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Excessive THD at 100Hz not present in the PROOF- mode test is usually caused
by problems in the bass timing module on Card # 5. Excessive THD at 500Hz is
often
caused
by
problems
in
the
master
timing
module
on
Card # 5. ( See
paragraph 2.d of Appendix B).
Atypical THD at
3kHz can be caused by several factors. Excessively high THD
can
be
caused
by
a failure
in
the
high- frequency
limiter
on
Card # 6 (see
paragraphs 4.a and 4.b in Appendix B), or
by failure
in the Hilbert- Transform
Clipper. ( 3kHz is still within the frequency range in which the Hilbert- Transform
Clipper
normally
produces
less
than
2%
THD
even
with
large
amounts
of
limiting.)
Please note that it is normal for the THD to increase as high as 2% between 4
and
5kHz
because of
the action
of the Hilbert- Transform
Clipper
as
discussed
immediately above.
4)
Now turn the HF LIMITING control to 10. Sweep the oscillator frequency up
from
lkHz,
and
determine
what
frequency
first
turns
the
front- panel
HF
LIMITING lamp ON. This frequency is typically 2.1kHz.
Steps ( 5)
and ( 6) below
provide a first- order
test of
the
dynamics
of
the
timing
circuitry in the compressor. However, there are many possible faults which these
tests will
not detect. These must be diagnosed by more sophisticated tests at the
factory.
5)
Turn
the BASS COUPLING control
to " 0". Switch the RELEASE
SHAPE
to
LIN, and make sure that the RELEASE TIME control
is at " 10". Make sure the
Loudness Controller is off. Set the oscillator frequency to 5kHz, and adjust the
oscillator
output
level
until
the
TOTAL
MASTER
G/R
meter
reads "- 5dB".
Abruptly reduce the oscillator output level 20dB, and measure the time required
for
the TOTAL
MASTER
G/R
meter
to
rise
from "- 5dB"
to "+ 5dB"
indicated
G/R. This time should be 7.5 seconds, + 1.5 seconds.
Switch
the RELEASE SHAPE to EXP, and repeat the test. The time should be
11 seconds, + 3 seconds.
6)
Change
the
oscillator
frequency
to
200Hz
and
adjust
the
oscillator
output
level
until
the
BASS
G/R
meter
reads "- 5dB"
G/R.
Abruptly
reduce
the
oscillator output level by 20dB, and measure the time required for the BASS G/R
meter
to
rise
from "- 5dB"
to "+ 5dB"
indicated
G/R.
This
time
should
be
3
seconds, + 0.75 second.
d)
Loudness Controller: The Loudness Controller
is
tested
by
applying
various
frequencies to the OPTIMOD-TV input and observing the level at the output of
the
compressor
with
the
normal
compressor
control
circuitry
defeated
so
that
only
the
Loudness
Controller
determines
the
amount
of
gain
reduction.
This
test can only be made as described if the Loudness Controller is
installed
in the main 8182A chassis and Jumper " A" on Card # 2 is in the correct ("FROM
CARD # 8/9")
position
for
this
configuration.
If
the
Loudness
Controller
is
installed in the 8182A/ST Accessory Chassis and you wish to perform this test,
it is necessary to temporarily reconfigure the system for single- chassis operation.
(Please refer to the 8182A/ST Manual.)
Set
the
operating
controls
as
follows ( controls
not
listed: DON'T
CARE):
CLIPPING
- 4
BASS COUPLING
10
RELEASE TIME
5
COMP PR/OP
PROOF
RELEASE SHAPE
EXP
LIMITER PR/OP
PROOF
0-4
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