E.F. Johnson Company 761X, Service Manual

Page: 45 / 99

VHF (7610) CIRCUIT DESCRIPTION
5A-2
November 1998
Part No. 001-7600-001
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.