Standard Horizon GX1250SA Service Manual page 6

2.2.4 Second lF Circuit
Refer to Figure 2-3.
The first lF signal through pin 20 of second lF IC QR60 is
applied to the second mixer in QR60. In the second mixer,
the first IF (21.4 MHz) and second local (21.855 MHz from
XP01) signals are mixed to produce a 455 kHz second IF
signal. The signal passes through pin 4 of QR60 and ap-
plied to ceramic filter FR60 to eliminate adjacent signals. It
is then applied to pin 6 of QR60 where it is amplified by a
second IF amplifier and fed to a quadrature detector where
it is converted to an audio signal and output from pin 11.
Figure 2-3 Second IF IC block diagram
2.2.5 Audio Circuit
A portion of the audio signal output from second IF IC QR60
is applied to de-emphasis circuit RR66 and CR65. Owing to
de-emphasis circuit RR66 and CR65 (-6dB / Oct.), the fre-
quency of the audio signal is compensated and then applied
to the AF volume RG02.
The level of the audio signal is adjusted by RG02 and is ap-
plied to the pop noise reduction circuit Q103.
It is applied to pin 1 of the AF power amplifier Q101, and then
output from pin 6. The audio signal passes the external speaker
socket J104 to drive the internal speaker EG30.
2.2.6 Noise Squelch Circuit
A portion of the audio signal from pin 11 of QR60 is applied to
the low pass filter comprised of RR64, RR65, RR63, CR63,
and CR64. The audio signal is then applied to pin 13 of QR60.
The noise component of the signal is output from pin 14 of
QR60. It is converted into DC voltage in noise detector QR61
to produce a squelch signal; the voltage of the squelch signal
is set by the squelch volume RG01 and is applied to pin 15 of
QR60. The squelch signal output from pin 15 passes through
the squelch circuit in QR60 and then output from pin 17.
When the output from pin 17 is "Open", the squelch is oper-
ated (no sound is emitted from the transceiver).
When the output from pin 17 is "Lo", the squelch is turned off,
noise is emitted from the transceiver.
When the squelch is operated, "Open" level from pin 17 is ap-
plied to pin 4 of the microprocessor QL01.
When the squelch is turned off, "Lo" level from pin 17 is also
applied to pin 4 of the microprocessor QL01. There, they con-
trol the scan operation, and applied to the AF power amplifier
via reverse-voltage protection circuit Q102.
5