Service manual
IMIOl5
To
set
the
supply
for
a
particular
line
voltage,
the
numbers
on
the
two
switches
are
added
together.
This
allows
the
supply
to
be
set
for
four
different
input
line
voltages.
The
position
of
the
switches
and
the
resultant
input
voltages
is
shown
in
the
schematic.
NOTE
When
changing
the
AC
input
voltage
from
100/120
to
220/240~
the
fuse
(F1) must
also
be changed
INPUT
VOLTAGE
100/120
220/240
FUSE
SIZE
3/4A 125V
SE
3/BA 250V
SE
The low
voltage
supply
uses
a
series-pass
regulator
designed
to
maintain
a
constant
DC
output
for
changes
in
input
voltage,
load
impedance and
temperature.
Also
included
is
a
current
limiting
circuit
designed
to
protect
transistors
connected
to
the
55V
output
of
the
regulated
supply
from
accidental
output
short
circuits
and
load
malfunctions.
The low
voltage
regulator
consists
of
Q2, Q124, Q125, Q126, Ql27 and
their
components.
R206
and
its
circuitry
control
the
current
limiting
feature.
The
primary
voltage
is
stepped
down
at
the
secondary
of
TI where
it
is
rec-
tified
by a
full
wave
bridge
rectifier
AI.
Capacitor
C2
is
used
as
a
filter
capacitor
to
smooth
the
rectified
output
of
AI.
Transistor
Q2
is
used
as
a
series
pass
stage
to
drop
the
rectified
voltage
to
+57
VDC
and
to
provide
a
low
output
impedance.
Approximately
7
volts
is
applied
to
the
base
of
Q127
through
a
divider
network
of
R209
and
211.
A
reference
voltage
from
zener
diode
VRl04
is
applied
to
the
emitter
of
Q127.
If
the
output
voltage
changes,
an
error
current
is
generated
through
Q127.
This
error
current
modulates
the
base
current
of
Q125.
Since
Q2
is
driven
by Ql26
(in
a
darlington
configuration),
output
drive
is
regulated
in
this
manner
to
bring
the
output
voltage
back
to
its
proper
level.
The
short
circuit
protection
or
current
limiting
action
can
be
explained
as
follows.
Assume
the
55
volt
bus becomes
shorted
to
ground.
This
reduced
out-
put
voltage
is
sensed
by
the
base
of
Q127,
turning
that
transistor
off
because
of
the
reverse
bias
across
its
emitter
base
junction.
Simultaneously,
the
in-
creased
current
through
R206
increases
the
forward
voltage
drop
across
the
base
emitter
junction
of
Q126
and
turns
it
on. The
increased
collector
current
through
Ql26
shunts
away
the
base
current
of
Q125.
Since
Q2
is
driven
directly
from Q125,
its
output
current
becomes
limited.
This
closed
loop
oPeration
continues
until
a
stable
point
is
reached
at
which
the
current
available
during
a
short
circuit
condition
is
maintained
at
approximately
100
rnA.
This
"foldback"
action
limits
dissipation
in
the
monitor
to
safe
levels
during
fault
conditions
and
prevents
needless
device
failures
due
to
accidental
short
circuits.
3-6