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Heathkit TA-16 - Page 43

Heathkit TA-16
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Page
39
The
setting
of
the
Reverb
control
determines
how
much
of
this
delayed
signal
is
coupled
through
capacitor
C26
to
the
base
of
reverb
amplifier
transistor
Q5.
After
the
delayed
signal
is
amplified
by
transistor
Q5,
it
is
coupled
through
capacitor
C28,
resistors
R52
and
R25,
and
capacitor
C15
to
the
base
of
mixer
transistor
Q6.
The
base
of
reverb
amplifier
transistor
Q5
is
connected
through
resistors
R49
and
R48,
to
a
foot
switch.
When
the
foot
switch
is
On
or
unplugged,
transistor
Q5
amplifies
normally.
When
the
foot
switch
is
Off,
the
base
bias
of
transistor
Q5
is
shorted
to
ground,
cutting
off
the
transistor
and,
thus,
stopping
the
signal.
TREMOLO
CIRCUIT
Both
of
the
signals
from
the
reverb
channel
(one
delayed
and
the
other
not
delayed)
are
applied
across
the
resistance
element
of
the
LDR
(Light
Dependant
Resistor).
The
LDR
is
in
effect
the
output
element
of
the
tremolo
circuit,
which
operates
as
follows:
Tremolo
oscillator
transistor
Q12
is
connected
in
a
subsonic,
phase-shift
oscillator
circuit.
This
circuit
develops
a
signal
that
can
be
variedfrom
approximately
4
to
14
Hz
by
the
Rate
control
in
the
phase-shift
network.
The
amplitude
of
the
oscillator
signal
from
the
collector
of
Q12
is
varied
by
the
Depth
control.
The
signal
is
then
coupled
through
capacitor
C32
to
the
base
of
tremolo
modulator
transistor
Q13.
Transistors
Q12
and
Q13
are
connected
to
a
common
emitter
resistor
to
provide
positive
feedback
to
sustain
oscillation.
The
LDR
unit
consists
of
a
low-current
lamp
and
a
light-dependent
resistance
element.
The
value
of
the
resistance
element
depends
upon
the
brightness
of
the
lamp:
As
the
lamp
glows
bright-
er,
the
resistance
decreases;
as
the
lamp
glows
dimmer,
the
resistance increases.
Transistor
Q13
draws
collector
current
through
the
lamp
element
in
the
LDR
unit,
causing
the
lamp
to
glow.
Since
transistor
Q13
is
amplifying
the
tremolo
oscillator
signal,
the
collector
cur-
rent
will
follow
this
signal,
causing
the
lamp
in
the
LDR
unit to
glow
correspondingly
brighter
and
dimmer.
Because
the
brightness
of
the
lamp
varies
in
accordance
with
rate
and
depth
of
the
tremolo
signal,
the
resistance
of
the
LDR
will
also
vary
in
the
same
manner,
from
a
very
low
resistance
(practically
a
short
circuit)
to
a
very
high
re-
sistance
(practically
an
open
circuit).
Since
the
LDR
is
connected
between
the
reverb
channel
sig-
nal
path
and
ground,
its
resistance
variations
will
modulate
the
reverb
channel
signal
with
the
low-frequency
tremolo
signal.
The
tremolo
circuit
can
be
turned
on
and
off
with
the
foot
switch,
which,
in
the
Off
position,
shorts
the
base
of
oscillator
transistor
Q12
to
ground.
The
tremolo
circuit
is
on
when
the
foot
switch
is
unplugged.
THE
AMPLIFIER
CIRCUITS
Both
the
normal
channel
signal
and
the
reverb
channel
signal
(with
or
without
reverberation
and
tremolo)
are
present
at
the
base
of
mixer
amplifier
transistor
Q6.
These
signals
are
com-
bined
and
then
amplified
in
transistor
Q6,
and
then
further
amplified
by
predriver
transistor
Q7
and
driver
transistor
Q8.
The
amplified
signal
from
the
collector
of
driver
transistor
Q8
is
coupled
directly
to
the
base
of
output
transistor
Q9
and
through
diode
D101
to
the
base
of
output
transistor
Q10.
Diode
D101
is
part
of
the
collector
load
for
Q8
and
also
determines
the
relative
base
bias
for
the
output
transistors
to
establish
the
correct
class
of
operation.
It
is
also
usedtoprovide
temperature
stability.
Output
transistors
Q9
and
Q10
are
connected
in
a
complementary
symmetry
output
circuit.
In
this
circuit,
PNP
transistor
Q9
and
NPN
transistor
Q10
are
connected
in
a
class
B
emitter
-follower
configuration.
A
positive-going
signal
at
the
base
of
the output
transistors
will
cause
Q10
to
conduct,
charging
capacitor
C22
through
the
voice
coils
of
the
speakers.
When
the
signal
goes
negative,
Q10
will
be
cut
off
and
Q9
will
conduct,
discharging
capacitor
C22
back
through
the
voice
coils
of
the
speakers.
The
charge
and
discharge
of
capacitor
C22
causes
the
speaker
cones
to
move
in
and
out
and
convert
the
electrical
signal
back
to
sound.

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