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Heathkit AR-14 - Page 82

Heathkit AR-14
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Page
72
Transistor
Q13
does
not
conduct
current
until
the
positive
peaks
of
the
19
kc
signal
are
applied
to
it.
Then
it
only
conducts
until
the
time
constant
of
resistor
R35 and
capacitor
C38
cause
it
to
stop
conducting.
The
current
pulses
that
result
are
applied
to
transistor
Q14
and
amplified.
These
pulses,
produce
an
average
level
of 6
volts
across
the
stereo
indicator
lamp.
This,
in
turn,
causes
the
lamp
to
light,
indicating
that
an
FM
stereo
signal
is
being
received.
MONOPHONIC
FM
OPERATION
The
monophonic
signal
is
coupled
from
the
ratio
detector
circuit
to
the
base
of
transistor
QB.
After
amplification,
this
signal
is
coupled
to
transistor
Q9. Q9
acts
only
as
an
emitter
follower
since
no 19
kc
signal
is
present.
The
monophonic
signal
is
then
coupled
through
capacitor
C52
to
the
emitters
of
transistor
Qll
and
Q12.
No
oscillator
signal
is
coupled
to
transistors
Qll
and
Q12,
since
the
38
kc
oscil-
lator
circuit
is
disabled
by
the
Source
switch.
Transistors
Qll
and
Q12
will
conduct
when no
38
kc
signal
is
applied
to
their
bases.
Then
the
same
monophonic
signal
is
present
at
the
col-
lectors
of
transistors
Qll
and
Q12.
These
signals
are
then
coupled
through
the
de-emphasis
cir-
cuits
to
the
Left
and
Right
channel
outputs.
I
~
fff&iffiiirJ
LEFT
CHANNEL
AMPLIFIER
The
complete
amplifier
section
of
the
Receiver
consists
of a
left
channel
amplifier
and
a
right
channel
amplifier.
The
left
and
right
amplifiers
are
identical;
therefore,
in
order
to
simplify
this
description,
only
the
left
channel
amplifier
will
be
discussed.
The
FM
signal
from
the
FM
tuner
section
is
applied
through
resistor
R105
and
Source
switch
contacts
12
and
3
to
the
base
of
tran-
sistor
QlOO.
Resistor
Rll
1
is
used
to
pro-
vide
the
proper
load
impedance
for
the
phono
cartridge.
Transistor
Ql
operates
as
a
high-
gain
low-noise
amplifier
to
increase
the
level
of
the
incoming
signal.
The
amplified
signal
from
the
collector
of
Ql
is
applied
directly
to
the
base
of
transistor
Q3.
Transistor
Q3
amplifies
the
signal
again.
From
the
collector
of Q3, a
portion
of
the
signal
is
applied
through
a
frequency
selective
net-
work, which
consists
of
capacitors
C105
and
Cl
09,
resistor
Rl25,
and
the
Source
switch.
This
network
provides
RIAA
equalization.
DC
feed-
back
is
applied
from
the
emitter
of
Q3
to
the
base
of
Ql
through
resistor
Rl
1
7.
The
equalized
signal
from
the
collector
of
Q3
is
applied
through
capacitor
C 107
to
Volume
control
R129.
The
amount
of
signal
required
to
produce
the
desired
listening
level
is
tapped
off
by
the
slider
portion
of
the
Volume
control.
This
signal
voltage
is
applied
through
the
Bass
and
Treble
control
circuits
to
the
base
of
amplifier
transistor
Q5.

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