TM 11-5840-281-35/1
b. Tr Tubes. Tr tubes V101 and V102 function as rf
the LOCAL position, 221k resistor R103 is substituted for
Control-Indicator C-6988/TPN-18 LO TUNE control
switches to protect the receiver section input circuits by
R614. This provides the same value of reflector voltage
blocking transmitter section rf power from the receiver
as the midpoint setting of LO TUNE control R614. Local
section.
adjustment can now be made by means of local LO
(1) The tr tubes are essentially spark gaps which
TUNE control R105. When REMOTE-LOCAL switch
are enclosed by an envelope in an air and water vapor
S103 is again placed to the REMOTE position after a
environment. A keep-alive electrode, with -920 volts
local adjustment, klystron V103 may appear to be
applied, is located near one of the spark gap terminals to
mistuned. This occurs because LO TUNE control R614
maintain a steady glow discharge near the tr gap. Some
is not set to the exact value of the 221k resistor. When
of the electrons from the discharge enter the tr gap,
this happens, it is necessary to adjust LO TUNE control
permitting the tr tube to fire more rapidly when the
R614 slightly. The klystron output is coupled to the mixer
transmitter radiates. A thermostatically controlled heater
assembly through a 90-degree step-twist section of
within the tr tubes prevents the water vapor inside the tr
wave- guide. The local oscillator energy is mixed with the
tubes from freezing during cold weather.
received signal, and a 60-MHz, i-f signal is produced.
(2) Normally the tr tubes may be expected to
d. Mixer Assembly Circuit (Fig. 7-7). The mixer
perform satisfactorily for a period of 1000 hours.
assembly includes both sum and difference mixers.
Performance of the tr tubes may be checked by
Local oscillator energy is distributed equally into each
measuring the recovery time (Recovery time is the time
mixer section' by the first (or center) short-slot coupler.
after ionization of the tr tube at which low level of
The short-slot coupler in the sum channel or in the
attenuation is 3 db greater than its attenuation in the
difference channel distributes its portion of local oscillator
unexcited state.) The tr tube may be considered as
energy equally into the two mixer chambers; thus, each
performing satisfactorily if the recovery time does not
exceed 3 sec. The relative performance of the tr tubes
mixer chamber receives one quarter of the local
oscillator power. An adjustable attenuator is used
may also be checked by measuring the voltage drop
between the local oscillator and the mixer assembly to
across the tr tube with the transmitter operating in the
provide optimum power into the mixer chambers. The
wide-pulse mode. The voltage drop across the tr tube
local oscillator power is monitored by crystal current
should be approximately 250 to 300 volts.
meter M101 and adjusted for 0.6 ma for each crystal.
The crystals are reversed in the balanced mixer to
(3) Excessive insertion loss is also a limiting
reduce the noise generated in the local oscillator. The
factor on the life of the tr tube. Over a period of time,
signal outputs of the rectifiers are cross-coupled in input
molecular deposits upon the glass inner wall can cause
transformer T1301 of Intermediate-Frequency Ampli- fier
film deposits which attenuate the received signal. Normal
AM3337/TPN-8 to cancel each other when out-of-phase;
indication of this condition is a gradual deterioration of
consequently, there will be low noise signal output if the
receiver sensitivity over a long period of time.
crystals are balanced. The envelopes produced by signal
c. Local Oscillator Circuit (Fig. 7-7). The local
and local oscillator mixing are displaced in-phase by 180
oscillator is a reflex klystron local oscillator. It is a single-
degrees at the two crystals and their detected outputs
resonator-type klystron with an integral external cavity.
are out-of-phase with each other. Since the two crystals
The klystron operates with +200 volts on the resonator
are, in effect, parallel with each other (through
and from -70 to -140 volts on the repeller. Broad tuning
transformer T1304 in Intermediate-Frequency-Amplifier
of klystron V103 is accomplished by an adjustment which
AM-3337/TPN-8), their combined impedance is reduced,
physically alters the size of its cavity, while fine tuning is
which also tends to produce an increased receiver-to-
accomplished by varying the reflector voltage. The
noise ratio.
klystron frequency is adjusted to 60 MHz above the
transmitting frequency. When REMOTE-LOCAL switch
e. Intermediate-Frequency Amplifier AM-S337/ T
S103 is placed to the REMOTE position, tuning is
PN-8 Cct (Fig 7-7 and 7-38) . The sum and difference
accomplished by means of LO TUNE control R614 on
channels of the receiver section employ
the front panel of Control-Indicator C-6988/TPN-18.
When REMOTE-LOCAL switch S103 is held to
2-17
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