Orban 424A Operating Manual page 26

Signal
Processing Circuitry:
Equipment
Required:
1)
Oscilloscope with
DC-coupled
display
2)
2O-2O,0OOHz bandpass
filter,
l8dB/octave slopes
3)
VTVM
or
DVM
4)
Harmonic
distortion analyzer with
built-in
400Hz
and
80kHz
filters
and
residual
THD
below 0.0015%
5)
Low-distortion oscillator
with
residual
THD
below 0.0015%
6)
OPTIONAL:
General-purpose
storage oscilloscope
(A Sound Technology I700A
or
H-P
339
will satisfy
(4)
and
(5))
For
the following
tests,
a
600
ohm
load
must be provided
across the
424A
output.
A
620
ohm
1/2
watt
5%
carbon
resistor
is
suitable.
IN
THE CASE OF THE
424A,
THE
TEST
PROCEDURE BELOW SHOULD
BE
FOLLOWED FROM
BEGINNING
TO END FOR
CHANNEL
"A",
THEN REPEATED
FOR CHANNEL
"B".
I)
Gain and
Distortion Tests and
VCA
Alignment
a)
Connect
the
oscillator to the
424A
input.
Connect
the
THD
meter/AC VTVM
between
the
424A
(+)
OUTPUT
and
circuit
ground. Set the controls
as follows:
INPUT
ATTEN
RATIO
ATTACK
TIME
RELEASE SHAPE
RELEASE
TIME
GATE THRESH
OUTPUT
TRIM
IDLE
GAIN
COMP/LIM OP/DEFEAT
DE-ESSER OP/DEFEAT
DE-ESSER THRESH
OUTPUT ATTEN
COUPLED/INDEP SWITCH
b)
Verify that the
GATED
lamp
is
ON,
indicating
normal
operation of the
gating circuitry
in
the
DEFEATed
mode
of
the compressor.
c)
Set
the oscillator
frequency
to
I
kHz
and
its
output level to
-20dBm.
Verifying
that
all
of
the control
settings
are
as specified
in (a)
above,
measure
the
output
level of
the
424A.
Turn
R60 (GAIN TRIM)
counterclockwise
until this level
no longer decreases, indicating saturation
of
the
VCA
gain control
circuitry.
Then
turn
R60
clockwise
until
the output
level of
the
424A
increases by
0.5dB.
d)
Set
the
IDLE
GAIN
control
to
"-I0". Verify that the
G/R
METER
reads lOdB
G/R,
+2dB.
e)
(424A
only):
Temporarily switch the
COUPLED/INDEP
switch to
COUPLED,
and
verify that the
Channel B
G/R
METER
also
reads lOdB
G/R,
+2dB.
Restore the switch
to
INDEP.
f)
Set the oscillator
frequency
to
I
kHz
and advance
its
output
level until
the
424A
VCA
LEVEL
meter
indicates
"-6".
10
infinity
5
lin
5
off
0
-25
defeat
defeat
"less"
fully clockwise
independent
22