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7)
Low
Frequency Spectrum Analyzer
with Tracking
Generator
(e.g.
Tektronix
5L4N
in
5111
Bistable
Storage
Mainframe;
an
FFT-based spectrum
analyzer
with
signal
source
[HP5561A],
or a
sweep
generator and
X/Y
oscilloscope
could
also
be
used)
8)
An
audio preamplifier with
BALANCE
control
(or
other
means
for
pan-
potting a
signal
continuously
from
right to
left)
[A
Sound Technology
17Q0A
or
H-P
339
will
satisfy
(4)
and
(5).]
NOTE:
The term "dBu"
is
used
in
the following
procedure.
In
a
600-ohm
circuit,
"dBu" and
"dBm"
are identical.
However,
"dBu", unlike
"dBm",
is
not
dependent
on
circuit
impedance:
it
is
simply
a
voltage.
Read
"dBu" on
the
"dBm/600ohms"
scale
of
your audio voltmeter.
Further, the
levels specified are read
differentially
across the input
or
output terminals
as
appropriate.
The
275A's outputs are balanced-to-ground.
If
your
AC
Voltmeter/Distortion Analyzer
is
not
equipped with
a
balanced
input,
measure
275A
output
levels
between
the 275A's "HI" output terminal
and
its
chassis
ground, subtracting
6dBu from
all
275A
output
levels
specified
in
the
procedure below.
1)
Audio Path Alignment
And
Verification
a)
Bypass
Mode
Gain Trim
And
Performance
Evaluation
Press the
AUTO
POLARITY ON
and
BYPASS
buttons. Verify that the
green
AUTO
POLARITY
and
red
BYPASS
lamps
light.
Connect
the
oscillator to
the
275A
left
input.
Load
the
275A
left
output with a
604-ohm
+1%
resistor.
Set the
oscillator
frequency
to
1kHz
and
its
output
level to
+4dBu
as
measured
across
the
275A
input.
Adjust
R145 (LEFT
GAIN
trim) for
+4dBu
as
measured
across the
left
output
of the
275A.
Increase the
oscillator level to
produce
+20dBu
at
the
275A
output.
Measure
the
harmonic
distortion
at
20Hz, 1kHz, and 15kHz.
Verify that
THD
does not
exceed
0.02%
at
any frequency.
Suppress the
oscillator,
and
measure
RMS
noise at the
275A
output. Verify that
noise
does not
exceed -80dBu
in a
20-20kHz
bandwidth.
Repeat
all
of step
a)
for
the
right
channel.
b)
Auto
Polarity Test
Leave
the
oscillator
connected
to
the right input of
the
275A.
Connect
the
oscillator to
the
left
input of the
275A
in
reverse
polarity,
and
verify that the
INVERTED
lamp
lights.
Then
reverse the
polarity of the
connection
to
the
left
input,
and
verify that
the
INVERTED
lamp
turns
off.
(If
the
circuit
does not
appear
to
be working, suppress the
oscillator
output
for 5
seconds
to reset the circuit.
Then
repeat
the
test.)
5-2
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