8. Temporarily remove the amplifier line cord
from the Variac and
turn
on
and adjust the
Variac
for
120
volts output. Turn off the
Variac, and reconnect the amplifier line cord.
9. Turn
on
the Variac, applying the full 120-volt
line to the amplifier, while observing the
VTVM.
If
the meter pointer swings off-scale
in either the positive
or
negative direction,
remove power and refer to the Trouble
Analysis Section.
10. If the meter indicates on-scale, but not at the
zero center established in step no. 6, remove
the access cover screw
(See
Figure 4) and
adjust potentiometer R19, using a small,
narrow-blade screwdriver,
so
that
a zero
cen-
ter
scale indication
is
obtained. This repre-
sents zero volts,
de.
D.
TOTAL
HUM
AND
NOISE
TEST
1. Connect a shorting plug to the INPUT jack
of amplifier module.
2. Connect
an
8-ohm resistive load and a dis-
tortion analyzer across the LOUDSPEAKER
terminals.
3. Set the distortion analyzer controls for volt-
age
measurement, and apply power to the
amplifier.
4. If the analyzer indicates greater than
0.0007
volts
(-90
dB), refer to the Trouble Analysis
Section. Typical noise values are
0.0002
volt
(-100
dB).
E.
OUTPUT
CLIPPING
TEST
6
1. Connect the audio generator to the INPUT
jack of
amplifier
module.
Set
frequency to
2000
Hz.
2. Connect the 4-ohm resistive load and the dis-
tortion analyzer to the LOUDSPEAKER ter-
minals.
3.
Set
distortion analyzer controls for voltage
measurement
on
the 30-volt scale.
4.
Tum
on
the amplifier and increase the out-
put of the audio generator, until clipping oc-
curs,
as
displayed
on
the oscilloscope.
See
Figure
5.
5. Vary the
output
of the audio generator while
observing the oscilloscope display. Verify
that the distortion analyzer indication
is
be-
tween 17 .32 and 18.0 volts at the point where
clipping
is
barely discernible.
If
the clipping-
point voltage is below 17.32 volts or above
18.0 volts, refer to the Output Trimming
Procedure Section of this manual.
6. Set the distortion analyzer controls
for
dis-
tortion measurement
on
the
10%
scale, at
2000
Hz.
7. Adjust the controls of distortion analyzer,
if
required, to obtain
an
oscilloscope display
similar
to
that
shown in Figure 6.
8. Observe the sharp clipping spikes at each
end of the pattern.
As
the audio generator
output is varied slightly, the spikes should
become visible at the same time, and should
have equal amplitudes.
If
the spikes do not
appear simultaneously,
or
have noticeably
different amplitudes, refer
to
the Output
Trimming Procedure.
F.
HARMONIC
DISTORTION
TEST
1. Connect the audio generator to the INPUT
jack of the amplifier module.
2.
Connect the 4-ohm resistive load and the
distortion analyzer to the LOUDSPEAKER
terminals.
3.
Set the frequency controls
of
the audio gen-
erator and the distortion analyzer to
2000
Hz.
4. Set the controls of the analyzer for voltage
measurement
on
the 30-volt scale.
5.
Adjust the generator output level until the
analyzer meter indicates 16.73 volts. (This
represents 70 watts into a 4-ohm load.)
6.
Measure the total harmonic distortion with
the analyzer and verify that
it
is less than
0.1
%.
See
Figure 7.
7.
Set
the frequency controls of the generator
and the analyzer to
20,000
Hz
and repeat
steps 4,
5,
and
6.
See
Figure 8.
8.
Set the frequency controls of the generator
and the analyzer to 20
Hz
and repeat steps
4,
5,
and
6.
See
Figure
9.
G.
PARASITIC
OSCILLATION
AND
SHORT
CIRCUIT
TEST.
1.
Set frequency controls of audio generator and
distortion analyzer to 20
Hz.
2. Set distortion analyzer controls
for
distortion
measuremenl
on
the 10 % scale.
3. Connect the 4-ohm resistive load and the dis-
tortion analyzer to the LOUDSPEAKER ter-
minals of the amplifier.
4. Adjust the generator output level until clip-
ping occurs,
as
displayed
by
the oscilloscope.
See
Figure 10.
5.
Verify
that
the display
is
free of parasitic
oscillations. If parasitics appear,
as
shown
in
Figure 11, refer to the Trouble Analysis
Section.
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