EFJohnson 761X, 764X service manual Second if System IC1 UHF

UHF (7640) CIRCUIT DESCRIPTION

Figure 5-4 Second IF System IC1 (UHF)

Figure 5-5 Audio Switching and Amplifier Circuit (UHF)

and applied to the limiter amplifier and quadrature detector. The quadrature detector demodulates the audio signal contained in the second IF signal. Com- ponents connected to pins 10 and 11 including X1 are part of the quadrature detector. The audio signal is fed out on pin 9 (IC1) and applied to the AF amplifier circuit.

5.5.5 AUDIO AMPLIFIER CIRCUIT

The AF (audio frequency) amplifier circuit pro- vides drive to the speaker (see block diagram in Figure 5-5). The audio signal from IC1, pin 9 is applied to AF amplifier IC7b and then fed to high-pass filter IC6c/ IC6d. 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 IC6d, pin 14 is applied to pin 8 of AF switching circuit IC11. The switched output signal on IC11, pin 9 is then applied to pin 16 of level controller IC12. This device allows the CPU to control the speaker volume level. The vol- ume controlled output on pin 15 is then applied to pin 2 of AF switching circuit IC11. The switched output on pin 1 is applied to pin 4 of de-emphasis circuit IC4a. The signal is then amplified by IC9 to provide drive to the speaker.

5.5.6 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	5B-8