Yaesu FT-901DM Maintenance Service Manual page 43

TECHNICAL NOTES
AF UNIT (PB-1705)
The audio signal from the AM detector is ampli-
fied by Qso7 (2SC1000GR) and fed to the first
audio amplifier Qs9; (2SC1000GR). On SSB, CW,
FM, and FSK, the audio signal is directly fed to
Qso1 from pin 15. The audio signal is amplified
through Qso; and Qso2 (both 2SC1000GR) and
is then fed through the APF switch and audio
volume control VRsa to the audio output ampli-
fier Qs93 (TA720SAP), which delivers 3 watts
of audio output to the speaker.
The audio spectrum is shaped by an active low-
pass filter of fp = 2.7 kHz — 12 dB/octave.
Operational amplifier Qso, (MC3403) and AF
amplifier Qsos (2SC1000GR) are placed into the
audio circuit by the APF switch on the front
panel. The frequency of this selective amplifier
is varied with the front panel APF control VR,a/b
between 400 Hz and 900 Hz, providing single-
signal, noise-free CW reception. VRgo; provides
adjustment of the selectivity of the APF circuit.
2-16
S22 Dh 7, 19
APF APF vRa(b) RxBV 13.5V