Yaesu FT-901DM Maintenance Service Manual page 45

TECHNICAL NOTES
TRANSMIT CIRCUIT
SSB
The output from the MIC jack J, is fed through
the MIC GAIN control VR3a to pin 13 of the
CARRIER UNIT.
CARRIER UNIT (PB-1706)
The microphone signal is amplified by microphone
amplifier Qo; (TA-7063P) and fed through relay
RL7o; to the ring modulator, consisting of Dros —
Dros (all 1S1007), where the signal modulates a
carrier signal delivered from Q793 (2SC1815SY).
The double sideband signal is fed through Tyo,
to the FILTER UNIT.
FILTER UNIT (PB-1716)
The 8987.5 kHz signal fed to pin 8 of the FILTER
UNIT is amplified by buffer amplifier Q3o, (2SK
19GR) and is passed through sideband filter XF3o4
by diode switches D399 (1S1555) and D307 - D3os
(both 181007) where the DSB signal is converted
to an SSB signal by removing the unwanted side-
band.
NB UNIT (PB-1703)
The SSB signal is fed through a buffer amplifier
Qo (2SC1815Y) to the bases of Qr92 and Qz3o5
(both 2SC1815Y). When the RF processor switch
is "OFF," the SSB signal is amplified by Q2o2
and fed to Q393 (3SK59GR). When the processor
switch is "ON," the SSB signal is amplified by
Qzos (2SC1815Y) and is further amplified by
the limiter Qo, (TA7060P), where the signals
that exceed the preset clipping. level are sliced out.
VR2093 is used to equalize the level of the clipped
and unclipped signals.
This highly clipped SSB signal is passed through
a selective filter XF39, to remove RF harmonics
that result from signal clipping. The filtered out-
put signal is amplified by Qj93 and fed to the
RF UNIT. The PROC LEVEL control VR;b,
located on the front panel, varies the resistance
2-18
between the collector and emitter of Qzo7 (2SC
1815Y), thus changing the input signal level to
the filter.
The CW, FSK, AM, and FM signals pass through
Q2o; (2SK19GR) and are fed to Q293.
The ALC (Automatic Level Control) voltage
detected at the grid circuit of the PA tube is fed
to the first gate of Q93 to reduce the gain of the
amplifier, thus preventing overloading and dis-
tortion.
RF UNIT (PB-1702)
The SSB signal from J,o2 of the printed board
is fed to the balanced mixer consisting of Qios
and Qy97 (both 3SK40M), where the SSB signal
is heterodyned to the desired RF frequency by
mixing with the local signal which is supplied
from the VCO UNIT. The mixer output is applied
through diode switch Dyoq4 (181555) to the
DRIVE UNIT.
DRIVE UNIT (PB-1714) PA UNIT (PB-1715)
The output from the RF UNIT is fed to the grid
of the transmitter driver V,60, (12BY7A) where
it is amplified to a level sufficient to drive the
final amplifier tubes V,79, and V,702 (6146B).
A portion of the RF signal is coupled through
Cy, to the cathode of the 12BY7A driver tube
so as to improve the linearity of the final amplifier
stage (RF negative feedback).
Neutralization of the power amplifier is accom-
plished by feeding back a small amount of the
output through TC, to the cold end of the T;/Lo
combination. The return of the grid circuit is
connected to the REC A UNIT (PB1708) to
produce ALC voltage to control the gain .of the
TX IF amplifier, thus preventing distortion caused
by overdrive.