Marantz 2240 - Phono and Tone Amplifiers; Power Amplifier

inputs of PD-2 is given the 19 KHz resulting from the frequency division completed by DIV-1
and DIV-3, whereas the other input gets the 19 KHz output contained in the composite signal,
and the output is provided with a DC output in proportion to the amplitude of the pilot signal.
This DC output is furnished through LPF-2 to the trigger amplifier which drives the stereo
indicator lamp and stereo switch. Therefore, insufficient supply of the pilot signal results in
failure to light the stereo indicator and to turn on the stereo switch located in the path of the
38 KHz switching signal, thereby avoiding a wrong stereo operation. H303 attached on the
outside of pin 8 is a switching transistor for automatic monaural-stereo switchover. When the
intensity of an incoming signal from an FM station is weaker than a predetermined level, this
H303 is turned on and pin 8 is grounded, thereby developing a condition for monaural
reception. For a forced monaural operation, switch the MODE switch to “MONO”, and H303
comes into an ‘‘On’’ condition with the positive bias voltage applied to the base, and pin 8 is -
grounded,
thereby
establishing
monaural
operation.
The
transistor
H302
connected,
externally
to
pin
14
is
intended
to
stop
the
76
KHz
oscillation
of
the
VCO
which
interferes
an
AM
signal
during
the
reception
of
an
AM
station.
When
the
function
switch
is
set
to
“AM”
position,
a
positive
bias
is
charged
on
the
base
of
H302,
H302
is
turned
on,
and
pin
14
is
grounded.
Thus,
the oscillation of the VCO is stopped, ending the interference with AM reception.
3.3 Suggestion for Trouble Shooting of FM Tuner
3.3.1 Symptom: No FM Reception
First
turn
on
the
power
switch
and
try
to
tune
FM
stations.
Rotate
the
fly-wheel
tuning
knob
slowly
and
observe
the
FM
signal
strength
meter.
If
the
signal
strength
meter
deflect
at
several
frequencies
received,
the
tuner
circuits
preceding
the
discriminator
circuit
may
have
no
failure.
When
no
reading
is
obtained
in
the
meter,
check
FM
local
oscillator
circuit,
using
a
RF
VTVM.
The
normal
local
oscillator
voltage
is
one
or
two
volts
(rms)
at
the
tuning
capacitor,
depending
on
the
tuning
capacitor
position.
If
the
local
oscillator
voltage
is
normal,
next
check
all
voltage
distribution
in
the
FM
Front
End
and
IF
amplifier
unit
and
compare
them
with
those
shown
in
the
circuit
diagram.
When
signal
strength
meter
deflects
but
no
sound
is
obtained,
check
audio
circuits,
using
high
sensitive
oscilloscope.
3.3.2 Symptom: No Stereo Separation :
First
check
the
‘“‘MONO”
switch
is
in
normal
out
position.
Connect
a
FM
RF
signal
generator
output
modulated
by
a
stereo-modulator
to
the
rear
FM
ANTENNA
terminals,
and
check
the
stereo
beacon
is
turned
on
or
not.
If
not
turned
on,
check
for
19
KHz
VCO
output
signal (J310), using an
oscilloscope
and
a
frequency
counter.
4. PHONO AND TONE AMPLIFIERS
Signals from the PHONO jacks are applied to the phono amplifier mounted on P400. The
amplified and RIAA equalized phono signals and signals from the tuner section, AUX and TAPE
MONITOR IN jacks are applied to the SELECTOR switch.
All signals selected by the SELECTOR switch (S001) are let to the balance and volume
controls through the MONO switch.
Singnals properly attenuated by the volume control are applied to the tone amplifier and
subjected to the tone control networks such as bass, mid, treble control and high and low cut
filters.
Thus controlled audio signals are then led to the PRE OUT jacks on the rear panel.
5. POWER AMPLIFIER
Differential
amplifier
consists
of
the
transistors
H702
and
H703
to
provide
satisfactory
D.C.
stability.
The
transistor
H707
drives
the
inverter
transistors
H714
and
H715
which,
in
turn,
drive
the
power
stage
consisting
of
HOO1
and
HO02.
Transistors
H710
and
H711
are
current
limiter