Furuno 1622 Service Manual page 12

2.4!Functions of Each Circuit board
RU-9458 Module
This module includes the circulator and MIC circuit that converts the microwave frequency to
the intermediate frequency.
The circulator switches between the transmitting and receiving microwave circuits so that the
transmitting radio energy may be efficiently transmitted to the antenna without leaking to the
receiver side and the receiving radar signal from the antenna may be sent with efficiency to the
receiver without leaking to the transmitter side.
The receiver side is provided with the waveguide to coaxial converter, diode limiter, and
balanced mixer. The diode limiter, set after the converter, protects the balanced mixer from the
excessive input power, and the mixer converts the input microwave signal to the 60-MHz IF
signal.
A voltage-controlled FET oscillator outputs the signal to the mixer.
MD Board (03P9270)
The power supply circuit and modulation circuit of the scanner unit (transceiver) are mounted on
the MD board.
There are provided two DC-DC converter power supply circuits: one supplies –6 V, +5 V, and
+12 V to the circuits except the magnetron and the other supplies power to the magnetron heater
and TXHV. The two DC-DC converters generate stable voltage outputs with Q803 and Q832
driven by the pulsewidth controllers U801 and U805.
The magnetron current, -6 V, +5 V, +12 V, +8 V (magnetron heater voltage), and TX HV can be
checked at the check points TP804 on the board.
In the magnetron modulation circuit, the transmitting pulse waveform is determined by the one-
shot multivibrator U851 which is triggered by the transmitting trigger pulse. The pulsewidth of
the multivibrator is set by the time constant which is determined by the pulsewidth control signal
from INT board. The output of the multivibrator inputs to the FET switching transistor Q857
through transistor buffer Q853 and variable resistor VR851. The VR851 presets the voltage for
driving Q857 to determine the magnetron output power.
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