Yaesu FT-901DM Maintenance Service Manual page 46

cw
In the CW, AM, and FSK modes, carrier oscillator
Qro2 (2SK19GR) on the CARR UNIT generates
a crystal controlled carrier signal of 8988.2 kHz.
The carrier is fed through buffer amplifier Q703
(2SC1815Y) to the ring modulator. Q792 also
oscillates for TUNE operation.
DC voltage is applied through diode switch Do,2
(181555) and RL79;, to unbalance the ring mod-
ulator for CW, AM, and FSK operation; the
resulting carrier signal is fed to T79,.
The output from T79; is amplified by Qyo4
(3SKS9Y) and fed to Qs9, (2SC181S5Y) on the
NB UNIT. 'The signal is then further amplified
by Q2o3 (3SK59GR), and is fed to the transmitter
mixer.
Keying of the transmitter is accomplished by
changing the bias voltage to Vi60:, Vi7o1, and
Vizo2- The tubes are cut off on "key up" by
application of —35V for V,6o9, and —110V for
Vizor and Vj792. These cutoff voltage are re-
duced to —0.1V and —60V, respectively, during
"key down" conditions.
The key is connected to pin 13 of the REC C unit
(PB1717). Under 'key down" conditions, the
base of Qigo3 (2SA733) is grounded, causing
Qisos (2SC181SGR) and Qys93 (2SA639) to
conduct. The voltage at pin 16 of PB-1717 reaches
zero when Qygo4 conducts. Since pin 16 of PB-
1717 is connected to pin 2 of the RECT A UNIT
(PB-1708), the bias voltage of Vigo1, Vi701, and
Vi702 places these tubes in the normal operating
condition.
On other modes, 12 volts is applied to pin 15,
causing Q;go4 to conduct, thus placing the operat-
ing bias on the tubes.
KEYER UNIT (PB-1728)
The built-in electronic keyer utilizes Q.79, (Curtis
8044 IC). The output of Qo79; is high during
"key down" conditions; this output is connected
to the base of Qs792 (2SC1815Y), the collector
current of which is connected to the base of
Qiso3s, causing Q;s93 to conduct during "key
down" conditions.
TECHNICAL NOTE:
Keyer speed is controlled by VR,b on the front
panel. VR27o; provides adjustment of the dot-
space symmetry, while VR2792 adjusts the keying
weight. VR2792 normally is set to minimum,
providing a 1:1 dot: space weight.
AM
The microphone signal is amplified by Qyos5
(TA7063P) and Q,,o (2SC1000GR) and fed to
Qos (3SKS9Y), where it modulates the 8988.2
kHz carrier signal. The output from T792 is fed
to the NB UNIT in the same manner as described
in the CW section previously.
FSK
The carrier oscillator Qj92 (2SK19GR) generates
a crystal controlled carrier frequency of 8988.2
kHz. When keying is applied to the FSK jack J,5,
the base voltage of Qro, (2SCI81S5Y) becomes
zero, causing Qy9, to cut off. In the "space"
condition, Qj, conducts, placing TCy94 in the
circuit and lowering the carrier frequency to
8988.03 kHz.
AMGC circuit
The AMGC circuit is located in the CARRIER
UNIT and is used to reduce, the gain of the micro-
phone amplifier when only low-level background
noise is present (no speech input from the opera-
2-19