Yaesu FT-902DM Instruction Manual page 29
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CW
In the CW, AM, and FSK modes, carrier oscillator
Q702
(2SKI9GR)
on the CARR
UNIT
generates
a crystal controlled carrier signal of 8988.2 kHz.
The
carrier is fed through buffer amplifier Q703
(2SCI815Y)
to
the
ring modulator.
Q7o2
also
oscillates for TUNE operation.
DC voltage is applied through diode switch Do,
(181555) and RL~9,, 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
Qvyoq
(3SK59Y)
and
fed to Qy9,
(2SCI815Y)
on the
NB
UNIT.
The
signal
is then
further amplified
by Q303 (3SK59GR), and is fed to the transmitter
mixer.
Keying
of
the.
transmitter
is accomplished
by
changing
the bias voltage
to Vi601,
Vi701,
and
Vi7o2-
The
tubes
are cut off on "key up" by
application
of —35V
for Vy69;
and —110V
for
Vizo1
and
V,792.
These
cutoff voltage are re-
duced
to --0.1V
and
—60V,
EeepECHNEDY: during
'key down"' conditions.
The key is connected to pin.13 of the REC C UNIT
(PB1717A). Under
"key
down"
conditions, the
base
of Qj803
(2SA733)
is grounded,
causing
Qisos
(2SC1815GR)
and
Qjgoq
(2SA639)
to
conduct. The voltage at pinl6 of PB-1717A reaches
zero when
Qigo9q4 conducts.
Since pin 16 of PB-
1717A is connected to pin2of the RECT A UNIT
(PB-1708), the bias voltage of Vi601, Vizo1, and
Vi702
places these tubes in the normal operating
condition.
On other
modes,
12 volts is applied
to pin 15,
causing Q;g94 to conduct, thus placing the operat-
ing bias on the tubes.
KEYER UNIT (PB-1728A)
The built-in electronic keyer utilizes Q79, (Curtis
8044 IC). The
output
of Qj79;
is high during
"key down'
conditions; this output is connected
to the base of Q.792, (2SC1815Y),
the collector
current
of which
is connected
to the base
of
Qiso3,
causing
Qiso3
to conduct
during
"key
down" conditions.
Keyer speed is controlled
by VR,b
on the front
panel.
VR379,
provides
adjustment
of the dot-
space symmetry, while VR379,
adjusts the keying
weight.
VR,792
normally
is set
to
minimum,
providing a 1:1 dot: space weight.
AM
The
microphone
signal
is amplified
by
Qyo5
(TA7063P)
and Q7,9
(2SC732TM)
and fed to-
Qr04
(3SK59Y),
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 Q79,
(2SK19GR)
generates
a crystal controlled
carrier frequency
of 8988.2
kHz. When keying is applied to the FSK jack J,;,
the base
voltage
of Qj9,
(2SCI815Y)
becomes
zero,
causing
Qj 9, to cut off. In the "'space"'
condition,
Q79,
conducts,
placing TC7 9, 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-
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