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.
23
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