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Co-Operator
MAINTENANCE
4-9
b)
c)
d)
e)
f)
g)
h)
i)
j)
k)
l )
m)
n)
Set
controls,
trimmers,
and jumpers
as
follows:
L
HF
LIMIT
L
and
R PRE-EMPHASIS
HF
LIMITER DIST
NULL
TRIMMER
R182
FET
BIAS
TRIMMER
R204
JUMPER A
JUMPER
B
OPERATE
150
CENTER
FULLY
CLOCKWISE
DEFEAT
FLAT
Connect
a
THD
analyzer,
audio
voltmeter,
and
oscilloscope
to
test
point
TP8.
Adjust
the
L INPUT ATTEN
to
produce
a level
of O.OdBu ±0.5dB
at
TP8.
Verify
that
the signal
at
TP8
is
a
sine
wave
of normal
appearance.
Mute
the
oscillator,
verify
that there
is
no
"popcorn"
noise or
oscillation,
then
restore the
signal.
Slowly
turn
L
FET BIAS trimmer
R204
counterclockwise
until
the level
at
TP8
begins
to
decrease.
Then
turn
R204
clockwise
until
the level
at
TP8
stops
increasing.
Turn
R204
clockwise about Vio-tum
further.
Set
the
oscillator's
frequency
to
5kHz
and
its
output
level to
OdBu.
Adjust
L HF
limiter DIST
null
trimmer
R182
for
minimum THD.
Verify
that
THD
does not exceed
0.06%
in
a
20-20,
000Hz
bandwidth
at
TP8.
Set
the
oscillator's
frequency
to
1kHz, and
verify that the
L HF
LIMIT
indicator
does
not
light.
Set
the
oscillator's
frequency
to
10kHz, and
verify that the
L HF
LIMIT
indicator
lights.
Set
the
L HF
LIMIT
button
to
DEFEAT, and
verify that the
L HF
LIMIT
indicator
goes
out.
Observe
the
level
at
TP8,
then
set
the
audio
oscillator's
frequency
to
1kHz,
and
verify
that
the level
at
TP8
increases
by
about lOdB.
Turn
the
L INPUT ATTEN
fully
counterclockwise.
9)
Check
high-frequency
limiters,
(part
2)
a)
Place
jumper
A
in
the
"OPERATE"
position,
and
jumper
B
in
the
"PRE-
EMPHASIZED"
position.
b)
Set
the
oscillator's
frequency
to
10kHz
and
its
output
level
to
+10dBu.
c)
Slowly
turn the
L INPUT ATTEN
control clockwise,
and
verify
(on
the
oscillo-
scope)
that
symmetrical
clipping occurs
at
5.5V
peak.
5.5V peak
is
approximately
equivalent
to
4V
RMS
or
+14dBu.
d)
Place
jumper
A
in
the
"DEFEAT"
position,
and
verify that the clipping disap-
pears.
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