Icom IC-751A Instruction Manual page 43

7 -12 MICROPHONE UP/DOWN
OPERATION
•FREOUENCY CONTROL
UPd
~ I T C H
•MEMORY CHANNEL SELECTION
7 -13 AGC OPERATION
AGC
7 -14 PREAMP/ATT SWITCH OPERATION
•PREAMP SWITCH OPERATION
•ATTENUATOR (ATT) SWITCH
OPERATION
1) Each push of the UP or DN (down) SWITCH on the supplied
microphone changes the operating frequency one increment up or
down respectively.
2) Holding either switch down changes the operating frequency
continuo~sly in the same manner as turning the TUNING CON-
TROL. The tuning rate must be set with the TUN ING SPEED
SWITCH.
This up/down function is effective for changing your operating
frequency when using the V FO mode, or when using the ME-
MORV CHANNEL mode with the DFS SWITCH pushed IN.
1) When using the MEMORY CHANNEL mode, the memory chan-
nel may be changed with the microphone up/down switches.
Also, in the VFO mode with the DFS SWITCH pushed IN, the
displayed memory channel number may be changed in t~e same
way.
2) The memory channel or the displayed memory channel number
changes every two seconds if the microphone switches are held .
down.
The I-C-751 A has a fast attack/slow release AGC system which
maintains the peak voltage of the rectified IF signal from the IF
amplifier circuit for a brief period of time. This circuit keeps irritat-
ing background noise from being heard during the short pauses in
speech. The damping effect of the AGC therefore allows accurate
S-meter readings to be taken of the peak IF signal strength.
1) For normal SSB reception, select the SLOW position.
2) For CW or AM signais, or for SSB signais with short interval
fading, select the FAST position. ln the FAST position, the time
constant of the circuit is shortened.
3) ln the OFF position, the AGC circuit is deactivated and the
S-meter gives no signal strength readings. ln this case, the RF
GAIN CONTROL is useful to reduce the receiver gain when
strong signais are received. Adjustment of the RF GAIN
CONTROL affects the meter reading as described in ltem@of
SECTION 3 - 1 FRONT PANEL.
Place the PREAMP/ATT SWITCH on the top panel in the PRE AMP
position when receiving weak signais. An RF preamplifier is inserted
in the receive path which increases receiver sensitivity.
Place the PREAMP/ATT SWITCH in the ATT position when receiv-
ing very strong signais. ln the ATT position, the RF preamplifier is
removed from the receive path and a 20dB attenuator is inserted.
This helps to reduce interference.
Leave the PREAMP/ATT SWITCH in the OFF position for normal
operation.
See SECTION 3 - 3 for the switch location.
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