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Signal
tracing
in
OPTIMOD-TV
is
facilitated
by
the
fact
that
much
of
the
circuitry is duplicated for stereo, and is arranged so that the bad channel can be
readily
compared
with
the
good
one,
which
serves
as
a " normal"
reference.
Power Supply Tests: Some circuitry is common to both channels, and failures will
therefore affect both channels in a symmetrical way. In particular, problems in the
power
supply may
affect
many circuits simultaneously. For this reason, the first
step
in
any
troubleshooting
procedure
is
to check
the
power
supply
for
normal
output. Gross changes in power supply voltage can be detected with the "+ 15VDC"
and "- 15VDC"
positions
on
the VU
meter.
Normal
readings
are
OVU + 0.5VU.
If
normal
readings are obtained, skip to the next
section on VU Meter Techniques.
If
either "+" or "-" power
supply output
is significantly low,
it
could
indicate
a
defect
in
the
supply
itself.
But
it
is
more likely
to
indicate
a shorted
IC
or
capacitor somewhere in the circuit that is overloading the supply and causing it to
current- limit.
The power supply
is electronically protected against
excessive
current
demand by
other
parts
of
the
circuitry.
If
a failure
causes
a high
current
demand
on
the
power
supply, its
output
voltage will drop
as
far
as necessary to reduce
output
current
to
approximately
0.75A.
If the
power
supply
voltage
is
observed
to
be
abnormally low, unplug each circuit
card in turn and check
if the power
supply
recovers by observing the "- 15VDC" meter position. ( The negative regulator tracks
the + 15V supply. So the - 15V supply will go down if the + 15V supply does, even if
the - 15V
supply
or
load
is
completely
normal.
A normal "- 15VDC"
reading
thus
assures
a normal "+ 15VDC"
reading.)
If
recovery
occurs,
then
troubleshoot
the
unplugged board. Ordinarily, the defective component will become very hot, and is
easily detected by touch. ( Wet your finger first to avoid burns!)
If
all cards are removed and an undervoltage problem does not disappear, examine
the meter card, motherboard, and chassis wiring before suspecting the supply itself.
(A wiring problem will be indicated by an ohmmeter's indicating very low resistance
between the "+ 15V" or "- 15V" power busses with AC power OFF.)
Even
if
power supply
voltages appear normal
on
the VU meter, subtle
problems
such
as
hum, noise, or oscillation
may still exist with the supply.
To check for
this, test the regulated DC with a well- calibrated DVM, scope, and AC VTV!v1 with
20-20kHz
bandpass
filter.
Voltages
should
be + 15.00V + 0.075V, - 15.00V + 0.375V.
Ripple must be less than 2mV r.m.s., 20-20,000Hz. There must be no high frequency
oscillation.
VU
Meter Technique: If
one
channel goes
dead,
the VU meter
provides a means
for fast signal tracing. Note, however, that problems other than gross gain changes
or
total
failure
to
pass
signal
may
not
be
detected
by
the
meter
alone.
First, switch through the first eight VU meter functions ( which monitor the audio
processing) to see where the signal disappears ( or the VU meter pegs, implying that
a defective IC opamp has latched up to the power supply rail.) Refer to the block
diagram ( p. 3-25) to locate the exact points in the signal path monitored by the
meter.
If
the
signal
is
normal
at
the
input
terminals
and
abnormal
in
either
INPUT
BUFFER position, then the problem
lies
with Card # 3 ( left
channel) or Card # 4
(right
channel),
or
with
the
incoming
audio
circuitry
prior
to
these
cards.
If
the
signal
is
normal
at
the
INPUT
BUFFER
positions
but
abnormal
in
the
COMPR
OUT position, then the problem probably lies with Card # 3 ( left channel),
with Card # 4 ( right channel), or, if both channels are equally affected, with Card
#5.
F-2
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