EFJohnson 761X, 764X service manual Audio Amplifier Circuit, Receiver Mute Circuit

VHF (7610) CIRCUIT DESCRIPTION

Figure 5-1 Second IF System IC3 (VHF)

Figure 5-2 Audio Switching and Amplifier Circuit (VHF)

are part of the quadrature detector. The audio signal is fed out on pin 9 (IC3) and applied to the AF amplifier circuit.

5.1.5 AUDIO AMPLIFIER CIRCUIT

The AF (audio frequency) amplifier circuit pro- vides drive to the speaker (a block diagram is shown in Figure 5-2). The audio signal from IC3, pin 9 is applied to AF amplifier IC7A and then fed to pin 1 of AF switching circuit IC4. The switched output signal on IC4, pin 2 is then applied to high-pass filter IC8A/ B. The cut-off frequency of this filter is controlled by the “AFHPF” line. When “AFHPF” is high, the cut-off frequency is shifted higher to attenuate the Call Guard (CTCSS or DTCS) signals.

The filtered output signal on IC8, pin 7 is applied to limiter IC7b and buffer IC7c. De-emphasis is pro-

vided by R145 and C182. The audio signal is then applied to pin 16 of level controller IC5. This device allows the CPU to control the speaker volume level. The level controlled signal is fed out of IC5 on pin 15 and applied to low-pass filter IC6b. It is then applied to AF switching circuit IC4 on pin 8, and the switched output is on pin 9. This signal is buffered by IC6d and then amplified by AF amplifier IC10 to provide drive to the speaker.

5.1.6 RECEIVER MUTE CIRCUIT

Noise Squelch

The noise squelch circuit turns off the audio sig- nal when no RF signal is being received. The squelch circuit is controlled by the amount of noise present in the audio signal.

November 1998
Part No. 001-7600-001	5A-2