Orban 424A Operating Manual page 27

g)
Measure
the
THD
at the
424A
output and
adjust
R26
(DISTORTION NULL)
to
minimize
it.
THD
should not
exceed 0.03%
if
measured
using a
20-20kHz
bandpass
filter
to
minimize
the effect of supersonic noise
upon
the distortion
reading.
h)
Temporarily reduce
the oscillator output
level
lOdB. Set the
424A
IDLE
GAIN
control
to
"0",
and
verify that the
424A
output
level
is
increased
lOdB
+_2dB.
Then restore the
IDLE
GAIN
to "-I0".
i)
Disconnect
the oscillator
from
the
424A
input,
and connect
it
to
the wiper
of
R64
(OUTPUT
TRIM).
Set the
oscillator
frequency
to
100Hz, and adjust the
oscillator
output
level until a distorted
100Hz
feedthrough
component
can be
seen on
the
scope monitoring the
AC
VTVM.
Adjust
RI9
(THUMP
NULL)
to
minimize
the
amount
of
feedthrough.
j)
Disconnect
the oscillator
from
the wiper
of
R64. Connect
the
DVM
to
the
output
of IC9B. Adjust
R28 (DE-ESSER
THUMP
NULL)
to secure
a reading of
OVDC.
2)
Noise Test
a)
Suppress
the oscillator and turn the
424A INPUT
ATTEN
to
"0". Measure
the
noise
at the
424A
output through
a
20-20kHz
bandpass
filter
with l8dB/oct
or
greater
slopes.
The
noise should not
exceed -68dBm.
NOTE:
If
this
specification
is
not
met, be
sure that
hum
or
RF
pickup due
to
poor
test
equipment
configuration
or
grounding
is
not causing
falsely high
readings.
3)
De-esser Operation
a)
Verify that the de-esser
THRESHOLD
control
is
set at "less",
and
that the
de-esser
OP/DEFEAT
switch
is
DEFEATED.
b)
Set the
oscillator
frequency
to 10kHz,
and
set
its
level to
produce
+
1
2dBm
at the
424A
output.
c)
Switch
the de-esser
OP/DEFEAT
switch
to
OP, and
verify that the
424A
output
level
is
reduced
to
+6dBm,
+|dB.
d)
Verify that
the
NORM
de-essing
lamp
is
ON.
Adjust the de-esser
THRESHOLD
control clockwise,
and
verify that the
HEAVY
de-essing
lamp
comes
on
when
the control
is
set at
approximately
"4".
e)
Adjust the
424A
de-esser
THRESHOLD
control
to
"more". Switch the
oscillator
frequency
to
3kHz, and
verify that the
424A
output
level
becomes
+7.0dBm,
+ldB.
f)
Switch the de-esser
OP/DEFEAT
switch
to
DEFEAT.
4)
Compressor/Limiter Operate Mode
Tests
a)
Suppress the
oscillator output. Verify that the
GATE THRESH
control
is
OFF
.
Switch
the
COMP/LIM OP/DEFEAT
switch
to
OP, and
verify that the
G/R
meter
recovers
to "0".
b)
Set the
oscillator frequency
to 100Hz,
and advance
the oscillator output
level
until the
424A G/R
meter
reads
20dB G/R. Measure
the
THD
at
the
424A
output, and
verify that
it is
less
than 0.03%. Observe
the distortion output
from
the
THD
analyzer on a scope, and
verify that the
distortion residual
looks
like Fig.
D-l. Suppress
the
oscillator output,
and
verify that the time
required
for
the
G/R
meter
to
move
from 20dB
to lOdB
G/R
is
2.0seconds,
+0.5seconds.
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