Honeywell Bendix/King KX 165A, Maintenance Manual

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BENDIX/KING KX 165A
Page 4-34 15610M00.RCD Rev 0, Jun/2000
The modulator gate bias is adjusted instantaneously to effect amplitude modulation of the carrier.
RF drive to the gate is controlled by the second control loop. This loop which comprises U1034-
D and Q1038 to drive Q4004, predistorts the signal fed to the final by accomplishing low level mod-
ulation of the predriver to a level of about 15% AM, allowing a very high degree of final modulation
with low distortion.
4.3.2.5 THERMAL CUTBACK
The transmitter estimates ambient temperature by reading the internal thermistor, U1010, and ap-
plying offsets for unit specific operation (TOFF) and model specific operation (TAOS). TOFF is
typically 0 because thermistor deviation is very small. TAOS is typically -7 for units with glides-
lope, and -4 for units without glideslope. The Transmitter will hold carrier power of 10 watts to
+55C, and fall within the window of 2.5 to 6 watts at +70C.
4.3.2.6 TIMEOUT TIMER
The transmitter will allow continuous transmissions for durations of up to 30 seconds, after which
the COMM frequency display will blink indicating timeout of the transmitter. This guarantees min-
imized interference from a stuck microphone condition which may not be noticed by the pilot.
4.3.3 NAV (VOR/LOC) RECEIVER
Refer to figures 6-21 for the Nav receiver schematic. Some Nav Audio receiver functions appear
on the main board schematic in figures 6-8.
4.3.3.1 Preselector and RF Amp
The front end of the Nav receiver employs a high-dynamic-range RF amplifier embedded in a 5-
pole-band-switched preselector. Nav Signals at the Nav antenna connector are applied to the first
two poles of preselector consisting of L3025, L3035 and associated circuitry. These two poles are
band switched in two bands. When the operating frequency is below 113.6 MHz, the base of
Q3029 is driven low by U3002 and U3001-F. This turns Q3029 on, turning on pin diodes CR3020,
CR3022 and CR3026, lowering the response of the first two poles. At frequencies of 113.6 MHz
or higher, these pin diodes are off, allowing the frequency of the preselector to rise. Q3028 is the
RF amplifier operating in a lossless-feedback configuration. The gain of the RF amp is controlled
by transformer T2 and Directional coupler T1 at about 15 dB. Q3027 is a constant current source
for the RF amp, limiting the current through Q3028 to about 30 mA. Three additional poles of pre-
selector filtering consist of L3023, L3027, L3036 and associated circuitry. When CR3021 and
CR3027 have a positive voltage on their anodes, they turn on to provide RF AGC action.
4.3.3.2 First Mixer
FET mixer U3022 is a high-level-doubly-balanced mixer that converts the input received frequen-
cy on pins 12 and 14 down to a 20.5 MHz first IF Frequency on pins 5 and 10. The first local os-
cillator signal is applied to pins 1 and 3 at a frequency 20.5 MHz below the desired operating fre-
quency.
4.3.3.3 First IF
The low output impedance of the mixer at the output of T3005 is impedance matched by L3033
and C3317 to the higher input impedance of the 20.5 MHz 8 pole crystal filter, FL11. The output
of the filter is likewise matched to the low input impedance of IF amp Q3035 by L3034 and C3312.
After being amplified by Q3035, the 20.5 MHz signal is passed to pin 18 of U3023, an AM receiver
IC.
4.3.3.4 AM Receiver IC and Second IF