Synthesize Mode - Orban 275A Operating Manual

c)
Synthesize
Mode
Alignment,
Noise,
And
Distortion
Connect
the oscillator to
the
275A
left
input only
.
Turn
the
SEPARATION
control
fully
counterclockwise. Press the
SYNTH
FROM
LEFT
button and
make
sure that the
corresponding red
lamp
lights.
Press the
NOISE
REDUCTION
DEFEAT
button and
make
sure that the
corresponding green
lamp
is
off.
Suppress the oscillator output.
Connect
the
DVM
to
TP3
and adjust
R132 (Thump
Null
Trimmer)
for
0.000
VDC.
Set the oscillator for
1kHz
and
+20dBu
(as
observed differentially at the
275A
input
terminals).
Adjust
R134 (GAIN
Trim)
for
+18dBu (measured
differentially)
at the
275A
left
output.
Measure
THD
at
20Hz, 1kHz,
15kHz
with
+18dBu
at
the
275A
output. Verify that
the
THD
does not
exceed 0.3%.
Suppress the
oscillator
output and
measure
the noise at the
275A
output. Verify
that
it
does not
exceed -67dBu
in
a
20-20kHz bandwidth.
(NOTE:
In a
properly
operating the
275A, the
amount
of noise
and
distortion
produced
in
either Synthesize
mode
is
a function of IC12's
performance,
since
it
is in
the signal path regardless of
whether
the
Noise Reduction
is
ON
or
OFF
.
If
distortion or noise
is
excessive, the
most probable cause
is
a defective 1C12.)
d)
Noise Reduction Test
Press the
SYNTHESIZE
FROM
LEFT, NOISE
REDUCTION
OPERATE,
and
NARROW
buttons on the
275A.
Verify
LED
responses.
The
SEPARATION
control
should
still
be
at
"0". Connect
the
5L4N
Tracking Generator output
to
the
275A
left
input.
Adjust
the output level of the
tracking generator until the
EXPANSION
lamp
just
barely
comes
on.
Observe
the
swept response
at
the
275A
left
output.
Fig.
5-1 shows
the
response
as
the input
is
stepped
downward
in
5dB
steps, starting at the
level
which
just lights
the
EXPANSION
lamp.
Note
the
downward
broadband expansion and the high frequency
rolloff.
Fig.
5-2 shows
the
frequency response
in
an
expanded
(2dB/div) scale at
an input
level
lOdB below
the expansion threshold.
Fig.
5-3 shows
the
normal frequency response of
the circuit
when
the
NOISE
REDUCTION
is
DEFEATed.
Note
that
it is
down approximately -2dB
at
20kHz.
e)
Synthesize
Mode Frequency Response
Turn
the
SEPARATION
control
to
"100"
(fully
clockwise). Press
the
NOISE
REDUCTION DEFEAT
button.
Observe
the
275A
left
output with the
spectrum
analyzer
set for
20-20kHz
log
span, lOdB/div.
Set the tracking generator output
level to
permit the
swept
response
to
be seen
easily.
Then observe
the right
output.
Fig.
5-4 shows
the
expected frequency responses
of the
two
channels
overlaid.
Note
the
complementary
comb
filter
characteristic.
Press
WIDE
button on the
275A. Observe the
left
and
right
outputs,
and verify
that they are similar to Fig. 5-5,
which shows
both channels overlaid.
5-3