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g)
Observe
the
output
of IC307A ( pin K of
the card connector) with
the AC
VTVM,
and adjust R376 ( MASTER VCA GAIN) to produce + 2.0dBm at this point.
h)
Readjust the oscillator frequency to 35Hz. If necessary, readjust the output
level of the oscillator until it is once again - 15dBm at TP1.
i)
Adjust R377 ( BASS VCA GAIN) until the AC VTVM indicates + 2.0dBm at pin
K. The gains of both " Master" and "Bass" VCA's are now standardized, assuring
card interchangeability.
NOTE:
The
following
distortion
and
balance
adjustments
are
made
without
disturbing the resistors jumpered into place in the steps above.
j)
Without
disturbing
the
oscillator
output
level,
set
its
frequency
to
5kHz.
Switch the AC VTVM into its distortion- measuring mode, and measure the THD.
Adjust R336 ( MASTER DIST NULL) to null the THD. It should not exceed 0.04%
if
a noise- limiting
80kHz
lowpass
filter
is
employed
in
the
measurement.
CAUTION!
Any stray audio picked up on the leads of the 137K jumper resistor
will
cross- multiply
with
the
desired
signal
in
the
VCA,
and
will
produce second- harmonic distortion which cannot be nulled with the
MASTER
DIST
NULL
control.
It
may
be
necessary
to
bypass the
R333 side of the 137K resistor to ground with a tantalum capacitor
larger
than
5uF
and
15VDC.
Ground
the "+"
terminal
of
the
capacitor.
k)
Set
the oscillator frequency to 50Hz. Measure the THD as above, and adjust
R351 (BASS DIST NULL) to null it. It should not exceed 0.04%.
1)
Remove the oscillator from the OPTIMOD-TV input. Ground the low side of
the
oscillator output
to
the OPTIMOD-TV
chassis and,
using a pair of jumper
leads, connect the high side of the oscillator output through a luF film capacitor
to the side of R333 away from IC305. ( The 137K resistor is already connected to
this point. Don't disturb it.) Set the output frequency of the oscillator to 100Hz,
and
its
level
to
produce
approximately 0.25V
rms
at
its
output.
Observe
the
output
of IC307A
with
the
AC
VTVM
at
high
gain.
You
will
see
a distorted
feedthrough
component
from
the
oscillator.
Adjust
R331 ( MASTER
VCA
BALANCE) to null the feedthrough.
m)
Move the lead from the luF capacitor from R333 to the corresponding side
of
R348.
Do not disturb
the resistor already
connected
to
this point.
Set the
oscillator output to approximately 0.25V rms. Continue to observe the output of
IC307A,
and
adjust
R346 ( BASS
VCA
BALANCE)
to
null
the
feedthrough
component observed.
n)
Remove
all jumper leads connected to Card # 3, and remove Card # 3 from
the extender.
o)
Insert Card # 4 in the extender ( still in slot # 3), and repeat steps e through
n.
E-3
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