2
-----------·~
current
of
H 153 which in
turn
control the bias current
of
the RF amplifier through the resistor
R179 and R151. A
part
of
IF signal
output
is
also applied
to
the diode H158 through a
capacitor C167 and rectified
to
obtain
DC
current
for
energizing the
AM
signal strength meter
M010.
2.1 Suggestions
for
AM
Tuner Trouble Shooting
Check
for
broken
AM
bar antenna, next
try
to
tune station by rotating fly-wheel tuning
knob
slowly and observe the
AM
signal strength meter whether
it
deflects
or
not
.
If
the signal strength
meter
gives
a deflection at
several
frequencies received, no failure may exist in the
stages
at
least
preceding final IF transformer L154. Next connect a oscilloscope
to
the pin terminal J162
or
J 157 and check
for
audio signals
with
the tuning meter deflected.
If
the signal strength meter
does
not
deflect, check the local oscillator circuit. Normal oscillating voltage
at
the
hot
end
of
the oscillator tuning capacitor
is
about 2
or
3 volts, varying
with
tuning capacitor position. When
measuring oscillating voltage
use
a
RF
VTVM,
no
circuit
tester
gives
correct indication.
If
the
local oscillator voltage
is
normal, check all voltage distribution in the
AM
circuits
by
using a
DC
VTVM
and compare
the
measured values
with
those given in the schematic diagram.
3. FM
TUNER
The FM Tuner section
of
Model 4430
is
divided
into
four
functional blocks: FM
Front
End,
IF
Amplifier
and Detector, Muting Control and MPX Stereo Decoding Circuit. FM signals
in-
duced by a FM antenna
are
led
to
FM antenna coil L101 through
ATTENUATOR
switch and a balun coil. These signals
are
then applied
to
the
FET
RF
amplifier
which
in
turn
applies its
output
to
the next
FET
Mixer H102 through the double tuned high selective circuits.
The
FET
Mixer convert its
input
signal into 10.7MHz intermediate frequency and amplifies
it
at
the
same
time. The H 103
is
a local oscillator and its
output
is
injected
into
the source
of
the
FET
Mixer, the injection voltage
is
about 700mV. The 10.7MHz
front
end
output
is
led
to
the
next
IF
amplifier
unit
through a coaxial cable.
The IF amplifier
unit
consists
of
five
stages
of
IF amplifier and one
stage
of
AGC amplifier.
Three pieces
of
dual elements ceramic filters
are
also
used
to
obtain high selectivity,
four
stages
of
symmetrical diode limiters
are
also employed
for
the best
limiting
characteristics, improved
capture ratio and good
AM
suppression.
A part
of
FM
Front
End
output
is
applied
to
the AGC amplifier H201 and rectified its
output
is
fed back
to
the gate
of
FET
RF
amplifier
to
decrease the gain
with
increased signal strength.
The
IF
signal sufficiently amplified through every
stage
of
IF amplifier
is
finally
applied
to
the
transistor limiter. The detected audio
output
is
led
to
the
buffer
amplifier H208 and its buffer-
ed
output
is
led
to;
(a)
noise amplifier
H471
through resistor R471 and capacitor C471, (b)
OUADRADIAL
jack on the rear panel through resistor R484, (c) MPX stereo decoding
circuit
through R483.
3.1
Audio
Muting
and Stereo Mode Auto-Selecting
Circuit
The muting
circuit
consisting
of
all solid-state electrical switching
has
been incorporated in
the Model
4430.
Three inputs control the muting function. The
first
is
related
to
singal strength,
the second
to
the noise
condition
at the detector and the
third
is
derived
from
the
DC
com-
ponent
of
the detector output. These inputs
are
properly matrixed and gated
to
provide muting
free
from
noise and transients.
The
first
input
of
DC voltage obtained by rectifying a part
of
IF
output
signal
from
the H206
is
applied
to
the
base
of
H306
and
turns on it,
if
the
IF
output
is
greater than predetermined level
(muting threshold level).
When
the H306
is
turned on the H307
is
turned
off,
allowing the
emitter-collector resistance increasing and the collector voltage
rises
about 9V. The increased
collector voltage increases the gate bias voltage and turns on the switching
FET
H308, decreasing
the source-drain resistance
to
near zero ohm and allowing the audio signal applied
to
the source
to
flow
to
the center
of
38KHz switching transformer through the source-drain path.
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