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4-6
MAINTENANCE
Orban Model
464A
e)
Verify a
level
of
+14.0dBu ±2.0dB
at
TP7.
f)
Verify
that
the signal
at
TP7
is
a
sine
wave
of normal
appearance.
g)
Adjust
L
DIST
NULL
trimmer
R
124
for
minimum
THD
at
TP7.
Verify
that
THD
is
below 0.05%
in
a
20-20,
000Hz
bandwidth.
h)
Adjust
L GAIN trimmer
R129
to
produce
a
+14.0dBu ±0.2dB
level
at
TP7.
i)
Disconnect
the
audio
oscillator
from
the
LEFT
IN HI terminal
and connect
to
pin
9 of
IC5.
Set
the
oscillator's
frequency
to
1kHz
and
its
output
level to
-30dBu.
j)
Adjust
L
THUMP NULL
trimmer
R119
to
minimize
1kHz
feedthrough (observe
the distortion residual
on
the oscilloscope).
k)
Disconnect
the
audio
oscillator
from IC5 and
connect
it
to
the
LEFT
IN
Hi
terminal.
Set
the
oscillator's
frequency
to
1kHz
and
its
output
level to
OdBu.
l)
Increase the
audio
oscillator's
output
level
until
clipping
is
observed
at
TP7.
Verify
that the oscillator's
output
level
is
at least
+18dBu when
clipping
is first
seem
m) Mute
the
audio
oscillator
and
verify
that
there
is
no
"popcorn"
noise or
oscil-
lation.
n)
Check
the right
channel
VCA
stage
by
repeating the
above
for the right channel.
Use
the right
channel
controls, terminals,
and
trimmers
instead
of
the
left,
and
substitute
TP10
for
TP7, and
pin 12 of
IC5
for
pin
9 of
IC5.
o)
Disconnect
the
THD
analyzer, the
audio
voltmeter,
and
the oscilloscope
from
the
Co-Operator.
6)
Check
gate
control
circuits.
a)
Connect
the
audio
oscillator to
the
LEFT
IN HI
terminal,
and ground
the
LEFT
IN
LO
terminal.
Set
the
oscillator's
frequency
to
1kHz
and
its
output
level to
-lOdBu.
b) Set the
MODE
button
to
DUAL
and
turn the
L
GATE THRESH
control
fully
clock-
wise.
The
L
INPUT
ATTEN
control
should
still
be
turned
fully
clockwise.
c)
Mute
the
oscillator.
Verify
that
the
L
GATE THRESH
indicator (the
LED
next
to
the
L GATE
THRESH
control)
lights.
d) Increase the
audio
oscillator's
output
level
until
the
L
gate thresh
indicator
goes
out.
Verify
that
this
occurs
when
the
level
at test
point
TP7
is
+5.0dBu
±1.0dB.
e)
Turn
the
L
GATE THRESH
control
fully
counterclockwise.
f)
Mute
the
oscillator,
verify
that
the
L GATE THRESH
indicator
does
not
light,
then
restore the signal.
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