Icom IC-751A Service Manual page 20

front panel which alters the emitter voltage of Q15. Q12, the
noise blanker gate, closes when noise is received. Q15 then
turns ON with the rectified voltage.
The noise blanker circuit contains a delayed pulse-width
circuit which consists of a Miller integrator and a
comparator. The output signals from this circuit combine
with an output signal from Q13 and the combined signal is
used for controlling the noise gate circuit. This allows the
noise blanker circuit to blank wide-width pulse noise called
woodpecker noise. When the NOISE BLANKER TIMING
SWITCH is set at the [WIDE] position, the noise blanker
does not function with pulse noise which is less than 4
millisecond of pulse width.
Q12 controls the blanking time and prevents blanking for
more than 1 to 2 milliseconds and 10 milliseconds when the
[NORMAL] and [WIDE] positions are selected, respectively.
This results in a distortion-free audio signal. The limits of
blanking time are determined by D16, R56, R57, C39, and
C40. Q10 prevents noise blanking times from exceeding the
above limits, automatically stopping the function when the
limits are reached.
4-1-5 NOTCH CIRCUIT
Anotch circuit is installed in the IC-751A with a 9MHz crystal
filter function. This circuit uses a bridge-type notch filter and
achieves very sharp attenuation and stability using a crystal
unit.
Notch frequency can be altered by a capacitance change of
varicap D110 which is installed in series with crystal unit X2.
When the notch circuit is not turned ON with the [NOTCH]
SWITCH, both the input and output of the notch circuit are
grounded by D109.
Monolithic filter F17 is connected to the next stage of the
notch circuit and eliminates spurious signals emitted from
mixer IC11.
4-1-6 DETECTOR CIRCUIT
Signals amplified in the receive IF circuit pass into 1C20 for
detection of the SSB, CW and RTTY modes, and into D145
for detection of the AM mode through buffer amplifier Q80.
The product detector for the SSB, CW and RTTY modes
functions in 1C20, a balanced mixer IC chip. BFO signais are
applied to |C20 for each mode. In the AM detector, D145
rectifies IF signals and the rectified signals are received at
Q79, a high impedance emitter follower circuit which
improves the distortion ratio of the rectified signals.
4-1-7 AGC CIRCUIT
The 1C-751A has a fast attack/slow release-type AGC
system which maintains the peak voltage of rectified IF
signals from the IF amplifier circuit for a brief period of time.
The AGC circuit provides a dual gate MOSFET in each IF
amplifier circuit. AGC voltage is applied to the PIN
attenuator in the RF UNIT, obtaining a dynamic range of
100dB.
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The time constant of the AGC circuit is selectable, and has
three positions, [FAST], [SLOW], and [OFF] for suitable
operation.
The received IF amplifier output signals are detected and
amplified at Q32. Then, the output of Q32 is connected to a
time constant circuit consisting of capacitors and resistors
which change the AGC release time. Q27 and Q28 control
the AGC voltage. The AGC voltage is set for approximately
4.4V when no signal is received. The voltage will decrease
depending on the signal strength of received signals.
When the [AGC] SWITCH is set in the [SLOW] position,
R137, C65 and C66 are added to the normal time constant
circuit to obtain a longer AGC release time. When the [AGC]
SWITCH is set in the OFF position, 8V is applied to the base
of Q28 through D64. Therefore, the AGC circuit does not
function. The AGC circuit also does not function in FM
mode.
4-1-8 FMIF AMPLIFIER AND DETECTOR CIRCUITS
When FM mode is operating, IF signals are passed through
FI6 (a 455kHz filter) and are amplified at 1C12 and Q45, and
then are limiter amplified at 1C13. After being amplified,
signals are fed into a detector circuit consisting of D1, D2,
and ceramic discriminator X3. Detected signals are
deemphasized by an integrated circuit consisting of R405
and C246, and are then applied to the AF preamplifier circuit.
4-1-9 AF AMPLIFIER CIRCUITS
Audio signals from 1C20 or Q76 are amplified at 1C19(b) and
fed into a tone control circuit consisting of R394 and C258 on
the MAIN UNIT and the [TONE] CONTROL on the front
panel. After passing through the tone control circuit, audio
signals pass through the [AF GAIN] CONTROL on the front
panel and are then fed into 1C18, the. power amplifier. 1C18
drives the speaker at more than 3W with 8 ohms of
resistance.
Q74 and Q75 comprise a squelch switching circuit which
cuts audio output. Output signals from iC19({b) are applied
to the rear panel AF OUT pin in the ACCESSORY SOCKET.