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Under
certain
unusual
circumstances,
the
regulator
may
lose
control
of
its
output
voltage,
yet the current
limiting circuit
may still work. If this
occurs,
F102
will
not
blow,
and
VR101
will
overheat
and
burn
out.
Fortunately,
its
failure
mode is a short-circuit. It will therefore still protect the OPTIMOD-TV
circuitry even in this exceptional circumstance.
8.c) - 15 Volt Regulator: ( on Card # PS -- rear chassis apron)
The - 15
Volt regulator is an operational amplifier containing a discrete power-
booster output stage with current limiting. It " amplifies" the output
of the + 15
Volt regulator by - 1, thus producing a - 15 Volt tracking supply. Shutdown of the
+15 Volt supply ( due to current limit conditions or to a fault which blows F102)
will also result in the - 15 Volt supply's shutting down.
The
basic
opamp
is IC102;
its input resistor R109 and feedback resistor R108
are
equal- valued, resulting in a gain of - 1 + 2%. IC102's negative supply comes
from
the
unregulated - 22
volt
supply.
The
common- mode
range
of
the
301A
opamp
includes its positive power supply, thus permitting operation with IC102's
positive
supply
at
ground. Under
normal
operating conditions, the "+"
input
of
IC102 is grounded, and its "-" input is within 10mV of ground.
0103 and
Q102
form
a conjugate emitter
follower which can boost
the
output
current
of IC102 to
more than
3/4 Amp.
The basic emitter
follower
is Q103;
Q102 is connected in a 100% negative feedback configuration to boost the current
output capability of Q103.
0104
is
a current- limit
transistor.
If
the - 15
Volt
supply
is
called
upon
to
deliver more than 3/4 Amp, sufficient voltage drop ( approximately 0.6 volts) will
occur across R104 to turn on 0104, thus shunting drive current away from Q103
into
the
load
and
protecting
0102/0103
from
burnout.
Under
these
conditions,
IC102 is protected by internal current limiting circuitry.
C113
frequency- compensates
the - 15
Volt
supply
to
protect
it
against
high
frequency oscillations. R102 increases the circuit's immunity to leakage in Q103.
The
rest of the circuitry is protected against a catastrophic failure of the - 15
Volt
regulator
by
means
of
zener
clamp
VR102,
CR106,
and
fuse
F103.
The
operation of this circuit is identical to the operation of the corresponding circuit
in the + 15 Volt regulator ( see 8.b).
8.d) Miscellanous Voltage Supplies:
The
operation
of
these
supplies
is
extremely
straightforward.
No
further
explanation beyond that given in Appendix A is required.
9) Loudness Controller: ( on Card # 2)
[NOTE:
This section should be read in close conjunction with the corresponding
section in Appendix A (System Description)].
The input
signal to be sensed by the Loudness Controller is applied to summing
amplifier IC201A and
associated components. Here, the left
and right
channels
are summed.
If
the
Loudness Controller
is
being
driven
from
the
preemphasized
output
of
Cards # 8/#9 ( as
is
normal
in
a single- chassis
configuration),
then
75us ( or,
optionally,
50us)
deemphasis
is
applied
by
means
of
feedback
elements
R204,
C201. If the Loudness Controller is being driven from the output of Cards # 3/#4
(in
the
split- chassis
configuration),
then
deemphasis
is
not
applied
and
gain
is
slightly
increased to normalize
the threshold at
which loudness control
occurs.
B-14
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