Honeywell Bendix/King KX 165A, Maintenance Manual

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BENDIX/KING KX 165A
Rev 0, Jun/2000 15610M00.RCD Page 4-35
U3023, includes an additional gain controlled 20.5 MHz amplifier, a second mixer, amplification
at 450 kHz, a detector and AGC circuitry. The 20.5 MHz signal applied to U3023 pin 18 is am-
plified then mixed down to 450 kHz, the second IF frequency. The 20.95 MHz reference oscillator
is applied to U3023 pin 16 from the Q3023 buffer-amp to serve as the second LO.
The 450 kHz second IF signal output at pin 1 of U3023 is impedance matched by L3038 and
C3314 to FL3008, a 450 kHz ceramic filter. The filtered signal is amplified by Q3034 and then re-
turned to U3023 at pin 3. Within the receiver IC, the 450 kHz is further amplified and passed to an
internal detector. C3285 and C3299 filter an internal reference voltage within U3023 and are im-
portant to maintaining accurate VOR bearing information. The gain of both the 20.5 MHz and 450
kHz amplifiers internal to U3023 is controlled by the AGC voltage on pin 7. AGC time constants
are established by C3230, C3248, C3249, and C3286. The amplified 450 kHz signal is available
at pin 12 of U3023 (TP3009) to allow easy viewing of the signal with an oscilloscope.
4.3.3.5 RF AGC
The AM receiver IC, U3023, outputs an IF AGC voltage on pin 7. This voltage will be about 1 VDC
with no input signal to the unit and rise to about 2 VDC with a strong input signal. U3020-B is used
to monitor the IF AGC voltage, and when it reaches about 1.7 VDC, the U3020 pin 7 voltage in-
creases to a positive voltage, turning on pin attenuator diodes CR3021 and CR3027 in the prese-
lector. As these diodes turn on, they attenuate the incoming signal to prevent the receiver from
being overloaded on strong signals.
4.3.3.6 Nav Synthesizer and LO Amplifier
Serial data from the micro processor is sent to program the synthesizer IC, U3018. U3018 com-
pares VCO output on pin 8 with the 20.95 MHz reference oscillator signal on pin 1 and generates
high or low pulses out of pin 5 to the active integrator, 3019-A. The combined action of U3006-B
and the RC loop filter apply a DC tuning voltage to CR3025 in the VCO. The voltage will vary from
about 3.5 V at 108 MHz to about 7 V at 117.95 MHz .
Internal to U3018 are two dividers which divide the 20.95 MHz reference frequency and the VCO
frequency down to a lower reference frequency of 50 kHz . The divided reference pulses appear
on U3018 pin 13 and the divided VCO frequency pulses appear on pin 14; these pulses are only
about 50 ns wide on pin 13 and 300 ns wide on pin 14 and will occur at 50 kHz rate. The VCO
frequency is 20.5 MHz lower than the receiver’s operating frequency. U21 buffers the VCO output
and sends it back to the synthesizer IC, U3018, to keep the loop locked. The output of U3021 is
also routed to Q3030, the Nav local oscillator amplifier. A lowpass filter on the output of Q3030
reduces the LO harmonics before T3003 transforms the low source impedance of Q3030 to a high
impedance on the balanced input of the mixer at pins 1 and 3 of U3022.
4.3.3.7 Nav Audio
Nav audio is output from the RECEIVER BOARD on the nav composite line. The audio then is
input to the main board’s nav audio filter which is a 4 pole 350Hz-2500Hz bandpass filter com-
posed of U1017-A&B and associated circuitry. The output of the audio filter is then amplified by
a gain of 1.6 by U1029-D before passing through the digital potentiometers U1016-B&D. After the
potentiometers, the signal routes to unity gain amplifier U1029-C and then to audio power ampli-
fier U1031-B which has a gain of about 35. T1002 transforms the signal in order to drive a 500
ohm load.
The ident filter, U1017-C&D and related components, form a 1020 Hz notch filter. When the notch
is enabled (front panel ident knob in), the 1020 Hz identification signal is attenuated by more than
15dB. The ident filter is disabled when the front panel ident knob is pulled out enabling the 1020
Hz signal to pass to the audio amplifiers.
4.3.3.8 VOR Converter