455 Khz Circuit; Tx/Rx Section: Agc, Af - Furuno FS-1503 Service Manual

R210 connected to Q39 (RX GAIN) adjusts the receiving gain according to AGC level.
In J3E (4 MHz), the R210 is adjusted so that the S-meter starts to deflect when the gain
volume is at maximum and an antenna input (SSG level) is +10 dBµV. A thermister RT4
compensates the temperature characteristic of the overall receiving gain.
The signal output from Q39 passes through FL5 (455 kHz), and enters AGC circuit and
AM and SSB demodulators (commonly used for transmitting and receiving).
54.455 MHz
CR10
U1
(MC1350D)
2Lo (54 MHz)

TX/RX Section: AGC, AF

In the AGC circuit, the receiving signal (455 kHz) is amplified by Q18, and detected by
CR18 and CR19. C77, R60, and R62 determine the AGC time constant.
C77 and R60 determines the AGC attack-time to be about 10 msec. R61 and R62 deter-
mines AGC decay-time in AGC-FAST to be about 100 msec and R62 determines that in
AGC-SLOW about 3 sec. U21 switches R61ON and OFF according to AGC-FAST and
SLOW, respectively.
U4-1/2, U3-1/2, and U3-2/2 are AGC signal amplifiers. The control signal from the gain
volume* is supplied to the AGC circuit to control the gains of U1 (MC1350D) of the first
IF amplifier and U2 (MC1350D) of the second IF amplifier. The controlling method is
forward AGC.
U4-2/2 amplifies the AGC voltage and detects the receiving signal strength "S". This sig-
nal not only deflects S-meter, but controls the scanning, sweeping, and squelching of the
receiving signal.
In the scanning and sweeping, the AGC is automatically turned ON.
*: The voltage from the gain volume (RF GAIN VR) is read by the CPU, A/D-converted,
and then processed for the volume curve. RF GAIN can also be adjusted by RFxx of MIF
command from a remote terminal. Operation of the RF gain volume cancels the MIF
command.
FL2
CR11
-12 2.4 k
FL3
TP3
0.4 k
FL4
6.0 k
U25
455 kHz
NB
Figure 2-17 455 kHz Circuit
FL5
U2 Q39
(MC1350D) 455 kHz
R210
RT4
RX GAIN
2-23
CR18, 19
U3, 4
RF GAIN VR
AGC OFF/FAST
Q18 AGC
S
CR20, 21, U5
H3E DET
Q19
CR22
Q20
3Lo (456.5 kHz)
AM AF
J3E AF