Display Unit - Icom IC-730 Maintenance Manual

inverter, then Pin6 of IC3 as a switching signal. The other
waveform-shaped signal is output from pin3 of IC2 and
differentiated by C12 and R10, then fed to pin1 of IC3,
inverter. The output signal from pin3 of IC3 is also
differentiated by C11 and R9, then fed to pins 5 and 12 of
IC3. To the other gates of IC3, the switching signal is applied
and controlled the differentiated signals. The output signals
from pins 4 and 11 of IC3 are fed to 2-input NOR gate of IC3
and combined as the UP signal. This UP signal is fed to CP
terminal of IC5, flip-flop, and it holds the output terminal, pin
13, H-level when the UP signal is present. This H-level signal
is then fed to B3 terminal of the CPU, and the CPU processes
to increase the operating frequency. When the tuning control
is turned counterclockwise, the B3 terminal is maintained in
the L-level.
Two differentiated signals at C12 and C11 are fed to pins 5
and 6 of IC2, OR gate, and combined signal is output from pin
4. The rotary encoder outputs 50 pulses per revolution, while
pin 4 of IC2 outputs both leading edge and trailing edge,
resulting in an output of 100 pulses per revolution. This output
signal is fed to a counter of 3-bit, consisting of IC4 and a half
of IC5.
This counter is capable of counting a maximum of 7 pulses,
and serves as a timing buffer between the encoder and the
display in relation to reading by the CPU. If there is any
output at the counter, the output of the diode OR gate,
consisting of D5 through D7, becomes H-level, input to
inverter, IC8, then input to the INT terminal of the CPU with its
level inverted to L-level. This terminal is a priority terminal
which gives priority to sensor processing by stopping other
operations. In the priority routine, counter data and up-down
data fed to B0 through B3 terminals are read by the CPU,
performing in this way all operations related to sensor
processing.
Immediately after the sensor data have been read, a reset
pulse is put out from I0 terminal, and the pulse is
differentiated by C5 and R46, then fed to clear terminals of
the counter and up-down latch to clear them.
The up-down control circuit consisting of Q3, Q4, R16 through
R21, C17 through C21, etc., provides frequency control from
the up-down switches on the microphone. When the UP or
DOWN button is depressed, the multivibrator, Q3, oscillates
and pulses are fed to pin 6 of IC4, and the 3-bit counter
counts the pulses the same as the pulses from the rotary
encoder.
After processing, the CPU outputs frequency data for the PLL
from the port E. This data is a 4-bit parallel data and it is
output in sequence according to its digits. The port E also
output frequency data for the display with time sharing. This
data is 4-bit parallel and 8-digit data. F0 and F2 terminals
output clock pulses for the display and F1 terminal outputs
clock pulse for the PLL. Data for 1kHz and lower digits are
output from the ports C and D, and fed to D/A converter
consisting of R35 through R43 to convert into an analog
signal. This analog signal is fed to the 2nd
local oscillator in the 2nd I F unit to get 10Hz step frequency
resolution.
3 – 4 DISPLAY UNIT
This unit is composed of DS1 display tube, IC1 driver IC and
DC-DC converter for the display tube and negative voltage
source.
The frequency data is fed to pins 2 through 5, input port S,
and timing control pulse is fed to pin 15. After processing,
segment data is put out from pins 16 through 22 with digit
control signal put out from pins 6 through 11. These signals
are fed to the display tube and light the tube with dynamic
lighting.
The DC-DC converter is composed of Q1, L1, R1 through R3,
C1 through C4, D1 through D4, etc. This converter generates
a negative voltage and filament voltage for the display tube,
and -5V for the AGC circuit and operational amplifiers.
3 – 8