Kenwood TK-940 service manual Microphone amplifier

CIRCUIT DESCRIPTION

4. Transmitter System

4-1.Microphone amplifier

The signal f r o m t h e microphone goes t o t h e nxcrophone mute switch (Q201 off). It then passes through the high-pass filter in IC205 (212) and the pre- emphasis/lDC circuit in IC205 (112). (If the option has been installed, the signal is mixed with the encode signal.)

The signal is applied t o the IC212 summing amplifier and mixed with QT and DQT from the CPU (IC209). It then passes through the splatter filter (the fourth low- pass filter) consisting of IC21 1 (112, 2/2), w h i c h removes unwanted harmonics.

The output from the low-pass filter is input to the D l

Aconverter (lC6) t o adjust the modulation.

	0201	IC205 (212)	IC205 (112)	IC212	IC211 (212)	IC211 (112)
					-
From	MIC	HPF + IDC + SUM --B-			LPF	LpF	MO"DIAConverte'
~ i c r o ~ h o n e -	MUTE						IC6

DATA

Fig. 3 Microphone amplification

4-2. Final amplifier 4-3. APC circuit

The signal from the PLL is amplified by t w o power modules (ICI2 and IC14)t o an output level of 15W, and goes through the harmonic filter and antenna switch D20, and on t o the antenna terminal.

IC13 (1/2) compares the DC input t o pin 2 with the reference voltage at pin 3 applied by IC8 (1121, amplifies the result, and controls the DC amplifier (Q22 and Q23) to keep the transmit final current constant, thus keeping the transmit output constant.

The direct current that flows through the final module (lC14) produces a voltage across resistors R108, R109, and R110. This voltage is applied to pin 6 of lC13 (2/2), and is input as the reference voltage difference of pin 5 and amplified.

DRIVE

POWER	D20
AMP
	ANT S W

8T

A A

DRIVER

DRIVER

LPF - -I)). -

I 025 0 2 6

I

I

		I	DC	DC
AMP	DET		SW	- S W	E l C 2 0 9
		I		-

Fig. 4 Transmit power circuit and APC circuit