Honeywell Bendix/King KX 165A Maintenance Manual page 33

BENDIX/KING
Digital to analog converter (DAC) U3005 drives the varactor tuned Com preselectors as well as
provides alignment voltages to the unit's master reference oscillator and sets the threshold of the
Com squelch. Another DAC, U3006, provides the alignment voltages for the Com IF filter adjust-
ments, the Com and the Nav RF AGC activation levels as well as the Com and the Nav VCO ad-
justments. A precision regulator, U3027, provides the DACs with a stable 8.4 VDC supply voltage.
4.3.1.3 Serial to Parallel Register
The outputs of the serial to parallel register, U3002, controls four functions. When the volume con-
trol knob is pulled out by the pilot, driving U3005 pin 2 from its normally low state to a high, the
squelch can be manually forced open. The Nav RF preselector is controlled by U3005 pin 2, which
will be high if the Nav operating frequency is below 113.6 MHz, and will be low at frequencies of
113.6 MHz and above. When U3002 pin 3 is high, the Com receiver audio level compressor is
disabled, a feature that may be used during some bench tests.
4.3.1.4 EEPROM
The non-volatile EEPROM, U3007, stores the alignment information for each RECEIVER
BOARD. This EEPROM is read by the microprocessor when the unit is first turned on. The values
in U3007 are updated when the technician adjusts any of the alignment functions controlled by
U3005 and U3006.
4.3.1.5 Voltage Regulator and T/R Inverters
Voltage Regulator and T/R Inverters
Voltage regulator U3009 provides a stable voltage for sensitive synthesizer circuitry.
The microprocessor sends transmit-receive commands to the RECEIVER BOARD via P3001 pin
2. This pin is high when the unit is in transmit. TX switch driver U3001-G, pin 10 is low in transmit
and high in receive. The output of inverter U3001-B is high in transmit and low in receive.
4.3.1.6 T/R Switch Drive and Interlock Attenuator
Received signals arrive at RECEIVER BOARD J3006 from the T/R switch located on the Trans-
mitter board. When Q3004 is turned on, a positive voltage appears at the center conductor of
J3006 to turn on the T/R diode located on the transmitter board. In transmit, a negative voltage is
developed at the center conductor of J3006 by a small amount of rectification in the T/R diode it-
self, holding the T/R diode off.
CR3004 and CR3005 form a 14 dB attenuator which is activated in transmit or when the unit's
interlock feature is enabled externally. Q3001, Q3002 and Q3003 drive the attenuator active when
the collectors of Q3002 and Q3003 are at a positive voltage. A negative voltage at the collectors
of Q3002 and Q3003 allows the incoming signal to pass through the attenuator with little attenu-
ation.
4.3.1.7 Preselector and RF Amp
L3006 and L3007 are part of the varactor tuned Com preselector, which feeds RF Amp Q3010.
The gain of Q3010 is maximum when its gate 2 is above 7 volts. At signals stronger than about -
85 dBm at the receivers input, the voltage on gate 2 will drop, reducing the gain of Q3010, provid-
ing RF AGC action. Q3010's output is fed to two more varactor tuned preselector poles consisting
of L3009 and L3011. The voltage on the varactors CR3006-CR3009 will vary from about 4 VDC
at 118.0 MHz to about 7.0 V at 136.9 MHz. In transmit, the preselector poles are detuned to re-
duce the overloading of the receiver.
4.3.1.8 First Mixer and First IF
Page 4-30 15610M00.RCD
KX 165A
Rev 0, Jun/2000