Orban 674A - Page 34

The
nominal
unregulated
voltage
is
+22 volts
DC at
rated
line
voltage.
This
will
vary
widely
with line
voltage
variations.
Regulator
dropout
will
occur
if
the
unregulated
voltage
falls
below
about
+17.8 volts.
Regulated
voltages
are
supplied by
a
pair
of
overrated
500mA "three
terminal
1C
regulators
VR80I,
VR802.
Because
they
are
operated
so
conservatively,
they
can be
expected
to
be
extremely
reliable.
Therefore,
before
replacing
the
regulators,
check
to
see
whether
other
abnormalities
in the
circuitry
(such
as a
shorted
1C)
have
caused
excessive
current
demand
which
is
causing
the
regulator
IC's to go
either
into
current
limiting
or
into
thermal
shutdown,
their
two
built-in
protective
modes.
If if
becomes
necessary
to
replace
a
regulator,
be
sure
to
replace
its heat
sink
securely.
The
regulators
VR80I
and
VR802 are
frequency-compensated
by
C803,
C804
at
their
outputs
to
prevent
high
frequency
oscillations.
If
C803
or
C804
are
ever
replaced, be
sure
to
use
low-inductance
aluminum
electrolytics.
Tantalums
can
fail
because
the
current-delivering
capacity
of the
power
supply
can
cause a
runaway
condition
if
the
dielectric
is
punctured
momentarily;
high-inductance
aluminums
can
fail to
prevent
the
regulators
from
oscillating.
It is
therefore
necessary to
check
for
oscillation
on
the
power
bus
with an
oscilloscope
if C803
or
C804
is
replaced.
CR805,
CR806
are
connected
from
ground
to
each
power
bus
in
reverse
polarity,
and
protect
the
rest
of the
circuitry
from a
fault
condition
that
might
otherwise
cause a
reverse
polarity
voltage
on
either
power bus.
In addition,
small
0.05uF/25V
ceramic
capacitors
bypass
the
power
busses
to
ground
locally
throughout
the
board
to
prevent
signal-carrying
IC's from
oscillating
due to
excessive
power-lead
inductance.
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