EFJohnson 764X, 761X service manual UHF Transmitter Circuit, Microphone Amplifier Circuit

UHF (7640) CIRCUIT DESCRIPTION

Part of the AF signal from pin 9 of FM IF circuit IC1 is applied to pin 24 of level controller IC12. This device allows the CPU to control the squelch threshold level. The level-controlled output signal on pin 23 is applied to IC1, pin 8 which is the input of an internal filter (see Figure 5-4). Noise frequencies are passed and then fed out on pin 7 and also applied to an inter- nal noise detector.

The detected noise signals are rectified and fed out on pin 13 without smoothing. The noise signal (NOIS) is then applied to pin 19 of CPU IC1. The CPU analyzes the noise condition and outputs the RMUT signal via I/O expander IC13. This signal con- trols AF mute switch IC11a (see Figure 5-5).

Call Guard (CTCSS/DTCS) Operation

The tone Call Guard squelch circuit detects AF signals and opens the squelch only when receiving a signal containing a matching subaudible tone or code (CTCSS or DTCS). When tone squelch is in use and a signal with a mismatched or no subaudible tone is received, the tone squelch circuit mutes the AF signals even when noise squelch is open.

A portion of the audio signal from amplifier IC7b passes through low-pass filter Q5 in the front unit to remove voice signals. It is then applied to CPU IC1 on pin 97 via the “CTCIN” line. The CPU detects the CTCSS or DTCS signaling and controls the AF mute switch via I/O I/O expander IC13.

5.6 UHF TRANSMITTER CIRCUIT

5.6.1 MICROPHONE AMPLIFIER CIRCUIT

The microphone amplifier circuit amplifies the microphone audio signal to the level required by the modulation circuit and also provides pre-emphasis.

The microphone audio signal is amplified by IC8b and limited by IC8a. Pre-emphasis is provided by R152 and C180. The signal is then fed to splatter filter IC7a and then to pin 11 of switching circuit IC11.

The output signal from IC11 on pin 10 is fed to pin 16 of level controller IC12. This device provides deviation control and is also used in the receive mode to provide volume control. The level controlled output on pin 15 is fed to pin 3 of switching circuit IC11. The switched output (MOD) on pin 4 is then applied to the PLL circuit (D21) where it frequency modulates the transmit signal.

Narrow band/wideband switch Q35 is connected to the input of the splatter filter IC7a and switched by the “NWC” signal from the I/O expander IC13. When “NWC” is high, Q35 shifts the filter cut-off frequency for narrow band deviation selection.

5.6.2 MODULATION CIRCUIT

The microphone audio signal modulates the fre- quency of transmit VCO Q25. The “MOD” signal from switch IC11, pin 4 changes the reactance of diode D21 and this modulates the frequency of trans- mit VCO Q25. The modulated VCO signal is ampli- fied by buffer amplifiers Q28 and Q30 and applied to the transmit driver circuit (Q21).

5.6.3 TRANSMIT DRIVER CIRCUIT

The transmit driver circuit amplifies the VCO oscillating signal to the level required by the power amplifier module. The RF signal from buffer amplifier Q30 passes through the T/R switch D18 and is ampli- fied by the buffer amplifiers Q21, Q20, and driver Q19. The signal is then applied to the power amplifier module IC5.

5.6.4 POWER AMPLIFIER MODULE

The power amplifier module amplifies the driver signal to the output power level. The RF signal from the driver Q19 is applied to the power module which produces 35 watts of output power at the antenna jack.

The amplified signal is fed through antenna switching circuit D3, a low-pass filter and APC detec- tor, and is then applied to the antenna jack. The collec- tor voltage of driver Q19 and the control voltage on pin 2 of the power module IC5 come from APC tran- sistors Q17 and Q18. Transmit switch Q16 turns off Q17 and Q18 when the transmitter is disabled.

	November 1998
5B-9	Part No. 001-7600-001