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8.a)
Unregulated
Power
Supply:
( on
chassis
outside
RF - tight
enclosure)
The
unregulated
power
supply
is
wholly
conventional.
It
consists
of
a dual-
primary
transformer T101,
two
full- wave
rectifiers
CR101, CR102
and
CR103,
CR104, and two energy storage capacitors C101, C102.
T101 's primary may be
switched for
115 Volt
operation by paralleling
its two
primaries, or for 230 volt operation by connecting its two primaries in series. RF
filtering
is provided on the AC line by means of FL101. In addition, VHF and
UHF RF
is filtered from the unregulated DC supply lines as they enter the main
chassis
by
means
of
C103,
C104,
C105,
C106,
C107,
L101,
L102.
The
RF
suppression scheme divides the chassis into three major sections. The section to
the
left ( unregulated
power
supply
chamber)
contains
the
AC
wiring
and
the
unregulated power supply, and is assumed to contain some RF. The card cage, to
the right, uses RF suppression on each line entering or leaving the area, and is
thus RF - free. ( The RF filter box, on the rear panel, interfaces the audio input
and
output
lines
with
the
outside
world.
It
contains
the
input
pads.
Its
connections to the main RF - tight compartment are all RF - filtered.)
8.b) + 15 Volt Regulator: ( on Card /MS -- rear chassis apron)
The + 15
Volt
regulator
is
the
main
reference
for
all
other
voltages
in
the
OPTIMOD-TV
system.
It
employs
a 723C
IC
voltage
regulator
IC101
in
conjunction
with
an
external
series- pass
transistor
Q101.
This
transistor
is
mounted
on
the
rear
apron
of
the
chassis,
which
serves
as
a heat
sink.
The 723C contains a reference voltage source, an opamp ( externally compensated
by means of C109 to prevent oscillation), and a current limiting transistor. The
reference voltage ( nominally + 7.15 Volts) is developed at pin 6. C108 filters high
frequency
noise
from
the
reference
voltage.
The
reference
voltage
is
directly
connected to the non- inverting input of the internal opamp, pin 5. Voltage divider
R105,
R106,
R107
develops
a precise
fraction
of
the
output
voltage
of
the
regulator at the wiper of R106. R106 adjusts this fraction. The wiper of R106 is
connected to
the inverting input
of IC101's internal
opamp. Negative
feedback
thus
forces
the
voltage
at
the
wiper
of
R106
to
be
equal
to
the
reference
voltage. Thus the output voltage of the regulator is always the reference voltage
divided by the voltage divider gain.
The output
current
flowing through Q101
develops a voltage
drop across R103.
When the current exceeds approximately 3/4 Amp, said voltage drop is sufficient
to turn on the current- limit transistor inside IC101, whose base- emitter junction
is
connected to pins 2 and 3 of IC101. The current- limit transistor then shunts
base
drive
current
from
the
external
series- pass
transistor
Q101
and
prevents
damage due to overheating.
If
a catastrophic failure in the + 15 Volt regulator causes it to lose control over
its
output
voltage, the rest of the circuitry must be protected against the full
unregulated
voltage,
or
the
entire
system
will
be
severely
damaged.
This
protection
is provided by zener
diode VR101, CR105,
and 1 Amp fast-blo
fuse
F102.
In
the event that the regulator loses control of the output voltage, VR101 will
conduct and limit the output voltage to approximately 16.5 Volts, which will not
damage
the
system.
Extremely
large
amounts
of
current
will
flow
in VR101.
However,
before
VR101
is
damaged,
this
current
will
blow
F102,
thus
disconnecting the circuitry from the unregulated supply. VR101's clamping action
will
also prevent the negative tracking supply from going any higher than - 16.5
Volts.
If the regulator
is operating properly, the current limiting circuitry
will
prevent
F102
from
blowing
even
if
the
regulator
output
is
short-circuited.
R-1 3
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