And IC12 outputs a DC voltage (RSSI) corresponding to the input of the IF amplifier. The CPU reads the RSSI signal via pin 93. IC19 determines whether to output sounds from the speaker by comparing the input voltage of pin 91 and pin 93 with the preset value.
IC12 : FM IF IC IF AMP DET Q4 NOISE AMP D4 DET 93

91 IC19 CPU

RSSI 12

BPF AMP

7

Fig. 4 Squelch circuit

SQ voltage

T SQ close T T T T

SSW H L H L

VC1 H L L L

VC2 L H L L

SP INT EXT MUTE MUTE

Preset Value

SQ open

ANT input level
RSSI voltage

ANT input level

Fig. 5 Squelch and RSSI voltage vs ANT input level

4. Transmitter System
4-1. Microphone amplifier
VP 5 Q304

The signal from the internal microphone goes through the mute switch (Q300). When the SP-MIC is not attached, the microphone switching

AF AM

SW VC1 2

INT.SP

terminal (MSW) on the universal connector becomes High, and mute switch (Q300) is turned ON. When the SP-MIC is attached, MSW is connected to GND at inside of SP-MIC. For this reason, Q300 is turned OFF, the internal microphone is muted, and only the input of the external microphone is supplied to the microphone amplifier of the TX-RX unit. The signal from microphone passes through the limitter circuit in D8, and Mic mute switch (Q17 is off in TX) and through the low-pass filter (IC25:1/2), the high-pass filter, the ALC circuit, the low-pass filter, the high-pass filter, and preemphasis/IDC circuit in IC13. When encoding DTMF, mute switch (Q13) is turned OFF for muting the microphone input signal. The signal passes through the D/A converter (IC8) for the maximum deviation adjustment, and enters the summing amplifier consisting of IC7 (1/2), and is mixed with the low speed data from the CPU (IC19) and 9600bps DATA from Optional Board Terminal. 7
S-Meter level

EXT.SP Q308 VC2 8 IC300 Q301 SSW

Fig. 3 Audio amplifier circuit