Orban 674A - Orban 674 A Parts List; Obtaining Replacement Parts

transistors
Q60I,
Q605
which
are
thermally
connected
to
their
associated
output
transistors
to
provide
thermal
feedback
which
stabilizes
the
output
stage
against
thermal
runaway.
Current
limiting
to
short-circuit-protect
the
output
stage
is
provided
by
CR60I,
CR603
in
conjunction
with R603,
R605.
When
the
voltage
drop
across R603,
R605
exceeds
the
turn
on
voltage
of its
associated
diode
(about
0.55
volts),
then
the
diode
conducts,
shunting
the
drive
current
away
from
Q603, Q607
and
into
the
load,
thus
protecting
Q603,
Q607
from
burnout.
1C
50
IB,
the
driver,
is
loaded
by
R607,
R609.
The
junction
of R607
and R609
is
bootstrapped
to
the
current-booster
stage
output
by C60I,
and
thus
looks
to IC50IB
like
a
constant-current
(infinite
impedance)
load.
This
reduces
distortion
in
IC50IB.
The
output
is
connected
to
the
outside
world
through
the
load-isolating,
RF-
suppression
network
R509,
C5, Cl.
The
output
impedance
of
the
equalizer
is
thus
47
ohms
in
parallel
with
lOOOpF.
When
the
optional
output
transformer
is
installed,
R509
is
omitted.
Lowpass
Filter;
Output
Buffer:
This
filter
consists
of
IC60I
and
associated
components
arranged
in
a
"Sallen
and
Key"
unity-gain
positive
feedback
configuration.
The
filter
characteristic
is
l2dB/octave
Butterworth.
The
gain
of
IC60I
is
+1
in
the
flat
part
of its
frequency
range.
The
output
of
IC60I
is
coupled
to
a
discrete
output
stage
which
is
included
within
the
overall
feedback
loop,
and
which
enables
the
stage
to
drive
600
ohm
loads.
For
troubleshooting
hints
and
a
reference,
see
"Highpass
Filter;
Output
Buffer"
immediately
above.
Overload
Indicator:
The
output
of
each
main-signal-path
amplifier
in the
674A
is
connected
to
its
own
pair
of
diodes.
One
diode
is
connected
to
a +10
volt
bus
(created
by
voltage
divider
R7I9,
R72I);
the
other
diode
is
connected
to
a
-10
volt
bus
(created
by R7I3,
R7II).
If
the
instantaneous
output
of
any
amplifier
exceeds
+
10.6
volts,
then
the
appropriate
diode
will
conduct
and
couple
a
pulse
onto
one
of
the
busses,
which
are
relatively
high
impedance.
Positive-going
overload
pulses
are
fed
into
transistor
inverter
Q703
and
appear
at
Q703's
collector
amplified
and
inverted
so that
they
are
negative-going.
Negative
going
overload
pulses
are
connected
directly
to
Q703's
collector.
Thus
any
overload
appears
at
Q703's
collector
as
a
negative-going
pulse,
and
is
coupled
through
C703
to
IC70I
and
associated
circuitry,
connected
as
a
one-shot
multivibrator.
Ordinarily,
IC70I
is
held OFF
(pin 6
LOW)
because
R705
holds
IC70l's
inverting
input
at
a higher
voltage
than
voltage
divider
R707,
R709
holds
its
non-inverting
input.
A
negative-going
pulse
transmitted
through
C703
pulls
IC70l's
inverting
input
down,
thus
briefly
switching
IC70i’s
output
HIGH.
This
in
turn
pulls
IC70l's
non-
inverting
input
HIGH
through
R703,
C70I,
and
latches
IC70l's
output
HIGH
until
C70
1 can
discharge
through
R707,
R703,
R709,
which
normally
takes
about
200
milliseconds.
While
IC70l's
cxjtput
is
HIGH,
the
OVERLOAD
lamp
is
illuminated
through
R70I
and
Q70I,
connected
as
a Zener
diode.
Thus
very
fast
overloads
are
'time
stretched"
and
can
be
easily
seen.
Under
continuous
overload
conditions,
it is
normal
for
the
OVERLOAD
lamp
to
flash
on
and
off.
Power
Supply:
Unregulated
voltage
is
supplied
by
two
pairs
of
full
wave
diode
rectifiers
CR80I,
CR803
and
CR802,
CR804
operating
into
a pair
of
energy
storage
capacitors
C80I,
C802.
The
power
transformer
Tl
is
strappable
for
either
115
volt
or
230
volt
operation;
the
two
sections
of
the
primary
are
paralleled
for
115
volt
operation
and
connected
in
series
for
230
volt
operation.
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