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
To Next Page
Loading...