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TM 11-5820-873-34
operating the press-to-talk switch on the handset, by
by amplifiers 1A3A2U1 and 1A3A2U2 and coupled to
grounding the keyline at AUDIO connector 1A8J2, or by
the inputs of amplifier 1A3A4Q1 and product detector
operating the CW key. Operating the CW key energizes
1A3A4Q6, 1A3A4Q8, 1A3A4Q9.
the CW tone oscillator, which produces a 1 kHz audio
(a)  The output from amplifier 1A3A4Q1 is
signal.  Input audio from the handset, the 600 ohm
coupled to emitter follower 1A3A4Q2 and AM detector
balanced line, or the CW tone oscillator is amplified and
1A3A4Q7, 1A3A4Q10.
used to modulate the 10.5 MHz, 3rd local oscillator
1.  The  output  from  emitter  follower
signal from synthesizer 1A4.  The resultant double
1A3A4Q2 is coupled to AGC detector 1A3A4Q3. The
sideband suppressed carrier signal is amplified and
DC output from 1A3A4Q3 is coupled to emitter follower
filtered in accordance with the setting of the mode
1A3A4Q4. The output of 1A3A4Q4 is the AGC voltage
switch. The remaining upper or lower sideband signal
RF gain control amplifier 1A3A4Q12 is connected
(AM transmission is actually upper sideband plus
across the AGC line, RF GAIN control 1A1R55 controls
carrier) is amplified and, if the mode switch is set to AM,
the gain of 1A3A4Q12 and established the minimum
mixed with the 10.5 MHz carrier signal.  The lower
level of AGC. With 1A1R55 fully counterclockwise, it
sideband, upper sideband, or upper sideband plus
should be possible to completely disable the receiver.
carrier signal is mixed with the 80.75 MHz of the 2nd
The AGC voltage is also used to drive S meter amplifier
local oscillator signal from synthesizer 1A4.
The output of 1A3A4Q5 drives meter
resultant signals are amplified and passed through a
91.25 MHz (sum frequency) filter.  The 91.25 MHz
2.  When extremely high signal levels are
signal is mixed with the 91.25 to 121.2499 MHz, 1st
present, peak signal level AGC amplifier 1A3A5Q2
local oscillator signal, from synthesizer 1A4.
limits the level of input signal that is applied to RF
difference frequency (1st local oscillator frequency
amplifier 1A3A1Q2.
minus 91.25 MHz) is amplified to the 100 watt level and
3.  With mode switch 1A1S13 set to AM
passed through a low pass filter to ANT jack 1A8J1.
(and relay 1A8K1 not energized) AM detector 1A3A4Q7,
b. Detailed Functional Operation.
1A3A4Q10 is enabled.  The 10.5 MHz signal from
(1)  Power supply 1A6 (para 2-7) provides the
1A3A4Q1 is detected by AM detector 1A3A4Q7,
regulated DC voltages required to operate the circuits in
1A3A4Q10. The audio signal from the AM detector is
the RT-1277/URC-92.
coupled to the inputs of 600 ohm audio line driver
(2)  TCXO  1A8U1  is  an  extremely  stable,
1A3A4U1 and audio amplifier 1A3A4Q11.
temperature controlled, crystal oscillator and provides
(b)  With mode switch 1A1S13 set to LSB, USB
the 5 MHz reference signal to the synthesizer.
or CW product detector 1A3A4Q6, 1A3A4Q8, 1A3A4Q9
(3)  Synthesizer 1A4 (para 2-8) generates three
is enabled.  The 10.5 MHz 3rd local oscillator signal
local oscillator frequencies.  The 1st local oscillator
from the synthesizer is mixed with the 10.5 MHz signal
frequency of 91.25 to 121.2499 MHz is controlled by the
from amplifier 1A3A2U1, 1A3A2U2 in product detector
selected  front  panel  FREQUENCY  switches  and
1A3A4Q6, 1A3A4Q8, 1A3A4Q9. The audio signal from
determines the operating frequency of the radio. The
the product detector is coupled to the inputs of 600 ohm
2nd local oscillator frequency is 80.75 MHz. And, the
audio  line  driver  1A3A4U1  and  audio  amplifier
3rd local oscillator frequency is 10.5 MHz.
(a)  The 3rd local oscillator signal is derived by
(16) The audiot signal from 600 ohm audio line
direct synthesis techniques (i.e. by dividing and mixing
driver 1A3A4U1 is coupled to AUDIO connector 1A8J2.
the output of 1A8U1). This local oscillator signal is used
The audio signal from audio amplifier 1A3A4Q11 is
as a carrier generator.
coupled through VOLUME control 1A1R51 to speaker
(b)  The 2nd local oscillator consists of a crystal
driver 1A1A1.  The output from 1A1A1 is coupled
oscillator at a nominal frequency of 80.75 MHz. This
through the normally closed contacts of PHONE jack
frequency is used in VHF mixer 1A3A1 to convert the
1A1J3 to speaker 1A1LS1.
1st IF of 91.25 MHz to the 2nd IF of 10.5 MHz. Since
the 2nd local oscillator is not referenced to 1A8U1, a
Transmitter Functional Operation
small frequency error can exist. However, because of
(Fig. FO-2).
the mixing scheme used, this same error appears on the
1st local oscillator signal and is therefore canceled at
The RT-1277/URC-92 can be used in the transmit mode
the output of the VHF mixer.
in any setting of the mode switch.  This paragraph
(c)  The VCO (1st local oscillator) is a phase
describes functional operation as a transmitter with the
locked oscillator covering the frequency range of 91.25
mode switch set to LSB, USB, AM, or CW For functional
to 121.2499 Hz in 100 Hz steps. The exact frequency of
operation with the mode switch set to CPLR TUNE KW
the 1st local oscillator is equal to 91.25 MHz plus the
setting of the selected FREQUENCY switches and
a. Overall  Functional  Operation.
The  RT-
1277/URC-92 is placed in the transmit mode by

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