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Page Title: Transmitter Functional Operation (cont)
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TM 11-5820-873-34
and the mode switch is not m CPLR TUNE KW, transmit
the difference between the frequency of the 2nd local
audio enable/disable switch 1A3A5Q1 is enabled. This
oscillator and 80.75 MHz.  The 1st local oscillator is
enables 3rd local oscillator amplifier 1A3A3Q1.  The
used to convert the 1st IF frequency of 91.25 MHz to the
10.5 MHz 3rd local oscillator signal from synthesizer
selected operating frequency.
1A4 is amplified by 3rd local oscillator amplifier
(4)  The RT-1277/URC-92 is normally in the
1A3A3Q1  and  applied  to  balanced  modulator
receive mode It is placed in the transmit mode when
1A3A3CR3 thru 1A3A3CR6.
relays 1A5A4K2 and 1A8K1 are energized (see para 2-
(7)  The balanced modulator modulates the 10.5
22 for a discussion of the keying circuits). The relays
MHz 3rd local oscillator signal with the audio signal from
are energized as follows:
either of the three audio sources ((5) above).  The
(a)  Operating the press-to-talk switch on the
output from the balanced modulator is a double
handset energizes relays 1A5A4K2 and 1A8K1 when
sideband suppressed carrier signal at 10.5 MHz. The
mode switch 1A1S13 is set to LSB, USB, or AM.
double sideband suppressed carrier signal from the
(b)  Grounding the keyline at AUDIO connector
balanced modulator is applied to IF amplifier 1A3A2Q4.
1A8J2 (600 ohm audio) energizes relays 1A5A4K2 and
(8)  IF amplifier 1A3A2Q4 is enabled by the
1A8K1 when mode switch 1A1S13 is set to LSB, USB,
transmit enable signal and amplifies the output from the
CW, or AM.
balanced modulator.  The output from 1A3A2Q4 is
(c)  Operating the CW key causes transistors
coupled to the inputs of two diode gates.
1A3AQ12 and 1A3A3Q14 to energize relays 1A5A4K2
(9)  The diode gates at the inputs and outputs of
and 1A8K1 when mode switch 1A3S13 is set to CW.
filters 1A3A2FL1 and 1A3A2FL2 are enabled by + 12
Transistors 1A3A3Q12 and 1A3A3Q14 hold the relays
volts from mode switch 1A1S13. The 10.5 MHz signal
energized for approximately 1 second after the CW key
is coupled through the selected filter to amplifier
is released. This prevents the relays from deenergizing
1A3A2Q1, 1A3A2Q2.
between  normal  CW  characters  and  words  but
(10) Because the RT-1277/URC-92 uses high side
automatically returns the unit to the receive mode when
conversion, the sidebands are reversed in the IF
the CW key has not been operated for 1 second. This
circuits.  That is, the lower sideband is at a higher
delay is adjusted by 1A3A3R50.
frequency than the upper sideband.
(5)  Audio is brought into the RT-1277/URC-92 as
(a)  With mode switch 1A1S13 set to LSB, diode
gate 1A3A2CR8, diode gate 1A3A2CR1, 1A3A2CR2
(a)  Audio from the handset is coupled through
and diode gate 1A3A2CR11, 1A3A2CR12 are enabled.
XMT GAIN control 1A1R58 to microphone audio
The lower sideband signal is coupled through upper
amplifier 1A3A3Q4 when mode switch 1A1S13 is set to
sideband filter 1A3A2FL1.  The center frequency of
LSB, USB, or AM. The amplified audio from 1A3A3Q4
1A3A2FL1 is 10.5016 MHz.
is applied to balanced modulator 1A3A3CR3 thru
(b)  With mode switch 1A1S13 set to USB or
CW, diode gate 1A3A2CR10, diode gate 1A3A2CR5,
(b)  Audio from the 600 ohm balanced input is
1A3A2CR6 and diode gate 1A3A2CR15, 1A3A2CR16
amplified by 600 ohm audio amplifier 1A3A3Q11. The
are enabled.  The upper sideband signal is coupled
amplified audio from 1A3A3Q11 is applied to balanced
through lower sideband filter 1A3A2FL3.  The center
modulator 1A3A3CR3 thru 1A3A3CR6.
frequency of 1A3A2FL3 is 10.4984 MHz.
(c)  Operating the CW key turns on the CW tone
(c)  The  AM  transmit  mode  of  the  RT-
oscillator, 1A3A3Q7 and 1A3A3Q8, when mode switch
1277/URC-92 is actually upper sideband plus carrier.
1A1S13 is set to CW. The 1 kHz tone from the CW
With mode switch 1A1S13 set to AM and relay 1A8K1
tone oscillator is applied to 600 ohm audio line driver
energized,  diode  gate  1A3A2CR9,  diode  gate
1A3A4U1, audio amplifier 1A3A4Q11, and emitter
1A3A2CR5, 1A3A2CR6 and diode gate 1A3A2CR15,
follower 1A3A3Q6.
1A3A2CR16 are enabled. The upper sideband signal is
1.  The audio signal from 600 ohm audio
coupled through lower sideband filter 1A3A2FL3. The
line driver 1A3A4U1 is coupled to AUDIO connector
upper sideband signal is combined with the 10.5 MHz
carrier signal by amplifier 1A3A2Q1, 1A3A2Q2 ((12)
2.  The audio signal from audio amplifier
1A3A411 is coupled through VOLUME control 1A1R51
(11) The gain of amplifier 1A3A2Q1, 1A3A2Q2 is
to speaker driver 1A1A1.  The output from 1A1A1 is
controlled by the output of ALC amplifier 1A3A3Q9,
coupled through the normally closed contacts of PHONE
1A3A3Q10 ((19) below).  The ALC voltage limits the
jack 1A1J3 to speaker 1A1LSL.
gain of 1A3A2Q1, 1A3A2Q2 when the transmitter output
3.  The audio signal from emitter follower
reaches 100 watts peak or there is a higher VSWR at
1A3A3Q6 is applied to balanced modulator 1A3A3CR3
the transmitter output. Amplifier 1A3A2Q1, 1A3A2Q2 is
thru 1A3A3CR6.
enabled by the transmit enable signal and amplifies the
(6)  When the transmit enable signal is present

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