Orban 674A - Highpass

Because
of
the
multiple
feedback
loops,
troubleshooting
the
resonator
is
usually
a
cut-and-try
affair.
Replace
IC205
first;
if this
does
not
solve
the
problem,
the
individual
components
must be
removed
and
tested
on an
impedance
bridge.
Of
course,
the
adjustable
controls
R243,
R24I
are
the
least
reliable
passive
components,
and
should be
investigated
first.
The
fixed
passive
components
are
highly
understressed,
and
failures
are
most
improbable.
EQ
IN/OUT
Switch:
The
final
summing
amplifier
associated
with the
band
8
equalizer
(1C 103)
can
be
switched
from
inverting
(EQ IN)
to
non-inverting
(EQ
OUT)
operation
in
order
to
maintain
consistent
input/output
polarity
as
S 10
1 ,
the
EQ
IN/OUT switch,
is
operated.
In EQ
OUT
mode,
S 10
IB
disconnects
the
output
of
the
equalizer
section
from
1C 103.
S 10
1 A
disconnects
the input
of
the band I
bandpass
resonator,
thus
defeating
band
I
equalization
but
retaining
IC2I7A,
with its
l8dB
gain.
S 10
1
A
simultaneously
connects
the
output
of
IC2I7A to
the
non-inverting
input
of
1C 103
through
voltage
divider
R
1 1
9,
R 1 2 1 .
The overall
gain of
1C 103
in
either
inverting or
non-inverting
modes
is
-4dB.
This compensates
for
the
4dB
gain
in
IC50IA and
IC50IB.
The
composite
capacitor
Cl ll
t
Cl
13,
Cl 15
eliminates any
DC offsets
which
may
have
accumulated
through
the
main
equalizers
while
preserving
maximum
audio
quality.
R I 17
assures
that
Cl
1
1,
Cl
13,
Cl 15
do
not build
up
a charge
in
the
OUT
mode,
which
could
cause a
pop
when
SI0I is
operated.
Rill
(OFFSET)
adjusts
the
quiescent
DC output
voltage
of
IC2I7A to
OmV,
assuring
that no
step change
of
DC
voltage
(and thus
no
click) will
occur
when
SI0I is
operated.
However,
if
SI0I is
operated
while
program
material
is
passing
through
the
equalizer,
a
click may
be
heard
because
the
program
is
interrupted
for
a
few
milliseconds
while
the
switch is
changing
state.
Highpass
Filter;
Output
Buffer:
The
highpass
filter consists
of
a "Sallen
and
Key"
positive feedback
active
filter
IC50IB
and
associated
circuitry.
A
discrete
complementary-symmetry
output
stage is
appended
to
IC50IB
to enable
this 1C to
drive
600 ohm
loads.
IC50IB
exhibits
4dB
non-inverting
gain in
its flat
response
region.
This
compensates
for
the
gain
loss in
1C 103.
To assure
constant
gain
regardless
of
the
position
of
HP
FILTER
IN/OUT
switch
S60I,
the
non-highpass-filtered
signal
driving the
OUT pole
of
S60I is
amplified
4dB
by
non-inverting
buffer
IC50IA.
The highpass
filter
is designed
to
provide a
"second-order
Butterworth"
response
with an
ultimate
slope
of
l2dB/octave.
R5I3,
R5I5 set
the
gain, and
thus the
"Q"
of
the filter.
R505,
R507,
R5II, C503,
C505,
C507,
C509
determine
the
tuning
and
further affect
the
"Q".
This
group
of
components
all
interact.
Therefore,
the best
way
to
troubleshoot
the
filter is:
a)
Replace
IC50I.
If this
does
not cure
the
problem,
then
b)
Check the
gain
in the
"flat"
part
of the
filter's
response
curve.
If this is
not
4.03dB
+2%,
R5I3,
R5
1 5
are
suspect.
If this
does not
cure,
then
c)
Check
the
other
passive
components
on
an
impedance
bridge.
In particular,
the
tracking
of
dual
pot
R507 is
important
to
assure
consistent
"Q"
as
the
filter
is
tuned.
R507 is
by far
the
most
suspect
component
because
it can
wear and
deteriorate
in
hostile
environments.
For
further
details,
see:
(Tobey,G.E.,
Graeme,
J.G.,
&
Huelsman,L.P.:
Operational
Amplifiers:
Design
and
Applications
,
pp.295-298
(New York,
McGraw-Hill,
1971)).
The
output
transistors Q603,
Q607
are
operated
as
emitter
followers
in a
class
AB
configuration.
R603,
R607
provide
local
DC
feedback
to
stabilize
the
quiescent bias
current
through
the
output
stage.
The
output
stage
is
biased by
diode-connected
28