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CATALOG OF TYPICAL SYMPTOMS
AND PROBABLE CAUSES
This
troubleshooting
guide
is
a catalog
of
some
possible
failure
modes
in
OPTIMOD-TV.
It
should be
used in conjunction with Appendices A and B to
aid
troubleshooting at the component level.
ALWAYS
BE
SURE
THAT
THE
PROBLEM IS NOT
IN
THE
SOURCE
MATERIAL
FEEDING OPTIMOD-TV.
Whistle is heard on air.
1.
Power supply oscillation. Suspect C111, C112, IC101, IC102.
2.
Whistle on one stereo channel only probably due to oscillating IC. Use signal
tracing techniques to isolate defective IC.
Buzz or hum.
1.
Improper
grounding.
Chassis
not
properly
grounded
to
rack.
Circuit
and
chassis
grounds
connected
through
excessively
long
path.
No
direct
connection
between OPTIMOD-TV circuit ground and circuit ground of exciter with balanced
input.
2.
RFI.
Improve
grounding
scheme.
Relocate
OPTIMOD-TV
chassis.
Change
length of input or output cables to retune them.
3.
Low line voltage causing regulator to drop out and pass ripple.
4.
C101, C102
in unregulated power supply failed, resulting in extremely
high
ripple.
Power
supply
regulator
drops
out
on
each
ripple
cycle
which
instantaneously goes lower than 17.5 volts.
Loss of modulation control.
1.
Make sure LIMITER PROOF/OPERATE switch ( on Card # 6) is in OPERATE.
2.
Check for tightly- controlled peak levels at
rear- panel
TEST jacks.
If levels
not well- controlled, check + 4.2V supply on Card # 6.
3.
If levels are well- controlled, check connection to aural exciter, including STL
(if used). Inadequate phase linearity and/or frequency response in the circuit path
after
OPTIMOD-TV
can
change
peak
levels,
causing
overshoots
and
loss
of
modulation
control.
This
problem
will
ordinarily
appear
immediately
upon
installation.
Bass incorrectly balanced.
1.
It
is
normal
when
operating
OPTIMOD-TV " independent"
to
have
it
accentuate bass on many different types of program material. If you want the
frequency
balance
between " Air"
and " Program"
to
be
substantially
identical,
operate the BASS COUPLING control closer to " wideband".
2.
Possible misalignment or failure in exponential converter circuitry for either
"Master" or " Bass" compressors. This will cause frequency response to be non- flat
even in PROOF mode. If this is the case, check circuitry associated with IC501,
IC502, IC510, IC511.
3.
Failure
in
Input
Conditioning
Filter ( on
Cards # 3 and # 4).
This
will
be
revealed in PROOF mode.
4.
Failure in either " Bass" or " Master" VCA, causing gain shift.
Insufficient high frequency response.
1.
Due
to
the
preemphasis curve, some high frequency loss is inevitable when
OPTIMOD-TV is operated aggressively for maximum loudness ( i.e., large amounts
of
clipping,
and
fast
release
time).
To
obtain
more
highs,
back
off
both
the
CLIPPING and RELEASE TIME controls.
2.
In " independent" mode,
the
increase
in bass response with certain program
material
may
cause
an
apparent
loss
of
highs.
Try
operating " wideband"
temporarily
to
see
if
the
highs
are
then
balanced
like
the
input
material.
F-7
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