Icom IC-751A Service Manual page 37

4-7-2 RELAY UNIT
The RELAY UNIT is located on the LOGIC UNIT and stops
relays on the FILTER UNIT while memory scan is operating.
The scan signal appears from pin 29 on 1C14 when memory
scan is selected. This scan signal turns Q17, Q18 and Q19
OFF. The relays on the FILTER UNIT are controlled by C15.
When Q19 is turned OFF, all the transmit relays are turned
OFF and are silent.
4-7-3 KEYER UNIT
The KEYER UNIT employs an electronic keyer circuit and a
cooling fan control circuit.
(a) DOT, DASH INPUTS AND ELECTRONIC KEYER
OUTPUT
When IC1 receives a dot signal at pin 1, |C1 outputs a DOT
and SPACE (ratio is 1:1). If |C1 receives the next dot signal
during output of the first DOT and SPACE, IC1 outputs only
the first DOT and SPACE.
DOT input 7
| aa The keyer accepts the
' next DOT input after
this point.
i
1
1
n l
'
Keyer out tL
"lat
DOT Space
1 1
Fig. 23
When IC1 receives a dash signal at pin 2, IC1 outputs a
DASH and SPACE (ratio is 1:3). If1C1 receives the next dash
signal during output of the first DASH and SPACE, [C1
outputs only the first DASH and SPACE.
DASH input
\_. The keyer accepts
the next DASH
input after this
point.
Keyer out
[- DASH |
3
Space
1
Fig. 24
If 1C1 receives a dash signal during a DOT and SPACE
output, |C1 outputs a DASH and SPACE after the DOT and
DASH output. This is called the DASH MEMORY.
DOT input
DASH input of
1
Keyer out tu |
>
| Koss |
DOT Space 3 Space
1 1 1
Fig. 25
If IC1 receives a dot signal during a DASH and SPACE
output, IC1 outputs a DOT and SPACE after the DASH and
SPACE output. This is called the DOT MEMORY.
DOT input it
DASH input Tb
Keyer out Jou
DASH —
r 3 ace Lot Space
1 1
1
Fig. 26
lf 1C1 receives the DOT and DASH signal continuously as
shown in Fig. 27, IC1 outputs the DOT and DASH
alternately. This is called the IAMBIC FUNCTION.
DOT input —
DASH input
HLUL
ale
DOT Space DASH Space DOT Space DASH Space DOT
1 1 3 1 1 1 3 1 1
Fig. 27
Keyer out
(b) SPD
These terminals determine the speed of the DOT, SPACE
and DASH.
(c) WEIGHT
These terminals determine the ratio of the DOT, SPACE,
AND DASH (DOT: SPACE: DASH).
(d) FAN MOTOR CONTROL
Thermal switches S1 and S2 detect the temperature of
output from Q4 and Q5 on the PA UNIT and control the
cooling fan while decreasing output power.
If the temperature of the transceiver increases in transmit
mode, S2 turns ON (at about 50° C) and the cooling fan starts
to rotate. The fan continues to rotate even after receive mode
is switched to, and will continue until the temperature drops
below 50°C. Fan rotation is somewhat slower in receive
mode than in transmit mode.
$1 turns ON if the temperature increases to about 90°C or
more due to antenna mismatching or other problems. The
rotation speed of the fan increases when S1 turns ON.
THERMAL $1 OFF OFF ON
SWITCH S2 OFF ON ON
COOLING FAN RECEIVE | OFF SLOW MED.
SPEED TRANSMIT] OFF |MED.HIGH| HIGH
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