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SECTION
5:
Alignment
And
Performance
Evaluation
1
.
GENERAL
This section provides
a series of
thorough bench
tests
which
will
usually verify
whether
or
not the
275A
is
operating normally.
The
275A
is
aligned
as
these
tests are
performed.
The
275A
has
five
trimmers
(which
standardize the
gains of
the
left
and
right
channels
in
Bypass
mode,
standardize the
gain of the synthesizer
relative to
"Bypass",
determine
the threshold of
mono
detection
in
the
automatic
recognition
circuit,
and
null
DC
offset).
Alignment
is
ordinarily
required only
when
IC12, IC51,
or
IC52
is
replaced.
Refer
to
the
drawings
in
Appendix E
for
the location of
test points
and
trimpots.
2.
POWER
SUPPLY
Equipment
Required:
1)
VTVM
or
DVM
2)
Oscilloscope
The
following
tests
will
verify
correct operation
of
the
Power
Supply:
1)
Unregulated
Power
Supplies
Using
the
DC
voltmeter,
measure
the voltage
from
circuit
ground
to
both
positive
and negative unregulated
supplies.
This
can be
readily
measured
across the
two
large
filter
capacitors. This voltage
may
be
expected
to
vary widely
depending on
line
voltage;
it
should
measure between
+18 and +26
volts
DC.
2)
Voltage Regulators
Measure
the voltage
at
TP4
(+15V)
and
TP6
(-15V).
If
the absolute value
of either
voltage
exceeds
15.75
VDC,
it
implies that
its
associated IC regulator
is
defective.
If
the absolute value
of either
voltage
is
less
than
14.25
VDC,
refer
to
Power
Supply
in
Section
6 for
troubleshooting
hints.
3)
Ripple
and Noise
Using the
oscilloscope,
measure
the ripple
and
noise
on
the regulated positive
and
negative
power
buses.
Ripple and
noise should
be
less
than
4mV
peak on each
bus.
3.
SIGNAL PROCESSING
CIRCUITRY
Equipment
Required:
1)
Oscilloscope with
DC-coupled
display
2)
20-20,
000Hz
bandpass
filter,
18dB/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)
A
pair
of
604-ohm
+1%
resistors
5-1
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