Yaesu FT-901DM Maintenance Service Manual page 40

The output from the IF second mixer is fed
through a monolithic filter XF4o92 to the two-stage
IF amplifier consisting of Qio3 and Qaog (both
3SK40M). The output from Q4o3 is coupled to
the rejection tuning circuit which eliminates inter-
ference within the IF passband. This rejection
function is accomplished by varying the resonant
frequency of a crystal with varactor diode Daos
(182209) on the SSB, CW, and FSK modes.
The amplified IF signal is fed to the CARRIER
UNIT. In the AM mode, the signal is fed through
a buffer amplifier Qg,9 (2SCI81SY) to the AM
detector Dao; (1N60). The signal is then fed to
the AF UNIT,
A portion of the output from Q4yo is rectified
by Day, and Dgy2 (1N60) to produce AGC voltage.
The AGC voltage is amplified by DC amplifier
Qaoo (28C373) and fed to the gate of RF amplfier
Qior and to IF amplifiers Qao3 and Qgog to control
the gain of these stages for AGC purposes. The
time constant of the AGC voltage decay is select-
able (FAST/SLOW) by switch S2o93. The AGC
voltage is further amplified by Qos (2SK19GR)
and Q4o7 (2SA5S64A) for signal strength indica-
tion on the front panel S-meter.
TECHNICAL NOTES
The RF GAIN control on the front panel varies
the AGC voltage level, providing manual control
of the gain of the RF and IF stages.
Qaiy (28C373) works as a relay driver for the IF
rejection tuning circuit. Delay transistor Qaj2
(2SC735Y) supplies the voltage to the drain of
Qao: and Qo. when the transceiver returns to the
receive mode after transmission.
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